Garbled I2C unless printf is used

[snoopy20]

Hi,

I have a very odd problem but one which is consistent.

As part of my lcd library I send several ''packets' of data interleaved with spacing from a timer. It typically looks like -

6 bytes - 50us - 6 bytes - 50us - 6 bytes ...

A second timer works in counter mode and is used to stop the transmission once the requisite amount of bytes are sent. It works well.

Now, the command that kicks such a process off (the data is held in a buffer) is this -

void lcdTransactionEnd (void) {
  
  if (dmaBuffer.writeBuffer->writeLocked) {
    dmaBuffer.writeBuffer->writeLocked = 0;
  }
  if (dmaBuffer.transferBuffer->size && ! dmaBuffer.transferBuffer->txLocked) {
    printf("transfering \n");
    __startTransfer();
  }
}

and startTransfer is

static void __startTransfer (void) {

  dmaBuffer.transferBuffer->txLocked = 1;
  CFG_TWIM_LCD_TX->TXD.PTR = (uint32_t) &dmaBuffer.transferBuffer->data[0];
  CFG_TWIM_LCD_TX->TXD.MAXCNT = dmaBuffer.transferBuffer->packetSize;
  CFG_TIMER_LCD_COUNTER->CC[0] = dmaBuffer.transferBuffer->size / dmaBuffer.transferBuffer->packetSize;
  //printf("tx %d, %d \n", dmaBuffer.transferBuffer->size, dmaBuffer.transferBuffer->packetSize);
  CFG_TWIM_LCD_TX->TASKS_STARTTX = 1;
}

Now here's the thing. In the ledTransactionEnd there is a printf statement. If I remove this statement I get garbage (or data corruption) over I2C. I thought maybe I have a timing issue, so I replaced it with various nrf_delay_ms values and that had no effect. I have absolutely no idea how using printf can cause this to happen but it's repeating every time.

Any ideas most welcome!

[ovrebekk]

Hi

One thing to check is whether or not the data buffer provided to the TWIM PTR register is static or global, or if it is an automatic stack variable that could be overwritten by other data once you return from the function where it was defined. 

This could explain why the printf call solves the issue, since it might delay the time before you return from the function so that the data is written to the I2C bus before the stack memory gets reused. 

Best regards
Torbjørn

[snoopy20]

struct dmaBufferProto {
uint16_t size;
uint8_t writeLocked;
uint8_t txLocked;
uint8_t packetSize;
uint8_t data[LCD_BUFFER_LENGTH];
};

volatile static struct {
struct dmaBufferProto buffers[LCD_BUFFER_ARRAY];
struct dmaBufferProto *writeBuffer;
struct dmaBufferProto *transferBuffer;
} dmaBuffer;

[ovrebekk]

Hi 

The only case where the PPI propagation could be delayed is if the 16MHz peripheral clock is suspended, but unless you enter sleep mode and have disabled all peripherals that require the peripheral clock (such as the UART or a timer in timer mode) this will not be the case. Then the delay of the PPI should be consistent, and only about a clock cycle long. 

Are you going to sleep in between events by the way?

Best regards
Torbjørn

[ovrebekk]

Thanks for sharing the code, can't see any issue there ;)

[snoopy20]

I've had to abandon it, the consistency wasn't there.

Regards, Andrew