Lcd Handshaking

8/12/2019 Lcd Handshaking

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Processor I /O Expans ion & Handsh ak ing 1 (Rev. 2/25/2013)

Sim ple Port I /OI/O ports can be used as parallel ports for exchanging information with an

external device. The number of bits that are grouped together depends upon theapplication Figure 1 demonstrates the simplest form of using I/O ports for parallel IO.This configuration assumes that the external device requires bi-directionalcommunications and it assumes that separate pins on the external device are used forreceiving and sending data. As noted on Figure 1, the only way for the external device toknow when the microprocessor has changed the data is to compare successive readings ofthe microprocessors output port. The same is tr ue for the microprocessor detectingchanges of output from the external device.

Figure 1. Configuration of using two ports for parallel IO

Handsh ak ing Techn iquesMore sophisticated devices require synchronization . Synchronization techniques

notify the microcontroller when new data is at an input port and notify external deviceswhen new data is at an output port. Synchronization takes two forms, strobes and state orlevel handshaking.

In general, handshaking negotiates secure and reliable transfer of data commonlycalled communications. It involves both knowledge and acknowledge. The sender needsa signal from the receiver to know when the receiver is ready to accept the data. Thesender supplies a signal line that is used to tell the receiver two distinct pieces ofinformation: then the receiver can read valid data and when the write cycle has finished.The receiver uses a signal line to tell the sender when it is ready to receive data and whenit has read the data. These handshaking requirements are summarized in the timing

1 Excerpts taken from http://hcs12text.com/part013a.html#Using%20Ports%20H%20and%20J%20for%20Parallel%20I/O
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diagram shown in Figure 2. When handshaking uses hardware wires, bits in a statusregister or bits within a stream of communications to implement the interaction betweensender and receiver as shown in Figure 2, the handshaking is called explicate. Somehandshaking schemes assume that the receiver is always ready and can accept the datawithin specified timing constrains. Such handshaking schemes are called implicate or

time-implicate handshaking.

S e n

d e r

R e c e

i v e r

Time

R e c e

i v e r

R e a

d y

N e w

D a

t a

R e a

d y

R e c e

i v e r

D a

t a

A c c e p

t e d

T r a n s a c t

i o n

C o m p

l e t e d

Figure 2. Signal action for fully explicate handshaking.

In strobe synchronization, an additional control signal accompanies the data. Inthe case of an input strobe, the external device pulses the strobe control line when newdata is present. A latch, either within or connected to the microcontroller captures thedata, and typically asserts an interrupt as shown in Figure 3. The microcontroller mustread the data before the next input strobe occurs. In the case of an output strobe, themicrocontroller generates a pulse on the strobe control line after it writes new data. Theexternal device uses the strobe to capture the data being sent. The external device must beready for new data the next time the strobe line is pulsed. In some cases the circuitry to

generate the output strobe is built into the microcontroller. In other cases the strobe can be generated via software using another output port pin.

Strobes may be either positive (normally low pulsed high) or negative (normallyhigh pulsed low). Generally strobe handshaking assumes that the transaction isconcluded on the trailing edge of the strobe pulse. Relative to the transaction edge, thedata must be held constant. The period that the data must be held constant prior to thetransaction edge is called the data setup time. The period that the data must be heldconstant after to the transaction edge is called the data hold time.


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State or level handshaking is used to indicate when the data lines are valid (not intransition). Data valid (DAV) handshaking can be indicated by either a high or low statedepending upon the application. The data setup time is relative to the assertion of theDAV signal and the data hold time is relative to the DAV signal returning to its inactivestate.

Figure 3. Strobe handshaking

Handshaking synchronization solves the problem of data being sent before thereceiving end is ready. A second (acknowledgment or busy) control line as shown inFigure 4 is used by the receiving device to signal the sending device that it is ready forthe next data transmission. For handshaking with an output device, an extra input port pinis used to receive the busy indication from the external device. This input is polled and nonew data is sent unless the pin is not asserted (not busy). A potential race condition existsif the microcontroller tests the busy bit before the output device asserts busy, however if

busy is asserted automatically by the output device hardware, the microcontroller will not be able to test the bit quickly enough.


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Figure 4. Handshaking write strobe with Busy control

Figure 5 below shows handshaking for an input device. The signals are the same but the roles of the microcontroller and external device are reversed. In this case a latchwas added so that busy would be asserted immediately when data is received. Themicrocontroller pulses an output port pin to reset the busy latch when it is ready for thenext data byte.

Figure 5. Handshaking from external device


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Potential race conditions can be avoided by employing full handshaking . Fullhandshaking uses both edges of the strobe and busy signals. Figure 6 shows the schemefor an input device. The microcontroller indicates to the device when it is ready for data.This is the inverse of the Busy signal we have seen before. The input device provides thedata and indicates its availability by raising its strobe signal, Data Available (DAV).

Unlike the preceding cases, the strobe signal is held high until the data recipient, themicrocontroller, acknowledges receipt by lowering its request for data signal. The inputdevice then knows that the data has been successfully received, removes the data, andwaits for the next data request.

Figure 6. Processor poll for data available

When IO ports are used to provide both data and handshaking controls and thecontrol signals are managed using software, the process is called bit banging. Thestates of the handshaking signals are explicitly assigned in software as opposed to thehandshaking signal generated using hardware as shown in Figure 5. Another form ofhandshaking embed the busy or ready signals into the data flow. Examples of this is theinterface to the character LCD modules based on the Hitachi HD44780 that will bediscussed below.


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Examp le: LCD Driven f rom IO Por ts 2

1. Introduction to LCDs Advances in the features, miniaturization, and cost of LCD (Liquid Crystal

Display) controller chips have made LCDs usable not only in commercial products butalso in hobbyist projects. By themselves, Liquid Crystal Displays can be difficult to drive because they require multiplexing, AC drive waveforms, and special voltages. LCDmodules make this driving simpler by attaching hardware to the raw glass LCD to assistin some or all of these rudimentary driving tasks.

LCD modules can be split into two groups: those that have built-in controller anddriver chips, and those that have only driver chips. LCD displays that do not havecontrollers are typically used with powerful hardware, such as a laptop computer, where avideo controller is available to generate the complex drive signals necessary to run thedisplay. Most color and large (greater than 320x240) monochrome displays are of thistype.

The category of display modules that have built-in controllers can be split againinto character LCD modules and graphic LCD modules. Character modules can displayonly text and perhaps some special symbols, while graphic modules can display lines,circles, squares, and patterns in addition to text. Some examples of graphic LCDcontroller chips are the Toshiba T6963, Seiko-Epson SED1330, and Hitachi HD61202.Here, we will be primarily concerned with character LCD modules that have the HitachiHD44780 controller built-in.

Nearly every pixel-based alphanumeric LCD module made today uses the HitachiHD44780 LCD controller chip, or a derivative such as the Seiko-Epson SED1278. Thisapparent standardization in character LCDs has become extremely beneficial to designengineers and hobbyists. Dozens of manufacturers produce literally hundreds of models

of LCD modules using this controller chip. The smallest of these displays is only one lineof 8 characters; the largest is four lines of 40 characters each.Other common sizes are 16x1, 20x1, 20x2, 20x4, 40x1, and 40x2 (characters x

lines). Fortunately, all HD44780-based displays (of any size) use the same standard 14-wire interface. Therefore, code and hardware made for one size/type display can be

painlessly adapted to work for any HD44780 compatible device. Information about thesedisplays can be easily obtained on the web by including HD44780 in your searchkeywords. Because of their widespread use, these displays can be purchased surplus withtypical prices of $3 for small displays to $20 for large ones.

2. Interfacing your LCD module

The microcontroller/microprocessor interface to HD44780 LCD modules(hereafter generically referred to as character LCD modules) is almost always 14 pins.

Table 1 shows the basic pinout. You may find that some displays have additional pins for backlighting or other purposes, but the first 14 pins still serve as the interface.

2 (Excerpts from:http://www.stanford.edu/class/ee281/handouts/lcd_tutorial.pdf#search=%22HD44780%20LCD%22 )
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The first three pins provide power to the LCD module. J1.5 and/or J2.5 providesthe power GND and is connected to the digital ground. Pin J1.6 and/or J2.6 is for theVCC power and should be connected to +5V power. is the LCD Display Bias. Somedisplays that are specially made to work over a large temperature range may require anegative voltage to achieve readable contrast.

Pins J2.1 J2.2 and J2.3 are the control lines for the LCD. These lines indicate whatkind of transaction you are proposing to do over the data lines DB0-7. The state of RSindicates whether you wish to transfer commands or display data. The R/W line indicateswhether you intend to read or write. Finally, the E line tells the display when you are actuallyready to perform the transaction. The control lines RS, R/W, and E, along with the data linesDB0-7 are standard digital logic inputs or outputs. Remember than when performing readsyou must set the port connected to DB0-7 to be input.

Figure 7 . Cerebot MX7ck interface with LCD on ECE 341 project PCB

Table 1. LCD pin out list

As shown in Figure 7 , the system used in the ECE 341 lab uses JB pint 1 through 4and 7 through 10 of the Cerebot MX7ck processor for data lines and JC pins 1 through 3of JC for the handshaking and controls lines. The handshaking and control lines are


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outputs from the Cerebot MX7ck can be bit-banged to implement the required control.The RS and R/W line are the control signals that are connect to the Cerebot MX7ck edgeconnector JC pins 1 and 2. The E (enable) line is the handshaking signal for this interfaceand it is controlled by the Cerebot MX7ck connector JC pin 3. Additional detailsconcerning the exact sequence required to execute a data transfer to or from the LCD will

discussed in later sections of this chapter. The contrast adjust can be used to make theLCD more readable as viewed from various angles of incidence. Some character LCDmodels use pin 15 for enabling additional lines. These units actually have dualcontrollers with shared RS and RW control lines but with separate enable handshakingsignals. Although the handshaking described here is implemented using direct bit control(bit banging) the code provided for Lab 6 uses the PMP (Peripheral Master Port) interfaceto control the LCD. Refer to the chapter 10 of the text for additional details.

3. Accessing your LCD module

Character LCD modules are accessed through only two registers, the Command

Register, and the Data Register. When you perform a read or write with RS low, you areaccessing the Command Register and giving the module instructions. When you read orwrite with RS high, you are accessing the Data Register and reading or writingcharacters/data from or to the display.

Table 2 contains pseudo-code examples of reads and writes to the two registers.The pseudo-code can be implemented on any microcontroller and assumes that DBPORTis the port to which DB0-7 are connected. Since some microcontrollers are very fast,delays may be required between the steps described in the pseudo-code. See the concisedatasheet on the course web page for more information.

Table 2. LCD register access pseudo code

4. Initializing and programming your LCD

Character LCD s based on the Hitachi HD44780 controller have two address pointers. The display RAM (DDRAM ) pointer contains the address of the position wherethe cursor is located and /or where the next character entered will be located. When datais written to the LCD with the RS control line asserted high, the information on the data


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lines will be displayed on the LCD. 3 Data is entered as ASCII encoded text. 4 Non ASCIIdata will be displayed as either special graphics characters built into the LCD or special

programmable characters. 5 Writing bit patterns to the character generator RAM(CGRAM ) creates programmable characters.

Character LCDs must be initialized after power-on and before writing data to the

display. The initialization must consist of at least a Function Set command, preferablyfollowed by an Entry Mode Set , Display Control , and a Clear Display . Issuing each ofthese commands after the Function Set ensures that you know the state of your display.Instructions on how to issue and use these commands can be found by looking at theconcise datasheet, or at the HD44780 controller datasheet for more information. Below isan example initialization sequence and hello world program assuming youve

programmed the C functions LCDCommandWr(unsigned char command) ; towrite commands to the LCD, and LCDputc(unsigned char data) ; to writedata to the LCD:

Listing 1. Peseudo code for initializing and writing to the LCD // initiatize lcdLCDCommandWr(0x38); // function set:// Busy flag is not validsw_delay_ms(5); // User supplied software delay function// 8-bit interface, 2 display lines, 5x7 fontLCDCommandWr (0x06); // entry mode set:// Busy flag is not validsw_delay_ms(1); // User supplied software delay function

// turn display on, cursor on, no blinkingLCDCommandWr (0x10); // shift mode / direction:// wait for not busy see Listing 5 .while(ck_bf_addr() & 0x80);// increment automatically, no display shiftLCDCommandWr (0x0E); // display control:// wait for not busy see Listing 5 .while(ck_bf_addr() & 0x80);LCDCommandWr (0x01); // clear display, set cursor position to zero

Using the user supplied LCDputc function (see Listing 4 ), characterscan be sent to the LCD by executing the following list of Cinstructions

// write hello world to the display LCDputc(H); LCDputc(e); LCDputc(l);

LCDputc(l); LCDputc(o); LCDputc( ); LCDputc(W); LCDputc(o);

3 http://home.iae.nl/users/pouweha/lcd/lcd0.shtml#visible_ddram .)4 http://www.lookuptables.com/ 5 http://home.iae.nl/users/pouweha/lcd/lcd1.shtml#_8051_lcd_userchar
http://home.iae.nl/users/pouweha/lcd/lcd0.shtml#visible_ddramhttp://home.iae.nl/users/pouweha/lcd/lcd0.shtml#visible_ddramhttp://home.iae.nl/users/pouweha/lcd/lcd0.shtml#visible_ddramhttp://www.lookuptables.com/http://www.lookuptables.com/http://www.lookuptables.com/http://home.iae.nl/users/pouweha/lcd/lcd1.shtml#_8051_lcd_usercharhttp://home.iae.nl/users/pouweha/lcd/lcd1.shtml#_8051_lcd_usercharhttp://home.iae.nl/users/pouweha/lcd/lcd1.shtml#_8051_lcd_usercharhttp://home.iae.nl/users/pouweha/lcd/lcd1.shtml#_8051_lcd_usercharhttp://www.lookuptables.com/http://home.iae.nl/users/pouweha/lcd/lcd0.shtml#visible_ddram


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LCDputc(r); LCDputc(l); LCDputc(d);

After initialization, the text can also be displayed on the LCD using the function:LCDputs(Hello World);

Where the function LCDputs is defined as:

Listing 2. C function for writing a string of ASCII tect to the LCDVoid LCDputs(char *s){

while(*s) LCDputc(*s++);}

Refer to http://home.iae.nl/users/pouweha/lcd/lcd.shtml 6 for additional details on theHD44780 based character LCD

5. Interface between the LCD module and the Cerebot MX7ck Microprocessor Table 3 and Table 4 shows the control line states and the data values to implement LCD

controls described in part 4 above. The function, LCDCommandWr , must set the RSand RW lines low prior to asserting the LCD enable pin high. The data must be written to

port E prior to asserting the LCD enable pin low. Figure 10 Shows the timing required towrite to and read from the LCD unit. Table 5 defines the timing parameters identified inFigure 10.

The four LCD initializing commands can now be interpreted in light of theinstruction set shown in Table 3. The LCDCommandWr function writes data to theLCD with the RS and RW controls low. After setting the data lines and control bits, theenable bit is toggled high and back low. The enable line must be set back low for theLCD to record the new control and display data.

The first command in Listing 1 , LCDCommandWr(0x38); calls a functionthat sets the data bits for 38h .The LCD register is selected by the bit position of the mostsignificant high bit of the data bus, which, for this case is D5. D4 controls the number ofdata lines used for interfacing between the microprocessor and the LCD unit. (CharacterLCD units can operate using either 4 or 8 bit data busses. This will be discussed ingreater detail later in this chapter.) For the LCD models used in the ECE 341 lab, D3must be set to a one and bit D2 is a dont care and may be either zero or one.

The second command in Listing 1 , LCDCommandWr(0x06); calls the samefunction described in the paragraph above to set the entry mode. When set high, bit D1specifies that the address pointer to the LCD display RAM should be incremented aftereach new display character is written to the LCD. If this bit were not set, the address

pointer would decrement after each new character and the text would appear to be enteredfrom right to left.. With bit D0 set to a zero, all of the characters on the display remainfixed in their positions. If bit D0 is set high, the display would shift. The direction ofshift would be specified by the Cursor/Display Shift instruction.

6 http://home.iae.nl/users/pouweha/lcd/lcd.shtml
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Table 3. HD44780 instruction set

Instruction Code

Description Executiontime** RS R/

W D7 D6 D5 D4 D3 D2 D1 D0

Clear display 0 0 0 0 0 0 0 0 0 1 Clears display and returns cursor to the home position (address 0). 1.64mS

Cursor home 0 0 0 0 0 0 0 0 1 * Returns cursor to home position (address 0).Also returns display being shifted to theoriginal position. DDRAM contents remainsunchanged.

1.64mS

Entry modeset 0

0 0 0 0 0 0 1 I/D S Sets cursor move direction (I/D), specifies toshift the display (S). These operations are

performed during data read/write. 40uS

DisplayOn/Off control 0

0 0 0 0 0 1 D C B Sets On/Off of all display (D), cursor On/Off(C) and blink of cursor position character (B). 40uS

Cursor/display

shift 0 0 0 0 0 1 S/

C

R/

L * *

Sets cursor-move or display-shift (S/C), shiftdirection (R/L). DDRAM contents remainsunchanged.

40uS

Function set 0 0 0 0 1 DL N F * * Sets interface data length (DL), number ofdisplay line (N) and character font(F). 40uS

Set CGRAMaddress 0

0 0 1 CGRAM address Sets the CGRAM address. CGRAM data is sentand received after this setting. 40uS

Set DDRAMaddress 0

0 1 DDRAM address Sets the DDRAM address. DDRAM data issent and received after this setting. 40uS

Read busy-flag andaddresscounter

0 1 BF CGRAM / DDRAM address Reads Busy-flag (BF) indicating internaloperation is being performed and readsCGRAM or DDRAM address counter contents(depending on previous instruction).

0uS

Write toCGRAM orDDRAM

1 0 write data Writes data to CGRAM or DDRAM. 40uS

Read fromCGRAM orDDRAM

1 1 read data Reads data from CGRAM or DDRAM. 40uS

Remarks: - DDRAM = Display Data RAM.- CGRAM = Character Generator RAM.- DDRAM address corresponds to cursor position.- * = Don't care.- ** = Based on F osc = 250kHz.

Table 4. Bit names for commands listed in Table 3Bitname Setting / Status

I/D 0 = Decrement cursor position 1 = Increment cursor position

S 0 = No display shift 1 = Display shift

D 0 = Display off 1 = Display on

C 0 = Cursor off 1 = Cursor on

B 0 = Cursor blink off 1 = Cursor blink on


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Bitname Setting / Status

S/C 0 = Move cursor 1 = Shift display

R/L 0 = Shift left 1 = Shift right

DL 0 = 4-bit interface 1 = 8-bit interface

N

0 = 1/8 or 1/11 Duty (1 line)

1 = 1/16 Duty (2 lines)

F 0 = 5x7 dots 1 = 5x10 dots

BF 0 = Can accept instruction 1 = Internal operation in progress

The third command in Listing 1 , LCDCommandWr(0x10); calls thefunction to enter a command that sets the cursor or display shift and the direction. With

bits D3 and D2 both set low, the cursor will shift right after each display character has been entered. If D3 is set high, all of the display characters will shift the directionspecified by D2 instead of the cursor.

The fourth command in Listing 1 , LCDCommandWr(0x0E); calls thefunction to enter a command that controls that turns the display off or on and controls the

cursor display mode. When bit D2 is set high, the LCD display is turned on. When bit D1is set high, the LCD cursor is turned on and when bit D0 is set high, the LCD cursor will

blink on and off.The final command in Listing 1 , LCDCommandWr(0x01); calls the

function to enter a command that clears the LCD, puts the cursor in the left column, andsets the display address pointer to zero.

To this point, all data has been written to the LCD. Data in both the CGRAM andthe DDRAM can also be read if both the RS and the RW bits are asserted high. If theLCD is read with the RS bit set low, (the RW bit must still be set high to read the LCD)the address pointer of either the DDRAM or the CGRAM can be read as well as the LCD

busy flag. In order to read or write the CGRAM address, a Set CGRAM Address

command must first be written to the LCD. Likewise, reading or writing to the DDRAMrequires that the Set DDRAM Address command be sent to the LCD. Sending theClear Display command accomplishes that same instruction as setting the DDRAM tozero.

The Busy flag is the most significant bit when reading either the DDRAM orthe CGRAM. The LCD should not be written to as long as the busy flag is high. The

busy flag must be read with the direction of the Cerebot MX7ck processor set for inputand the control bits set as for reading the DDRAM or CGRAM. The Cerebot MX7ckPort E bits 0 through 7 must be read while the LCD enable bit is high. The enable linemust always be set back low again before any new data can be read from the LCD. Datafrom the LCD cannot change as long as the enable line is in the high state regardless of

any internal changes of address or busy flag.One final point to note from Table 3 is the instruction execution times listed in the

far right column. These execution times represent the worst case conditions during whichthe busy flag will be read as high.


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5. Managing the LCD

Eight basic functions are required to efficiently manage the LCD. These are:initializing, writing commands, reading the address and busy flag, positioning the cursorand writing display character data. There is also a software delay required during the

LCD initialization sequence. The collaboration diagram shown in Figure 8 describes therelationship of these six functions. Other functions may be necessary if you go beyonddisplaying simple text messages on the LCD. The actions of the six basic functions will

be explained in detail below. Application

Program

LCD TextDisplay

LCDInitialization

LCDWrite

Commands

SWDelay

LCDRead BF

and Address

LCDDisplay

Character

P o s i t i o n

S e t u p

Control code

A d d

r e s s

/ B F

Control code

delay

ASCII Char

Text string

A S C I I C

h a r

B F

LCDCursor

Position

P o s i t i

o n

C o n t r o l c o d e

B F

Figure 8. Collaboration diagram for basic LCD control functions

Initialize LCDThis function must write a sequence of commands to the LCD with RS bit low. The

basic setup is described by the code in Listing 1 but lack the prescribed delays betweeneach write command. This delay is necessary because the busy flag is not valid untilafter the initializing sequence has completed. It is recommended that a 30ms delayfollow the set entry mode command and 2 ms following all subsequent commands in theinitializing sequence. Generally, each of the commands are written by calling a specificfunction that asserts the RS and RW bits low as discussed in the following section.


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Write Command after initializationTo write an LCD command, execute the followingsequence:

Listing 3. Write command sequence of tasks

1. Check busy flag until not busy and read DDRAMaddress (usually accomplished in a separate function see Figure 8. )

2. RS (Port B bit 15) and RW (Port D bit 4) pins areasserted low

3. Port E bits 0 through 7 direction is set for output4. Write the command data to Port E5. The LCD enable pin (Port D bit 5) is asserted high6. The LCD enable pin (Port D bit 5) is asserted low

This sequence of operations meets the orderingrequirements shown in Figure 10. The timingconstraints from Figure 10 and Table 5 require that aminimum of 40 ns (t SU) be between steps 2 and 4.(Steps 3 and 4 can be executed in reverse order

provided that the RS and RW signals are asserted 40ns before the enable signal is asserted high.) As longas the data is written to Port E prior to step 5, thedata lines can be written at any time hence the datasetup time is specified by the parameter (t W - ttD)Most of the minimum timing requirements listed inTable 5 are easily met when the RS, RW, and Enablecontrols are bit-banged on relatively slowmicroprocessors. However, the Enable cycle timespecification requires a 500 ns interval between twoconsecutive rising edges. It is possible that thePIC32 processor will violate that constraint unless adelay function is employed. The Enable signal linemust be held high at least 220 ns and the data linesmust not change for 20 ns after the Enable line hasreturned to a low to satisify the data holdrequirements..

LCD_Cmd_Wr

Wait fornot Busy

SetRS=0RW=0

Set Port Aas Output

WriteCommandto Port A

Pulse Encontrol

Return

After LCDinitialization

Figure 9. Flow diagram for the

LCDCmdWr function.


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Figure 10. LCD Writer Cycle Timing Diagram

Table 5. LCD control timing for timing diagram shown in Figure 10

Write Display CharacterThe sequence for writing display data to the LCD is much the same as writing LCDcontrol commands. There are, however, two notable exceptions. The first exception isthe need to filter out ASCII control characters. The LCD control actions to implementcommon ASCII control characters are listed in Table 7.


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Table 6. LCD controls actions to implement ASCII control characters.Hex

Value Name Action LCD Control Sequence

0X00 NULL Do nothing LCD will display programmable graphich character atCGRAM address 0

0x08 Back

space

Move cursor one

position to the left.(\010)

1. Test address for 1 st position of line 1

a. Yes do nothing b. No Continue2. Test address for 1 st position of Line 2

a. Yes set DDRAM address for end ofline 1

b. No decrement DDRAM Address andwrite new address to LCD

3. If backspace is non destructivea. No do nothing

b. Yes write space character and repeatsteps 2 and 3 above.

0x0A New line \n Send LCD instruction for clear display (0x01)0x0C Form

Feed

New Page ( \011) Same as New Line above ( \n)

0X0D Carriagereturn

\r Send LCD instruction to put cursor in home position(0x02)

0x07 Bell Ring the bell Make funny noises (manual command)

In general, the LCD character display requires the following sequence. The timingrequirements shown in Figure 1 and Table 5 must be observed. From personal experience,I can guarantee that your LCD will have inconsistent and erratic behavior (including thedisplay) unless the sequences and timing constraints are observed.

Listing 4. Sequence of tasks for writing a display character or ASCII controlcharacter1. Check busy flag until not busy and read DDRAM address2. Test character for control character

a. send appropriate write control function b. exit function

3. Test address for end of linea. send new DDRAM address to write control

b. test busy flag until not busy ignore cursor addressc. continue to write display character

4. Assert RS (Port B bit 15) high5. Assert RW (Port D bit 4) low6. Set Port E bits 0 through 7 for output pins7. The data is written to Port E8. Assert the LCD enable pin (Port D bit 5) high

9.

Assert the LCD enable pin (Port D bit 5) low10. Set Port E bits 0 through 7 for input


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Read Busy FlagThis function must be called after writing every of a displaycharacters or control instructions following the initializationsequence. The LCD must be read with the RW bit set highand the RS bit set low. The timing requirements shown in

Figure 1 and Table 5 must be observed for this operation aswell. Since reading the busy flag also results in reading theDDRAM or CGRAM address, the function can return thedata as a single byte or as two separate variables. Thefollowing steps are needed to read the busy flag.

Listing 5. Sequence of tasks for reading the busy flag1. Assert RS (Port B bit 15) low2. Assert RW (Port D bit 4) high3. Set Port E bits 0 through 7 for input4. Assert the LCD enable pin (Port D bit 5) high5. Read Port E

6. Assert the LCD enable pin (Port D bit 5) low

Should the LCD need to be read again, such as may berequired when polling the busy flag, steps 4 through 6 must

be repeated in sequence otherwise the LCD output will notchange. The timing for the read operation shown in Figure 11uses parameters defined in Table 5 .

Figure 11. LCD read cycle timing

LCD_read_bf

Set Port Afor Input

SetEN, RS=0

RW=1

SetE=1

Read Port A

SetE=0

Return BF|Addr

Figure 12. Flow diagram forthe LCD check busy and readcursor address function.


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6. Programmable Graphics Character Generation 7

When you send the ASCII code for a character like "A" to an LCD module, themodule's controller looks up the appropriate 5x8-pixel pattern in ROM (read-onlymemory) and displays that pattern on the LCD. That character-generator ROM contains

192 bit maps corresponding to the alphabet, numbers, punctuation, Japanese Kanjicharacters, and Greek symbols.The ROM is part of the main LCD controller (e.g., HD44780, KS0066, etc.), is

mask-programmed, and cannot be changed by the user. The manufacturers do offeralternative symbol sets in ROM for European and Asian languages, but most U.S.distributors stock only the standard character set shown in the LCD Serial Backpackmanual.

Alphanumeric LCD controllers do not allow you to turn individual pixels on oroff-they just let you pick a particular pattern (corresponding to an ASCII code) anddisplay it on the screen. If you can't change the ROM and you can't control pixels, howdo you create graphics on these LCDs? Easy.

There's a 64-byte hunk of RAM (random-access memory) that the LCD controlleruses in the same way as character-generator (CG) RAM. When the controller receives anASCII code in the range that's mapped to the CG RAM, it uses the bit patterns storedthere to display a pattern on the LCD. The main difference is that you can write to CGRAM, thereby defining your own graphic symbols.

A character LCD that uses the HD44780 controller allow for eight programmablecharacters. The character patterns must be first written to the CGRAM of the LCD. First,the CGRAM must be select by setting the CGRAM address for the character that is to be

programmed. Each eight consecutive addresses starting at CGRAM address zero areassigned to one programmable graphical character. Each bit of the data at each addresssets the pixel on (1) or off (0). The pixels on a particular row at the right of the characterdisplay have the lowest binary value. The rows of pixels start at the top and move downthe character as the addresses increase in value as shown in Figure 13.

7Excerpts taken from http://www.geocities.com/dinceraydin/lcd/custom.htm
http://www.geocities.com/dinceraydin/lcd/custom.htmhttp://www.geocities.com/dinceraydin/lcd/custom.htmhttp://www.geocities.com/dinceraydin/lcd/custom.htm


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Figure 13. Example bit map for a programmable graphical character.

A WEB based character calculator can be use to help determine the list of bytesthat need to be written to the CGRAM. 8 9 Be advised that the calculator referenced in 9only generates seven bytes of data and does not specify data for the bottom row of pixels.Writing to CG RAM is a lot like moving the cursor to a particular position on the displayand displaying characters at that new location. The steps are:

Listing 6. Steps to program graphical characters

Reset RS and R/W pins of the LCD to prepare the LCD to accept instructions Set the CG RAM address by sending an instruction byte from 64 to 127 (locations 0-63 in CG

RAM). Switch to DATA MODE by changing setting the RS pin Send bytes with the bit patterns for your symbol(s). The LCD controller automatically increments

CG RAM addresses, just as it does cursor positions on the display. To leave CG RAM, switch to COMMAND MODE to set address counter to a valid display

address (e.g. 128, 1st character of 1st line); the clear-screen instruction (byte 1); or the homeinstruction (byte 2). Now bytes are once again being written to the visible portion of the display.

To see the custom character(s) you have defined, print ASCII codes 0 through 7.

8 http://www.geocities.com/dinceraydin/lcd/charcalc.htm 9 http://www.quinapalus.com/hd44780udg.html
http://www.geocities.com/dinceraydin/lcd/charcalc.htmhttp://www.geocities.com/dinceraydin/lcd/charcalc.htmhttp://www.geocities.com/dinceraydin/lcd/charcalc.htmhttp://www.quinapalus.com/hd44780udg.htmlhttp://www.quinapalus.com/hd44780udg.htmlhttp://www.quinapalus.com/hd44780udg.htmlhttp://www.quinapalus.com/hd44780udg.htmlhttp://www.geocities.com/dinceraydin/lcd/charcalc.htm

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