Repeat start sequence in I2C interface - PCF8574.

I am forwarding your question to someone with more I2C experience.

Hi Shruthi,

Here is a link to a thread, which could be helpful:

https://e2e.ti.com/support/microcontrollers/hercules/f/312/t/520353

In the above thread, there is a link to a code that have repeated mode reading:

/cfs-file/__key/communityserver-discussions-components-files/312/06636.sys_5F00_main.c

Regards

Fouad

Hi,

Thanks for the link. I went through the link but was unable to read the value from register.

here is the code snippet.

uint8 i2cReadByte(uint8 memAddress)
{
control_word.mem_address=memAddress;
uint8 data;
uint8 byteData;

delay2(1000);
//i2cREG1->MDR=(0xEA0); // disable repeat mode
i2cREG1->MDR |= I2C_REPEATMODE;

/*....................Set as read function andSet memory address to read...................*/

i2cSetSlaveAdd(i2cREG1, SC16lS740_ADDR);
i2cSetDirection(i2cREG1, I2C_TRANSMITTER);
i2cSetCount(i2cREG1,0x01);
i2cSetMode(i2cREG1, I2C_MASTER);
i2cSetStart(i2cREG1);
delay();
i2cSendByte(i2cREG1,0x18); //register address of SC16lS740
//i2cSendByte(i2cREG1,0x91);
delay();
//i2cSetStop(i2cREG1);
//while(i2cIsBusBusy(i2cREG1) == true);
//delay();
//while(i2cIsStopDetected(i2cREG1) == 0);
//i2cClearSCD(i2cREG1);


/*....................Receive Data....................................*/

i2cSetSlaveAdd(i2cREG1, SC16lS740_ADDR);
i2cSetDirection(i2cREG1, I2C_RECEIVER);
i2cSetCount(i2cREG1, 1);
i2cSetMode(i2cREG1, I2C_MASTER);
i2cSetStart(i2cREG1);
delay();
sciSendByte(scilinREG,'S');
data = i2cReceiveByte(i2cREG1);
//delay();
sciSend_hexdata(scilinREG, (uint8)data);

i2cSetStop(i2cREG1);
while(i2cIsBusBusy(i2cREG1) == true);
delay();
while(i2cIsStopDetected(i2cREG1) == 0);
i2cClearSCD(i2cREG1);

/*......................stores data...........................*/

byteData=data;
return byteData;

}

SLAVE ADDRESS IS SET TO 0x48.
its looping in the while loop inside i2cRecieveByte... BAUDRATE is 100KHz.,Can you please tell me where i am going wrong.

Thanks and Regards
Shruthi

Hi Shruthi,

Few points while I will try to go through code snippet:

1- Do you have an oscilloscope or some tool to spy on the electrical signals and have a snapshots?

2- Are you using PCF8574A? As this one can't take 0x48 as slave address (it should be 0x78 unless you use PCF8574)

3- I assume you meet I2C bus requirements by using the required pull up resistors!

4- And I assume you match A0, A1 and A2 to the slave address you are using, by connecting those pins to GND and VCC -as needed-.

5- Which HDK you are using (microcontroller), can you show your HALCoGen configurations in screen snapshots?

6- Do you do any pin multiplexing?

Regards

Fouad

Hi Fouad,

I m using PCF8574A and slave address is set to 0x38,sorry for wrong address as i was working on other I2C device in parallel.

pull up resistors are there.(1K on both SDA and SCL)

I am working on RM46L854ZWT which is interfaced with PCF8574A

These are my settings.

74A.

Thanks and Regards

Shruthi

Hi Shruthi,

It seems the HW interface (assuming that A0, A1 and A2 connected correctly to GDN and VCC).

HALCoGen configurations seem fine too.

But the slave address doesn't look right, here is a snapshot of the datasheet for PCF8574A, the adreess needs to be 0x78 (lower four bits are configured as per A0, A1 and A2 connection).

Please let me know if I am missing any point!

Regards

Fouad

Hi Fouad,

AO,A1,A2 address pins are connected to ground..so the 7-bit slave address comes upto 0x38..

In general how do i read the value from register?

sequence goes as such SLAVE_ADDR + W+ START + REG_ADDR +SLAVE_ADDR+R+START+ VALUE. + STOP...

Do i have to enable the RM bit in MDR before setting the start condition?

Thanks and Regards,
Shruthi

Hi Shuruthi,

BTW I am not TI I2C expert, I am just trying to help.

Looking at the datasheet I have, it seems it is not needed to provide registry address, all needed is to have the slave address (which I believe is 0x70) with "write" then the data to be written, then stop. When we want to read, it is the slave address with "read" then stop bit. I don't think you RM is needed.

You mentioned in your thread that you want to write to (write_addr) where is it in the datasheet?

So I think the sequence should be:

1- To write:

Mode: master

Direction: transmitter

Address: slave address

byte to send: data to be written to the output port (if that is the functionality needed)

Start bit: i2cSetStart

Wait till transaction is finished.

Stop bit

2- To read:

 

Mode: master

Direction: receiver

Address: slave address

Start bit: i2cSetStart

byte to be received: i2cReceiveByte

wait till transaction is finished

Stop bit

I hope this help, unfortunately I don't have access to PCF8574 so I can do some test.

regards

Fouad

Sorry for jumping in here without really understanding,  but when I look at the datasheet for the PCF8574 I don't see where the repeated start comes from.

Just to clarify, here is a picture of a read from the PCF8574 datasheet:

If it were showing a repeated start, there would be an "S" between the A and the next byte, and the next byte would be the address again not the data...  Like it's shown below:

Hi ,

I was able to read from data ports..of pcf8574a .by introducing some delay between start and sendbyte functions..

I was parallel working on other I2C device (SC16IS740PW) I2C-UART with same controller which requires sequence as such.when i send the register address and change it to master---receiver---start---receivebyte its stuck in receivebyte function...

Anyone worked on this device before?i could not find any links..

Thanks and Regards,
Shruthi