PERIPHERAL INTERFACINGUNIT III

Mr. S. VINODLECTURER

EEE DEPARTMENT

• Study of Architecture and programming of ICs: 8255 PPI,8259 PIC, 8251 USART, 8279 Key board display controller and

8253 Timer/ Counter – Interfacing with 8085 - A/D and D/A converter interfacing.

Block Diagram of 8255

It has a 40 pins of 4 groups.

1. Data bus buffer2. Read Write control logic3. Group A and Group B controls4. Port A, B and C

Data bus buffer: •This is a tri state bidirectional buffer used to interface the 8255 to system data bus. Data is transmitted or received by the buffer on execution of input or output instruction by the CPU.

• Control word and status information are also transferredthrough this unit.

Read/Write control logicThis unit accepts control signals ( RD, WR ) and also inputs from address bus and

issues commands to individual group of control blocks( Group A, Group B).It has the following pins.

a) CS – Chipselect : A low on this PIN enables thecommunication between CPU and 8255.b) RD (Read) – A low on this pin enables the CPU to readthe data in the ports or the status word through data bus buffer.c) WR ( Write ) : A low on this pin, the CPU can writedata on to the ports or on to the control register throughthe data bus buffer.d) RESET: A high on this pin clears the control registerand all ports are set to the input modee) A0 and A1 ( Address pins ): These pins in conjunctionwith RD and WR pins control the selection of one of the3 ports.

Group A and Group B controls

These block receive control from the CPU and issues commands to their respective ports.

• Group A - PA and PCU ( PC7 –PC4)• Group B - PCL ( PC3 – PC0)• Control word register can only be written into

no read operation of the CW register is allowed.

PORTSa) Port A: This has an 8 bit latched/buffered O/P and 8bit input latch. It can be programmed in 3 modes – mode 0,mode 1, mode 2.b) Port B: This has an 8 bit latched / buffered O/P and 8bit input latch. It can be programmed in mode 0, mode1.c) Port C : This has an 8 bit latched input buffer and 8 bitout put latched/buffer. This port can be divided into two 4bit ports and can be used as control signals for port A andport B. it can be programmed in mode 0.

Modes of Operation of 8255• These are two basic modes of operation of 8255. I/O

mode and Bit Set-Reset mode (BSR).• In I/O mode, the 8255 ports work as programmable

I/O ports, while in BSR mode only port C (PC0-PC7) can be used to set or reset its individual port bits.

• Under the I/O mode of operation, further there are three modes of operation of 8255, so as to support different types of applications, mode 0, mode 1 and mode 2.

• BSR Mode: In this mode any of the 8-bits of port C can be set or reset depending on D0 of the control word. The bit to be set or reset is selected by bit select flags D3, D2 and D1 of the CWR as given in table.

Mode 0• I/O Modes :a) Mode 0 ( Basic I/O mode ): This mode is also called

as basic input/output mode. This mode provides simple input and output capabilities using each of the three ports. Data can be simply read from and written to the input and output ports respectively, after appropriate initialization.

Programming 8255

Mode 1:

— Ports A and B are programmed as input or output ports— Port C is used for handshaking

PA[7:0]STBAIBFAINTRAPC3

PC5PC4

PB[7:0]STBBIBFBINTRBPC0

PC1PC2

PC6, 7

8255

PA[7:0]OBFAACKAINTRAPC3

PC6PC7

PB[7:0]OBFBACKBINTRBPC0

PC1PC2

PC4, 5

8255

11-15

Programming 8255 Mode 2:

— Port A is programmed to be bi-directional— Port C is for handshaking— Port B can be either input or output in mode 0 or mode 1

PA[7:0]OBFAACKA

INTRA

PC4PC6PC7

STBA

IBFA

PC0

PC3PC58255

PC0PC0

PB[7:0]

CONROL WORD

11-18

example 1 -Keyboard

11-19

Bouncing Problem

11-20

Bouncing

example 2 –stepper motor• Unipolar full step Stepper motor: • INPUT SEQUENCE: X Y X Y

1 1 0 0 0 1 1 0 0 0 1 1 1 0 0 1

• 8255 CONTROL WORD: 1 0 0 0 0 0 0 0 =80H• control register – 0F• PORT ADDRESS

port A - 0Cport B - 0Dport C - 0E

MVI A, 80 DELAY: MVI C, FFH OUT 0F LOOP:DCR C

START: MVI A, 0C JNZ LOOP OUT 0C RETCALL DELAY MVI A, 06 OUT 0C CALL DELAY MVI A, 03 OUT 0C CALL DELAY MVI A, 09 OUT 0C JMP START

TRAFFIC LIGHT CONTROLLER• Common Cathode: 1 – ON

0 – OFF • uses 8255 . • To make all ports as O/P ports: Control word 80H• 8255 port addresses: • If 8255 Chip is loaded in PCB left port:• Port A 0CH• Port B 0DH• Port C 0EH• Control register Address 0FH• • If 8255 Chip is loaded in PCB right port:• Port A 14H• Port B 15H• Port C 16H• Control register Address 17H

MVI A, 80OUT 0F

S: MVI C, 03LXI H, 4200

L: MOV A, MOUT 0CINX HMOV A,MOUT 0DINX HMOV A, MOUT 0EINX HDCR CJNZ LJMP S

DELAY: MVI B, 05S1: MVI D ,FFS2: MVI E, FFS3: DCR E

JNZ S3DCR DJNZ S2DCR BJNZ S1RET

TEST PROGRAMMVI A, 80OUT 0FMVI 01OUT 0C

DATA 01, 02, 04, 08, 10, 20, 40, 80

Address Data

4200

4201

4202

4203

4204

4205

4206

4207

4208

LOOP UP TABLE:

8279Programmable Keyboard/Display

Interface

• A programmable keyboard and display interfacing chip.• Scans and encodes up to a 64-key keyboard. And Controls up

to a 16 digit numerical display. • Keyboard section has a built-in FIFO 8 character buffer.• The display is controlled from an internal 16x8 RAM that

stores the coded display information.• 8279 has 8 control words to be considered before It is

programmed

I/O InterfaceBasic Description of the 8279

• A0: Selects data (0) or control/status (1) for reads and writes between 8085 and 8279.

Output that blanks the displays.

• CLK: Used internally for timing. Max is 3 MHz.

• CN/ST: Control/strobe, connected to the control key on the keyboard.

• Chip select that enables programming, reading the keyboard, etc.DB7-DB0: Consists of bi-directional pins that connect to data bus from 8085.

I/O Interface

• IRQ: Interrupt request, becomes 1 when a key is pressed, data is available.

• OUT A3-A0/B3-B0: Outputs that sends data to the most significant/least significant of display.

• : Connects to 8085 WR or RD signal, reads data/status registers.

• RESET: Connects to system RESET.

• RL7-RL0: Return lines are inputs used to sense key depression in the keyboard matrix.

•Shift: Shift connects to Shift key on keyboard.

• SL3-SL0: Scan line outputs scan both the keyboard and displays.

8279 INTERNAL ARCTHITECTURE

Architecture and Signal Descriptions of8279

• I/O Control and Data Buffers : The I/O control section• controls the flow of data to/from the 8279. The data buffers interface the

external bus of the system with internal bus of 8279.• The I/O section is enabled only if CS is low. The pins A0, RD and WR

select the command, status or data read/write operations carried out by the CPU with 8279.

• Control and Timing Register and Timing Control :• These registers store the keyboard and display modes and other operating

conditions programmed by CPU. The registers are written with A0=1 and WR=0. The Timing and control unit controls the basic timings for the operation of the circuit. Scan counter divide down the operating frequency of 8279 to derive scan keyboard and scan display frequencies.

Scan Counter : • The scan counter has two modes to scan the key matrix and refresh the display.

In the encoded mode, the counter provides binary count that is to be externally decoded to provide the scan lines for keyboard and display (Four externally decoded scan lines may drive upto 16 displays). In the decode scan mode, the counter internally decodes the least significant 2 bits and provides a decoded 1 out of 4 scan on SL0-SL3(Four internally decoded scan lines may drive up to 4 displays). The keyboard and display both are in the same mode at a time.

Return Buffers and Keyboard Debounce and Control:• This section for a key closure row wise. If a key closer is detected, the

keyboard debounce unit debounces the key entry (i.e. wait for 10 ms). After the debounce period, if the key continues to be detected. The code of key is directly transferred to the sensor RAM along with SHIFT and CONTROL key status.

Architecture and Signal Descriptions of8279

FIFO/Sensor RAM and Status Logic: In keyboard or• strobed input mode, this block acts as 8-byte first-in-first out (FIFO) RAM.

Each key code of the pressed key is entered in the order of the entry and in the mean time read by the CPU, till the RAM become empty.

• The status logic generates an interrupt after each FIFO read operation till the FIFO is empty. In scanned sensor matrix mode, this unit acts as sensor RAM. Each row of the sensor RAM is loaded with the status of the corresponding row of sensors in the matrix. If a sensor changes its state, the IRQ line goes high to interrupt the CPU.

Display Address Registers and Display RAM : The• display address register holds the address of the word currently being

written or read by the CPU to or from the display RAM. The contents of the registers are automatically updated by 8279 to accept the next data entry by CPU.

Architecture and Signal Descriptions of8279

Modes of Operation of 8279The modes of operation of 8279 are as follows• 1. Input (Keyboard) modes.• 2. Output (Display) modes. Input ( Keyboard ) Modes : 8279 provides three input modes. These modes are as follows:1. Scanned Keyboard Mode : This mode allows a key matrix to be

interfaced using either encoded or decoded scans. In encoded scan, an 8*8 keyboard or in decoded scan, a 4*8 keyboard can be interfaced. The code of key pressed with SHIFT and CONTROL status is stored into the FIFO RAM.

2. Scanned Sensor Matrix : In this mode, a sensor array can be interfaced with 8279 using either encoded or decoded scans. With encoded scan 8*8 sensor matrix or with decoded scan 4*8 sensor matrix can be interfaced. The sensor codes are stored in the CPU addressable sensor RAM.

3. Strobed input: In this mode, if the control lines goes low, the data on return lines, is stored in the FIFO byte by byte.

(i)Scanned Keyboard mode with 2 Key Lockout :• In this mode of operation, when a key is pressed, a debounce logic comes

into operation. During the next two scans, other keys are checked for closure and if no other key is pressed the first pressed key is identified.

• The key code of the identified key is entered into the FIFO with SHIFT and CNTL status, provided the FIFO is not full, i.e. it has at least one byte free. If the FIFO does not have any free byte, naturally the key data will not be entered and the error flag is set.

• If FIFO has at least one byte free, the above code is entered into it and the 8279 generates an interrupt on IRQ line to the CPU to inform about the previous key closures. If another key is found closed during the first key, the keycode is entered in FIFO.

• If the first pressed key is released before the others, the first will be ignored. A key code is entered to FIFO only once for each valid depression, independent of other keys pressed along with it, or released before it.

• If two keys are pressed within a debounce cycle (simultaneously ), no key is recognized till one of them remains closed and the other is released. The last key, that remains depressed is considered as single valid key depression.

(ii)Scanned Keyboard with N-Key Rollover• In this mode, each key depression is treated independently.

When a key is pressed, the debounce circuit waits for 2 keyboards scans and then checks whether the key is still depressed. If it is still depressed, the code is entered in FIFO RAM.

• Any number of keys can be pressed simultaneously and recognized in the order, the keyboard scan recorded them. All the codes of such keys are entered into FIFO.

• In this mode, the first pressed key need not be released before the second is pressed. All the keys are sensed in the order of their depression, rather in the order the keyboard scan senses them, and independent of the order of their release.

(iii). Scanned Keyboard Special Error Mode :

• This mode is valid only under the N-Key rollover mode. This mode is programmed using end interrupt / error mode set command. If during a single debounce period ( two keyboard scans ) two keys are found pressed , this is considered a simultaneous depression and an error flagis set.

• This flag, if set, prevents further writing in FIFO but allows the generation of further interrupts to the CPU for FIFO read. The error flag can be read by reading the FIFO status word. The error Flag is set by sending normal clear command with CF = 1.

(iv)Sensor Matrix Mode :

• In the sensor matrix mode, the debounce logic is inhibited. The 8-byte FIFO RAM now acts as 8 * 8 bit memory matrix. The status of the sensor switch matrix is fed directly to sensor RAM matrix. Thus the sensor RAM bits contains the row wise and column wise status of the sensors in the sensor matrix.

• The IRQ line goes high, if any change in sensor value is detected at the end of a sensor matrix scan or the sensor RAM has a previous entry to be read by the CPU. The IRQ line is reset by the first data read operation, if AI = 0, otherwise, by issuing the end interrupt command. AI is a bit in read sensor RAM word.

Output (Display) Modes : 8279 provides two output modes for selecting the display options. These are discussed briefly.

1. Display Scan : In this mode 8279 provides 8 or 16character multiplexed displays those can be organized as dual 4- bit or single 8-bit display units.

2. Display Entry : ( right entry or left entry mode ) 8279 allows options for data entry on the displays. The display data is entered for display either from the right side or from the left side.

Command Words of 8279• All the command words or status words are written or read

with A0 = 1 and CS = 0 to or from 8279. This section describes the various command available in 8279.

1.Keyboard Display Mode Set 2. Programmable clock3. Read FIFO / Sensor RAM 4. Read Display RAM5. Write Display RAM 6. Display Write Inhibit/Blanking7. Clear Display RAM 8. End Interrupt / Error mode

1.Keyboard Display Mode Set

2. Programmable clock• The clock for operation of 8279 is obtained by dividing the external clock

input signal by a programmable constant called prescaler.• PPPPP is a 5-bit binary constant. The input frequency isdivided by a decimal

constant ranging from 2 to 31,decided by the bits of an internal prescaler, PPPPP.If pin 3 of the 8279 is being clocked by a 2 MHz signal, ppppp should be set to 10100 to divide the clock by 20 to yield the proper 100 KHz operating frequency.

3.Read FIFO / Sensor RAM

4. Read Display RAM

5.Write Display RAM

6. Display Write Inhibit/Blanking• The IW ( inhibit write flag ) bits are used to mask the individual set as

shown in the below command word. The output lines are divided into two set ( OUTA0 – OUTA3 ) and ( OUTB0 – OUTB3 ), those can be masked by setting the corresponding IW bit to 1.

• Once a set is masked by setting the corresponding IW bit to 1, the entry to display RAM does not affect the set even though it may change the unmasked set. The blank display bit flags (BL) are used for blanking A and B nibbles.

• Here D0, D2 corresponds to OUTB0 – OUTB3 while D1 and D3 corresponds to OUTA0-OUTA3 for blanking and masking.

• If the user wants to clear the display, blank (BL) bits are available for each set as shown in format. Both BL bits will have to be cleared for blanking both the set.

7.Clear Display RAM

8.End Interrupt / Error mode

Rolling Message In Seven Segment Display

FORMAT FOR SEVEN SEGMENTS:

d c B a dp e g f

Common anode: 0 – ON

1 – OFF

command word

1.Keyboard Display Mode Set 10h2. Write Display RAM 90h

START : LXI H, 412CHMVI D, 0FHMVI A, 10HOUT 01MVI A, 90HOUT 01

LOP1: MOV A, MOUT 00CALL DELAYINX HDCR DJNZ LOP 1JMP START

DELAY: MVI B, A0LOP3: MVI C, FFLOP2: DCR C

JNZ LOP2DCR BJNZ LOP3RET

Address Opcode Comment

412C FF Blank

412D FF Blank

412F FF Blank

4130 FF Blank

4131 FF Blank

4132 FF Blank

4133 FF Blank

4134 FF Blank

4135 98 H

4136 68 E

4137 7A L

4138 C8 P

4139 FF Blank

413A FF Blank

413B FF Blank

413C FF Blank

Flashing Message In Seven Segment Display

START : LXI H, 4200HMVI D, 06HMVI A, 10HOUT 01MVI A, 90HOUT 01

LOP: MOV A, MOUT 00INX HDCR DJNZ LOP CALL DELAY MVI D, 06H

LXI H, 4300HLOOP; MOV A, M

OUT 00INX HDCR DJNZ LOOPCALL DELAYJMP START

DELAY: MVI B, FFLOP3: MVI C, FFLOP2: DCR C

JNZ LOP2DCR B

JNZ LOP3RET

Address Opcode Comment

4200 98 H

4201 68 E

4202 7A L

420342044205

C8FFFF

PBlankBlank

4300 FF Blank

4301 FF Blank

4302 FF Blank

430343044305

FFFFFF

BlankBlankBlank

ACCEPT A KEY AND DISPLAYMVI B, 08MVI A, 00OUT CNT ; set mode and display

MVI A, 0CC;set clear display

OUT CNTMVI A, 90 ; write display

OUT CNTMVI A, 0FF ;clear the display

BACK: OUT DATDCR BJNZ BACK

LOP: IN CNT ; lop for the pressing of key

ANI 07 JZ LOPMVI A, 40 ; set to read FIFO

OUT CNTIN DATANI 0F ; get the

corresponding

MOV L, A : code from look up table

MVI H, 42MOV A,MOUT DATJMP LOP

Address Opcode

4200 OC 9F 4A 0B

4204 99 29 28 8F

4208 08 09 88 38

420C 6C 1A 68 E8

• CNT C2 01• DAT C0 00

8259 Programmable Interrupt Controller

8259 Programmable Interrupt Controller

• The 8259 programmable interrupt controller (PIC) adds eight vectored priority encoded interrupts to the microprocessor.

• It accepts request from the peripheral equipment, determine which of the incoming requests is of the highest importance

• Special features of 8259:• Eight level priority controller• Expandable to 64 levels• Programmable interrupt modes• Individual request mash capability

It is packaged in a 28-pin DIP, uses NMOS technology and requires a single a5V supply. Circuitry is static, requiring no clock input.

The 8259A is designed to minimize the software and real time overhead in handling multi-level priority interrupts.

It has several modes, permitting optimization for a variety of system requirements

The interrupts at the IR input lines are handled by two registers in cascade, the Interrupt Request Register (IRR) and the In-Service (ISR). The IRR is used to store all the interrupt levels which are requesting service; and the ISR is used to store all the interrupt levels which are being serviced.

PRIORITY RESOLVERThis logic block determines the priorities of the bits set in the IRR. The highest priority

is selected and stroed into the corresponding bit of the ISR during INTA pulse.

INTERRUPT MASK REGISTER (IMRThe IMR stores the bits which mask the interrupt lines to be masked. The IMR operates

on the IRR. Masking of a higher priority input will not affect the interrupt request lines of lower quality.

INT (INTERRUPT)This output goes directly to the CPU interrupt input. The V level on this line is

designed to be fully compatible with the 8080A, 8085A and 8086 inputlevels.

INTERRUPT REQUEST REGISTER (IRR) AND IN-SERVICE REGISTER (ISR)

INTA (INTERRUPT ACKNOWLEDGE) INTA pulses will cause the 8259A to release vectoring information onto the data bus. The format of

this data depends on the system mode (mPM) of the 8259A.DATA BUS BUFFER

This 3-state, bidirectional 8-bit buffer is used to interface the 8259A to the system Data Bus. Control words and status information are transferred through the Data Bus Buffer.

READ/WRITE CONTROL LOGICThe function of this block is to accept Output commands from the CPU. It contains the Initialization Command Word (ICW) registers and Operation Command Word (OCW) registers which store the various control formats for device operation. This function block also allows the status of the 8259A to be transferred onto the Data Bus.

CS (CHIP SELECT)A LOW on this input enables the 8259A. No reading or writing of the chip will occur unless the device is selected.

WR (WRITE)A LOW on this input enables the CPU to write control words (ICWs and OCWs) to the 8259A.

RD (READ)A LOW on this input enables the 8259A to send the status of the Interrupt Request Register (IRR), In Service Register (ISR), the Interrupt Mask Register (IMR), or the Interrupt level onto the Data Bus.

A0This input signal is used in conjunction with WR and RD signals to write commands into the various command registers, as well as reading the various status registers of the chip. This line can be tied directly to one of the address lines

.

The events occur as follows in an MCS-80/85 system:

1. One or more of the INTERRUPT REQUEST lines (IR7±0) are raised high, setting the corresponding IRR bit(s).2. The 8259A evaluates these requests, and sends an INT to the CPU, if appropriate.3. The CPU acknowledges the INT and responds with an INTA pulse.4. Upon receiving an INTA from the CPU group, the highest priority ISR bit is set, and the corresponding IRR bit is reset. The 8259A will also release a CALL instruction code (11001101) onto the 8-bitData Bus through its D7±0 pins.5. This CALL instruction will initiate two more INTA pulses to be sent to the 8259A from the CPU group.6. These two INTA pulses allow the 8259A to release its preprogrammed subroutine address onto the Data Bus. The lower 8-bit address is released at the first INTA pulse and the higher 8-bitaddress is released at the second INTA pulse.7. This completes the 3-byte CALL instruction released by the 8259A. In the AEOI mode the ISR bit is reset at the end of the third INTA pulse. Otherwise, the ISR bit remains set until an appropriate EOI command is issued at the end of the interrupt sequence.

INTERRUPT SEQUENCE

The 8259A accepts two types of command words generated by the CPU:1. Initialization Command Words (ICWs): Before normal operation can begin, each 8259A in the system must be brought to a

starting point -by WR pulses.2. Operation Command Words (OCWs): These are the command words which

command the 8259A to operate in various interrupt modes. These modes are:

a. Fully nested modeb. Rotating priority modec. Special mask moded. Polled mode

The OCWs can be written into the 8259A anytime after initialization.

PROGRAMMING THE 8259A

Initialization Sequence

INITIALISATION COMMAND WORDSICW1

• A5±A15: Page starting address of service routines .In an MCS 80/85 system, the 8 request levels will generate CALLs to 8 locations equally spaced in memory. These can be programmed to be spaced at intervals of 4 or 8 memory locations, thus the 8 routines will occupy a page of 32 or 64 bytes, respectively. The address format is 2 bytes long (A0±A15). When the routine interval is 4, A0±A4 are automatically inserted by the 8259A, while A5±A15 are programmed externally. When the routine interval is 8, A0±A5 are automatically inserted by the 8259A, while A6±A15 are programmed externally.

• T: If LTIM e 1, then the 8259A will operate in the level interrupt mode. Edge detect logic on the interrupt inputs will be disabled.

• ADI: CALL address interval. ADI = 1 then interval = 4; ADI e 0 then interval e 8.

• SNGL: Single. Means that this is the only 8259A in the system. If SNGL = 1 no ICW3 will be issued.

• IC4: If this bit is setÐICW4 has to be read. IfICW4 is not needed, set IC4 = 0.

ICW 2

This word is read only when there is more than one 8259A in the system and cascading is used, in which case SNGL e 0. It will load the 8-bit slave register.

The functions of this register are:• a. In the master mode (either when SP = 1, or in buffered mode when M/S = 1 in ICW4) a ``1''

is set for each slave in the system. The master then will release byte 1 of the call sequence (for MCS- 80/85 system) and will enable the corresponding slave to release bytes 2 and 3 (for 8086 only byte 2) through the cascade lines.

• b. In the slave mode (either when SP e 0, or if BUF e 1 and M/S e 0 in ICW4) bits 2±0 identify the slave. The slave compares its cascade input with these bits and, if they are equal, bytes 2 and 3 of the call sequence (or just byte 2 for 8086) are released by it on the Data Bus. d only when there is more than one 8259A in the system and cascading is used, in which

ICW 3

If BUF=0,M/S is to be neglected.

Programmable IntervalTimer 8253

• The Intel 8253 is a programmable counter / timer chip designed for use as an Intel microcomputer peripheral. It uses N-MOS technology with a single +5V supply and is packaged in a 24-pin plastic DIP.

• It is organized as 3 independent 16-bit counters, each with a counter rate up to 2 MHz . All modes of operation are software programmable.

• Clock This is the clock input for the counter. The counter is 16 bits. The maximum clock frequency is 1 / 380 nanoseconds or 2.6 megahertz. The minimum clock frequency is DC or static operation.

• Out This single output line is the signal that is the final programmed output of the device. Actual operation of the out line depends on how the device has been programmed.

• Gate This input can act as a gate for the clock input line, or it can act as a start pulse, depending on the programmed mode of the counter.

Block diagram of 8253

Data Bus Buffer :• This tri-state, bi-directional, 8-bit buffer is used to interface the 8253/54 to the

system data bus. The Data bus buffer has three basic functions.1. Programming the modes of 8253/54.2. Loading the count registers.3. Reading the count values.

Read/Write Logic : The Read/Write logic has five signals : RD, WR, CS and the• address lines A0 and A1. In the peripheral I/O mode, the RD, and WR signals are

connected to IOR and IOW, respectively. In memory-mapped I/O, these are connected to MEMR and MEMW. Address lines A0 and A1 of the CPU are usually connected to lines A0 and A1 of the 8253/54, and CS is tied to a decoded address. The control word register and counters are selected according to the signals on lines A0 and A1.

Control Word Register : This register is accessed when lines A0 and A1 are at logic 1. It is used to write a command word which specifies the counter to be used (binary or BCD), its mode, and either a read or write operation.

Counters : These three functional blocks are identical in operation. Each counter• consists of a single, 16 bit, pre-settable, down counter. The counter can

operate in either binary or BCD and its input, gate and output are configured by the selection of modes stored in the control word register. The counters are fully independent. The programmer can read the contents of any of the three counters without disturbing the actual count in process.

Programming the 8253/54 : • Each counter of the 8253/54 is individually programmed by writing a control word

into the control word register (A0 - A1 = 11).

WRITE Operation :1. Write a control word into control register.2. Load the low-order byte of a count in the counter register.3. Load the high-order byte of count in the counter register.READ Operation :

In some applications, especially in event counters, it is necessary to read the value of the count in process. This can be done by two possible methods:

1. Simple Read : It involves reading a count after inhibiting the counter by controlling the gate input or the clock input of the selected counter, and two I/O read operations are performed by the CPU. The first I/O operation reads the low-order byte, and the second I/O operation reads the high order byte.

2. Counter Latch Command : In the second method, an appropriate control word is written into the control

register to latch a count in the output latch, and two I/O read operations are performed by the CPU. The first I/O operation reads the low-order byte, and the second I/O operation reads the high order byte.

MODES OF 8253• Mode 0 : Interrupt on terminal count• MODE 1 : Hardware Retrigger able One-shot• MODE 2 : Rate generator• MODE 3 : Square Wave Rate Generator• MODE 4 : Software Triggered Strobe.• MODE 5 : Hardware triggered strobe (Retrigger able).

Mode 0 : Interrupt on terminal count

• 1) The output will be initially low after the mode set operation.• 2) After the count is loaded into the selected count Register the output

will remain low and the counter will count. • 3) When the terminal count is reached the output will go high and

remain high until the selected count is reloaded.• 1)Gate = 1 enables counting.• 2) Gate = 0 disables counting.

Let us set channel 0 in mode 0START: MVI A, 30

OUT 0CEHMVI A, 05OUT 0C8HMVI A, 00OUT 0C8HHLT

Observe, using an CRO that the output of channel 0 is initially low. After giving six clock pulse the output goes high

MODE 1 : Hardware Retrigger able One-shot

a) Normal operation1) The output will be initially high2) The output will go low on the CLK pulse following the rising edge at the

gate input.3) The output will go high on the terminal count and remain high until the

next rising edge at the gate input.b) Retriggering

The one shot is retrigger able, hence the output will remain low for the full count after any rising edge of the gate input.

c) New countIf the counter is loaded during one shot pulse, the current one shot is not affected unless the counter is retriggered. If retriggered, the counter is loaded with the new count and the one-shot pulse continues until the new count expires.

START: MVI A, 32OUT 0CEHMVI A, 05OUT 0C8HMVI A, 00OUT 0C8HOUT 0D0H ;triggers gate 0HLT

MODE 2 : Rate generator

This mode functions like a divide by-N counter.a) Normal Operation1) The output will be initially high.2) The output will go low for one clock pulse before the terminal count.3) The output then goes high, the counter reloads the initial count and the

process is repeated.4) The period from one output pulse to the next equals the number of input

counts in the count register.b) Gate Disable1) If Gate = 1 it enables a counting otherwise it disables counting (Gate = 0 ).2) If Gate goes low during an low output pulse, output is set immediately

high. A trigger reloads the count and the normal sequence is repeated.c) New count The current counting sequence does not affect when the new

count is written. If a trigger is received after writing a new count but before the end of the current period, the new count will be loaded with the new count on the next CLK pulse and counting will continue from the new count. Otherwise, the new count will be loaded at the end of the current counting cycle.

Using mode 2, divide the clock preset at channel 1 by 10

START: MVI A, 74OUT 0CEH ;channel 1 in mode 2MVI A, 0AH ;LSB of countOUT 0CAHMVI A, 00H ;MSB of countOUT 0CAHHLT

Mode 3 Square Wave Rate Generatora)Normal operation1) Initially output is high.2) For even count, counter is decremented by 2 on the falling edge of each clock pulse. When the

counter reaches terminal count, the state of the output is changed and the counter is reloaded with the full count and the whole process is repeated.

3) If the count is odd and the output is high the first clock pulse (after the count is loaded) decrements the count by 1. Subsequent clock pulses decrement the clock by 2. After timeout, the output goes low and the full count is reloaded. The first clock pulse (following the reload) decrements the count by 3 and subsequent clock pulse decrement the count by two. Then the whole process is repeated. In this way, if the count is odd, the output will be high for (n+1)/2 counts and low for (n-1)/2 counts.

b) Gate DisableIf Gate is 1 counting is enabled otherwise it is disabled. If Gate goes low while output is low, output is set high immediately. After this, When Gate goes high, the counter is loaded with the initial count on the next clock pulse and the sequence is repeated.

c) New CountThe current counting sequence does not affect when the new count is written. If a trigger is received after writing a new count but before the end of the current half-cycle of the square wave, the counter will be loaded with the new count on the next CLK pulse and counting will continue from the new count. otherwise, the new count will be loaded at end of the current half-cycle.

To generate a square wave of frequency 150 KHz at channel

START: MVI A, 36OUT 0CEHMVI A, 0AOUT 0C8HMVI A, 00OUT 0C8HHLT

Vary the frequency by varying the count . Here the maximum count is FFFF. Thus with the clock frequency of 1.5 MHZ, which corresponds to 0.6 micro sec

MODE 4 : Software Triggered Strobea) Normal operation1) The output will be initially high2) The output will go low for one CLK pulse after the terminal count (TC).3) And become high againb) Gate Disable• If Gate is one the counting is enabled otherwise it is disabled. The Gate

has no effect on the output.c) New count• If a new count is written during counting, it will be loaded on the next CLK

pulse and counting will continue from the new count. If the count is two byte then

1) Writing the first byte has no effect on counting.2) Writing the second byte allows the new count to be loaded on the next

CLK pulse.

MODE 5 : Hardware triggered strobe Retriggerable

a) Normal operation• 1) The output will be initially high.• 2) The counting is triggered by the rising edge of the Gate.• 3) The output will go low for one CLK pulse after the terminal count (TC).b) Retriggering

If the triggering occurs on the Gate input during the counting, the initial count is loaded on the next CLK pulse and the counting will be continued until the terminal count is reached.

• c) New count• If a new count is written during counting, the current counting sequence

will not be affected. If the trigger occurs after the new count is written but before the terminal count, the counter will be loaded with the new count on the next CLK pulse and counting will continue from there.

ADC • ADC0808/ADC0809 8-Bit μP Compatible A/D Converters with

8-Channel Multiplexer• The 8-bit A/D converter uses successive approximation as the

conversion technique.• The 8-channel multiplexer can directly access any of 8-single-

ended analog signals.• Key Specifications:

Resolution 8 BitsSingle Supply 5 VDCLow Power 15 mWConversion Time 100 μs

successive approximation

The successive approximation circuit typically consists of four block:– A sample and hold circuit to acquire the input voltage (Vin). – An analog voltage comparator that compares Vin to the

output of the internal DAC and outputs the result of the comparison to the successive approximation register (SAR).

– A successive approximation register block designed to supply an approximate digital code of Vin to the internal DAC.

– An internal reference DAC that supplies the comparator with an analog voltage equivalent of the digital code output of the SAR for comparison with Vin.

The successive approximation register is initialized so that the (MSB) is equal to a digital 1. This code is fed into the DAC which then supplies the analog equivalent of this digital code (Vref/2) into the comparator circuit for comparison with the sampled input voltage. If this analog voltage exceeds Vin the comparator causes the SAR to reset this bit; otherwise, the bit is left a 1. Then the next bit is set to 1 and do the same test, continuing this binary search until every bit in the SAR has been tested. The resulting code is the digital approximation of the sampled input voltage and is finally output by the DAC at the end of the conversion (EOC).

Program for analog to digital conversion

MVI A, 10H ; CHANNEL 0OUT CEMVI A, 18H ; CONTROL WORD OUT CEMVI A, 01 ;START PULSEXRA A XRA A XRA A XRA AMVI A, 00 ;STOP PULSE

LOOP: IN D8 ;END OF CONVERSIONANI 01CPI 01JNZ LOOPINC0STA 4200HLT

Program for digital to analog conversionsquare wave

START: MVI A, 00OUT C0HCALL DELAYMVI A, FFOUT COH ; port address

DACCALL DELAY JMP START

DELAY: MVI B, 05L1: MVI C, 0FFHL2: DCR C

JNZ L2DCR BJNZ L1RET

Program for digital to analog conversionSaw tooth wave form

START: MVI A, 00L1: OUT 0C0H ; port address of DAC

INR AJNZ L1

Program for digital to analog conversionTriangular waveform

START: MVI L, 00HL1: MOV A, L

OUT C0HINR LJNZ L1MVI L, 0FFH

L2: MOV A, LOUT C0DCR LJNZ L2JMP START

Universal Asynchronous Receiver Transmitter

(UART - 8251)

Synchronous vs. Asynchronous

A communication protocol is a convention for data transmission that include such functions as timing, control, formatting, and data presentation. There are two categories depending on the clocking of the data on the serial link:

• Synchronous protocols--each successive datum in a stream of data is governed by a master clock and appears at a specific interval in time.

• Asynchronous protocols--successive data appear in the data stream at arbitrary times, with no specific clock control governing the relative delays between data.

There are special IC chips made for serial data communications. These chip is called UART (universal asynchronous receiver transmitter) and USART (universal synchronous-asynchronous receiver-transmitter) 8251.

Transfer Types

• DTE– data terminal equipment– e.g. computer, terminal

• DCE– data communication equipment– connects DTE to communication lines– e.g. modem

• RS-232C A widely accepted interface standard originally developed to foster data communication on

public telephone network through a modemThis has been adapted to the communication of terminals (PCs) directly to computers.

DTE Connections

25-Pin RS232 Connector

9-Pin RS232 Connector

TTL to RS-232

RS-232 Frame Format

…0b0 1b nb p 1s 2sStart bit

ASCII Parity Stop bit

111101000001111

Idle A

Example

8251 Block Diagram

• The UART is a universal asynchronous receiver/transmitter, which is modeled on the real-world Intel® 8251 peripheral interface adapter component. In the model we are considering, the UART consists of three main blocks.

• a serial transmit block• a serial receive block and• a CPU Interface (I/F) block.

Mode Instruction (Asynchronous)

Mode Instruction (Synchronous)

Command Register

Status Register