TWIM Buffer Behavior When Debugging vs. Not Debugging

Hi,

Can you post the code used for sending data through the TWIM peripheral?

Have you made sure that the buffer is in scope during the full transfer, to make sure nothing else in the application overwrites the stack area before the transfer is completed?

Best regards,
Jørgen

Hi Jorgen,

Data buffer setup, where "bank1"  is a structure containing a uint8_t array "data" of length equal to 3.

pcs_wire_inst *bank1 = (pcs_wire_inst*)malloc(sizeof(pcs_wire_inst));
static uint16_t debugData = 0x2233;
bank1->data[0] = (uint8_t)(0x04);                       //0x04
bank1->data[1] = (uint8_t)(debugData & 0xFF);           //0x33
bank1->data[2] = (uint8_t)((debugData >> 8) & 0xFF);    //0x22

Access function to TWIM peripheral: 

wire_pcs_write(0x3C, bank1->data, sizeof(bank1->data));

Function def:

uint32_t wire_pcs_write(uint8_t target_address, uint8_t const *data, size_t size)
{
	uint32_t err_code;
	uint8_t buffer[size];											//temp buffer for data
	memcpy(buffer, data, size);								//place incoming data into buffer
	
	err_code = nrfx_twim_tx(&m_twim, target_address, buffer, sizeof(buffer), false);
	return err_code;			//will return 0 if successful
}

Following the nrfx_twim_tx function all the way to task trigger...

As seen above, p_xfer_desc's buffer contains all three bytes of data. At this point, if I execute one step through the code I get 0x04 - 0x33 - 0x22 on my Arduino Serial Monitor. However, if I do not place a breakpoint here and simply let the code execute, the last byte is always 0x00 on the Serial Monitor. Again, I can simply change the length of the "data" buffer from 3 to 2 bytes, and everything works fine but my application requires 3 bytes minimum.

Am I misunderstanding where the data is actually contained, not in p_xfer_desc primary buffer?

Thank you for your help.

-Cody

Hi,

The data is not stored in p_xfer_desc, this is just a pointer to the RAM buffer where the data is actually stored.

In your wire_pcs_write function, you are copying the data from "data" pointer to a local "buffer" array. This buffer is not global or static, and will be stored on the stack. Once you exit the function call, the memory area of the "buffer" will be released back to the stack and can be used by other function calls. nrfx_twim_tx() is a async function, which starts the transfer, but the transfer will likely not finish before you have exited the wire_pcs_write() function. This means that other parts of your application can overwrite the data before the transfers is done with accessing the data in RAM.

Try to declare the "buffer" array as static, to prevent it from being overwritten when you exit the function, or move it's declaration to the global application scope.

Best regards,
Jørgen