Inter-integrated Circuit (I2C) Protocol and

DS1307 RTC Interfacing

Presented by:-Bhargav Kakadiya (140070110006)

The I2C Protocol

• Developed by Philips in late 1980s• Version 1.0 published in 1992

– Supports standard (100 Kbps) and fast (400 Kbps) mode• Version 2.0 published in 1998

– High-speed mode (3.4 Mbps) added• Classifies devices into slave and master• Allows multiple masters to be attached to the same bus• The master device uses either a 7-bit or 10-bit address to specify

the slave device as its partner of data communication.• Supports bi-directional data transfer• Allows multiple masters (microcontrollers) to share the same

peripheral devices

I2C Signal Level

• Float high and driven low• Use the SCL signal to carry clock signal to synchronize

data transfer• Use the SDA signal to carry data and address• The SDA and SCL pins of I2C devices (masters and

slaves) are open-drain and need external pull up resistors.

• The resistors 2.2 K and 1 K are recommended for 100 Kbps and 400 Kbps baud rate.

+VDD

RP RP

CLK1OUT

CLK1IN

Data1OUT

Data1IN

CLK2OUT

CLK2IN

Data2OUT

Data2IN

Device 1 Device 2

SDA lineSCL line

Figure 11.1 Connecting standard- and fast-mode devices to the I2C bus

Signal Components

• I2C data transfer consists of 5 signal components:– Start (S)– Stop (P)– Repeated Start (R)– Data– Acknowledge (A)

SDA

SCL

Figure 11.2 I2C Start condition

Start Condition

• Used to indicate that a device would like to transfer data on the I2C bus

• Represented by the SDA line going low when the clock (SCL) signal is high

• Will initialize the I2C bus

SDA

SCL

Figure 11.3 Stop (P) condition

Stop Condition

• A condition that a device wants to release the I2C bus• Is represented by the SDA signal going high when the

SCL signal is high• Once the stop condition is complete, both the SCL and

SDA signals are high. This is the idle bus.

SDA

SCL

Figure 11.4 Restart conditionstart condtion data transfer restart

condition

Repeated Start (R) Condition

• A Start signal generated without first generating a Stop condition to terminate the communication

• Used by the master to communicate with another slave or change data transfer direction without releasing the bus

• Also referred to as Restart condition

SDA

SCL

Figure 11.5 I2C bus data elementsNote. Data bit is always stable when clock (SCL) is high

Data

• It represents the transfer of eight bits of information.• Data on the SDA line is considered valid only when the SCL signal

is high.• When the SCL signal is low, the data is allowed to change.• The eight-bit data may be a control code, an address, or data.

SDA

SCL

Figure 11.6 ACK condition

SDA

SCL

Figure 11.7 NACK condition

Acknowledge (ACK) Condition

• Data transfer needs to be acknowledged either positively (A) or negatively (NACK).

• A device acknowledges a byte it receives positively by bringing the SDA line low during the ninth clock pulse of SCL.

• If the device allows the SDA line to float high, it is transmitting a negative acknowledge (NACK).

Synchronization (1 of 2)

• All masters generate their clocks on the SCL line to transfer messages on the I2C bus.

• A defined clock is needed for the bit-by-bit arbitration procedure to take place.

• Most microcontrollers generate the SCL clock by counting down a programmable reload value using the instruction clock signal.

• Clock synchronization occurs when multiple masters attempt to drive the I2C bus and before the arbitration scheme can decide which master is the winner.

• Clock synchronization is performed using the wired-AND connection of I2C interfaces to the SCL line.

• The high-to-low transition on the SCL line causes the devices concerned (masters) to start counting off their low period.

Synchronization (2 of 2)

• A master device that is counting off their low period will hold the SCL line low until the counter is count down to 0. At this point, the device will release the SCL line to high.

• If there are other devices holding the SCL low, then the SCL line will remain low until all master devices have counted down to 0. At this point, the SCL line will go high and all devices will start to count high.

• The SCL line will be held low by the device with the longest low period.

• By the same reasoning, the high period of the SCL signal is determined by the device with the shortest high period.

waitstate

start countinghigh period

counterreset

CLK1

CLK2

SCL

Figure 11.8 Clock synchronization during thearbitration procedure

Handshaking

• The clock synchronization mechanism can be used as a handshake in data transfer.

• Slave device can hold the SCL line low after completion of one byte transfer (9 bits).

• Slave halts the bus until it gets ready for the next operation and then release the SCL line.

master 1 loses arbitrationData 1 SDA

SCL

Data1

Data2

SDA

Figure 11.9 Arbitration procedure of two masters

Arbitration

• In the event two or more master devices attempt to begin a transfer at the same time, an arbitration scheme is employed to force one or more masters to give up the bus.

• The master devices continue to transmit data until one master attempts to send a high while the other transmits a low.

• Since the SDA bus has open drain, the master device that attempts to send a high will detect a low. At this point, it will stop driving the bus.

• The arbitration process does not slow down the winning master’s transfer and no data gets lost.

A6 A5 A4 A3 A2 A1 A0 R/W

Figure 11.13a 7-bit I2C address

1 1 1 1 0 A9 A8 R/W A7 A6 A5 A4 A3 A2 A1 A0

Figure 11.13b 10-bit I2C address

I2C Addressing Methods

• I2C protocol allows master devices to use either the 7-bit and 10-bit address to specify the slave device for data communication.

• The 7-bit addressing uses the upper 7 bits of the address byte for address and the least significant bit to specify the data transfer direction. The format is shown in Figure 11.13.

• The 10-bit addressing uses two bytes to carry the address information. – The bit 0 of the high byte is used to indicate the data transfer direction.– The upper 7 bits have the pattern of 1111 0xx with xx representing the

most significant two address bits of the slave. – The second byte carries the lower 8 address bits.

S Slave address R/ W A Data A Data A/ A P

data transferred(n bytes + acknowledge)'0' (write)

from master to slave

from slave to master

A = acknowledge (SDA low)A = not acknowledge (SDA high)S = start conditionP = stop condition

Figure 11.10 A master-transmitter addressing a slave receiver with a 7-bit address. The transfer direction is not changed.

Data Transfer Format (7-bit Addressing) (1 of 2)

• Master transmitter to slave receiver – shown in Figure 11.10• Master reads slave immediately after the first byte (address byte) –

shown in Figure 11.11• Combined format. A master may transfer some data to the slave and

then generate a restart condition to read data from the slave or send/read data to/from other slave-- shown in Figure 11.12.

S Slave address R/ W A Data Data P

data transferred(n bytes + acknowledge)'1' (read)

Figure 11.11 A master reads a slave immediately after the first byte

AA

S Slave address R/ W A Data Data

read orwrite

Figure 11.12 Combined format

A/ A R Slave address R/ W A/ A P(n bytes +

ack.) repeatedstart

read orwrite

A(n bytes +

ack.)*direction oftransfer maychange at thispoint

* not shaded because transfer direction of data and acknowledge bits depends on R/ W bits

Data Transfer Format (7-bit Addressing) (2 of 2)

TWI

TWBR

TWBR selects the division factor for the bit rate generator. The bit rate generator is a frequency divider which generates the SCL clock frequency in the Master modes.

TWBR = Value of the TWI Bit Rate RegisterTWPS = Value of the prescaler bits in the TWI Status Register

TWSR

Bits [7:3] – TWS: TWI StatusThese five bits reflect the status of the TWI logic and the Two-wire Serial Bus. The different status codes are described later in this section. Note that the value read from TWSR contains both the 5-bit status value and the 2-bit prescaler value. The application designer should mask the prescaler bits to zero when checking the Status bits. This makes status checking independent of prescaler setting. This approach is used in this datasheet, unless otherwise noted.Bit 2 – Reserved BitThis bit is reserved and will always read as zero.Bits [1:0] – TWPS: TWI Prescaler BitsThese bits can be read and written, and control the bit rate prescaler.

TWCR

Bit 7 – TWINT: TWI Interrupt FlagThis bit is set by hardware when the TWI has finished its current job and expects application software response. If the I-bit in SREG and TWIE in TWCR are set, the MCU will jump to the TWI Interrupt Vector. While the TWINT Flag is set, the SCL low period is stretched.Bit 6 – TWEA: TWI Enable Acknowledge BitThe TWEA bit controls the generation of the acknowledge pulse. If the TWEA bit is written to one, the ACK pulse is generatedBit 5 – TWSTA: TWI START Condition BitThe application writes the TWSTA bit to one when it desires to become a master on the Two wire Serial Bus. The TWI hardware checks if the bus is available, and generates a START condition on the bus if it is free.Bit 4 – TWSTO: TWI STOP Condition BitWriting the TWSTO bit to one in Master mode will generate a STOP condition on the Two-wire Serial Bus. When the STOP condition is executed on the bus, the TWSTO bit is cleared automatically.

Bit 3 – TWWC: TWI Write Collision FlagThe TWWC bit is set when attempting to write to the TWI Data Register – TWDR when TWINT is low. This flag is cleared by writing the TWDR Register when TWINT is high.• Bit 2 – TWEN: TWI Enable BitThe TWEN bit enables TWI operation and activates the TWI interface. When TWEN is written to one, the TWI takes control over the I/O pins connected to the SCL and SDA pins, enabling the slew-rate limiters and spike filters. If this bit is written to zero, the TWI is switched off and all TWI transmissions are terminated, regardless of any ongoing operation.• Bit 1 – Reserved Bit This bit is a reserved bit and will always read as zero.• Bit 0 – TWIE: TWI Interrupt EnableWhen this bit is written to one, and the I-bit in SREG is set, the TWI interrupt request will be activated for as long as the TWINT Flag is high.

TWDR

These eight bits contain the next data byte to be transmitted, or the latest data byte received on the Two-wire Serial Bus.

In Transmit mode, TWDR contains the next byte to be transmitted. In Receive mode, the TWDR contains the last byte received.

TWAR

These seven bits constitute the slave address of the TWI unit.

The TWAR should be loaded with the 7-bit slave address (in the seven most significant bits of TWAR) to which the TWI will respond when programmed as a slave transmitter or receiver. Inmultimaster systems, TWAR must be set in masters which can be addressed as slaves by other masters.

1X1

2X2

3VBAT

4GND

8

7

6

5 SDA

SCL

SQWOUT

VCC

DS1307

Oscillatorand divider

X1 X2

squarewave outSQWOUT

powercontrol

VCCVBATGND

serial businterface

SCLSDA

controllogic

addressregister

RTC

RAM(56x8)

Figure 11.27 DS1307 pin assignment and block diagram

The Serial Real-Time Clock DS1307

• Uses BCD format to represent the clock and calendar information• Has 56 bytes to store critical information• Clock calendar provides seconds, minutes, hours, day, date, month, and year information• Operates in either the 24-hour or 12-hour format with AM/PM indicator• Has built-in power sense circuit that detects power failure and automatically switches to the

battery supply• The SQW output frequency may be 1 Hz, 4 KHz, 8 KHz, and 32 KHz.

$00

$07$08

$3F

secondsminuteshoursdaydate

monthyear

control

RAM56 x 8

Figure 11.28 DS1307 address map

$01$02$03$04$05$06

CH 10 seconds seconds10 minutes0

0

minutes

hours1224

10 HRA/ P 10 HR

0 0 0 0 0 day0 0

0 0 0

10 date10

month

date

month

year10 yearout sqwe0 0 0 0 RS1 RS0

Figure 11.29 Contents of RTC registers

Bit 7 Bit 0

- Bit 6 of the hours register selects whether the 12-hour or 24-hour mode is used.- Bit 5 of the hours register selects whether the current time is AM or PM if 12-hour mode is selected.

DS1307 Address Map

Interfacing with ATMega32

Table 11.7 Square wave output frequencyRS1 RS0 SQW output frequency0011

0101

1 Hz4.096 KHz8.192 KHz32.768 KHz

DS1307 Control Register

• Bit 7 controls the output level of the SQWOUT pin when the square output is disabled.

• The SQWE bit enables/disables the SQWOUT pin output.

• Bits 1 and 0 select the output frequency of the SQWOUT pin.

DS1307 Write Operation

DS1307 Read Operation

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