I2C random read of off-board device from within PRU not working correctly

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1

Does anyone know the correct programming sequence for doing an I2C random read from within the PRU? I’ve tried this nicely written driver:

https://github.com/LinuxDroneLab/pru-i2c-lib

It works less than my code does…

My code works fine if I call my routine once, on the scope I see the write followed by the read of the 2 bytes I want… Call my function again and all I see is the read.

Code and scope grabs attached - the documentation on the I2C module from within the AM335xTRM pdf is shocking and this problem is driving me mad…

/*

  • read_wiper - returns the 2nd byte of a 2-byte read from the specified wiper
  • ‘reg’ contains the write command to read the required wiper
    */
    uint8_t read_wiper(uint8_t reg)
    {

uint8_t dataByte;

//initialise i2c2 device
init_i2c();

//set I2C2_CNT register to 1 for initial write command
I2C2_CNT = 1;

while (!(I2C2_STATUSRAW & 0x0100));
//poll ‘buss free’ bit in I2C2_STATUSRAW register (bit 8) until it is non-zero
//set transmit mode
I2C2_CON = 0x8601;
//load data register with write command
I2C2_DATA = reg;

while (!(I2C2_STATUSRAW & 0x0004));
//poll ‘access ready’ bit in I2C2_STATUSRAW register (bit 4) until it is non-zero
//set byte count to 2
I2C2_CNT = 2;
//set receive mode
I2C2_CON = 0x8403;

while (!(I2C2_STATUSRAW & 0x0008));
//poll ‘receive ready bit’ in I2C2_STATUSRAW register (bit 3) until it is non-zero

dataByte = I2C2_DATA; //first byte is always 0x00

//zero the receive ready bit once we have read the 1st byte into our local variable
I2C2_STATUSRAW |= 0x0008;

while (!(I2C2_STATUSRAW & 0x0008));
//poll ‘receive ready bit’ in I2C2_STATUSRAW register (bit 3) until it is non-zero

dataByte = I2C2_DATA; //second byte is the value we want

//zero the receive ready bit once we have read the 2nd byte into our local variable
I2C2_STATUSRAW |= 0x0008;

while (!(I2C2_STATUSRAW & 0x0100));
//poll ‘buss free’ bit in I2C2_STATUSRAW register (bit 8) until it is non-zero

return dataByte;
}

void init_i2c()
{
/* Enable I2C2 clock signal generation */
while (!(CM_PER_I2C2 & 0x2))
CM_PER_I2C2 |= 0x2;

/*

  • set I2C2_PSC register to 0x0B
  • set I2C2_SCCL register to 0x0D
  • set I2C2_SCCH register to 0x0F
  • set I2C2_CON register to 1000 0110 0000 0000 (0x8600)
  • set I2C2_SA register to 0x2E (address of MCP4641)
    */

I2C2_PSC = 0x000B;
I2C2_SCLL = 0x000D;
I2C2_SCLH = 0x000F;
I2C2_CON = 0x8600;
I2C2_SA = i2cPotAddress;
I2C2_BUF = 0x0000;

}

20180913135820.png

20180913135835.png

2

I love the TRM documentation its very detailed best to follow its recommended sequences and roll your own code using examples as a template.

I’d look at the i2C slave data sheet carefully especially the timing/setup diagrams.
you have the scope looks like your close.Another idea look for example code for device even if another micro. Looking at it might give you a clue what’s wrong. Last attempt try something quick and simple if you have available no OS barebones one processor I bet your code works on it.

Good Luck

20180913135835.png

20180913135820.png

3

Got this working with a few code changes:

/*

  • read_wiper - returns the 2nd byte of a 2-byte read from the specified wiper
  • ‘reg’ contains the write command to read the required wiper
    */
    uint8_t read_wiper(uint8_t reg)
    {

uint8_t dataByte;

//initialise i2c2 device
init_i2c();

//set I2C2_CNT register to 1 for initial write command
I2C2_CNT = 1;

while (I2C2_STATUSRAW & 0x1000);
//poll ‘busy bit’ in I2C2_STATUSRAW register (bit 12) until it is zero
//set transmit mode
I2C2_CON = 0x8601;
//load data register with write command
I2C2_DATA = reg;
while (!(I2C2_STATUSRAW & 0x0010));
//poll XRDY bit in I2C2_STATUSRAW register (bit 4) until it is non-zero

//data has transmitted, write 1 to clear the interrupt
I2C2_STATUSRAW |= 0x0010;

while (!(I2C2_STATUSRAW & 0x0004));
//poll ‘access ready’ bit in I2C2_STATUSRAW register (bit 4) until it is non-zero
//set byte count to 2
I2C2_CNT = 2;

//set receive mode
I2C2_CON = 0x8400;

//set start and auto stop condition
I2C2_CON |= 0x0003;

while (!(I2C2_STATUSRAW & 0x0008));
//poll ‘receive ready bit’ in I2C2_STATUSRAW register (bit 3) until it is non-zero

dataByte = I2C2_DATA; //first byte is always 0x00

//data has been received, write 1 to clear the interrupt
I2C2_STATUSRAW |= 0x0008;

while (!(I2C2_STATUSRAW & 0x0008));
//poll ‘receive ready bit’ in I2C2_STATUSRAW register (bit 3) until it is non-zero

dataByte = I2C2_DATA; //second byte is the value we want

//data has been received, write 1 to clear the interrupt
I2C2_STATUSRAW |= 0x0008;

while (!(I2C2_STATUSRAW & 0x0100));
//poll ‘buss free’ bit in I2C2_STATUSRAW register (bit 8) until it is non-zero

//clear all pending interrupts
I2C2_STATUS = 0x7FF;

return dataByte;
}

Note the clearing of pending interrupts after each check and the final clear of the status register (not status_raw) at the end. 5 byte writes now working every time…