TMS320F28035: Ensuring completion of data transmit before toggling the direction gpio in half duplex communication

Soham Karyakarte19

Part Number: TMS320F28035

I intend to use my controller in a half duplex system. I have a GPIO20 connected to the direction control of the RS485 interface IC.

At all times the GPIO20 is output 0 indicating RX state. Only when I want to send something over RS485 network, I set the GPIO20 to switch to TX state.

I am unable to understand, how should I ensure completion of transmission before resetting the GPIO20 pin.

I tried polling:

int RS232_Send(char *buf) {

	int a = 0;

	GpioDataRegs.GPASET.bit.GPIO20 = 1;



	while ((buf[a] != 0) && (SciaRegs.SCIRXST.bit.RXERROR != 1)) {
		if (SciaRegs.SCIFFTX.bit.TXFFST < 4) {
			SciaRegs.SCITXBUF = buf[a++];
		}
	}

	while (SciaRegs.SCIFFTX.bit.TXFFINT == 0);

	GpioDataRegs.GPACLEAR.bit.GPIO20 = 1;

	if (SciaRegs.SCIRXST.bit.RXERROR == 1) {  // if there is an error reset the SCI
		RS232_Reset();
		GpioDataRegs.GPACLEAR.bit.GPIO20 = 1;

		return 1;
	}


	return 0;
}

I tried using interrupt method

int RS232_Send(char *buf) {


	int a = 0;


	GpioDataRegs.GPASET.bit.GPIO20 = 1;


	while ((buf[a] != 0) && (SciaRegs.SCIRXST.bit.RXERROR != 1)) {
		if (SciaRegs.SCIFFTX.bit.TXFFST < 4) {
			SciaRegs.SCITXBUF = buf[a++];
		}
	}

	SciaRegs.SCIFFTX.bit.TXFFIENA = 1;


	if (SciaRegs.SCIRXST.bit.RXERROR == 1) {  // if there is an error reset the SCI
		RS232_Reset();
		GpioDataRegs.GPACLEAR.bit.GPIO20 = 1;

		return 1;
	}


	return 0;
}

__interrupt void tx_comp(void){

	GpioDataRegs.GPACLEAR.bit.GPIO20 = 1;

	SciaRegs.SCIFFTX.bit.TXFFINTCLR = 1;
	SciaRegs.SCIFFTX.bit.TXFFIENA = 0;
	PieCtrlRegs.PIEACK.all = PIEACK_GROUP9;
}

In both the above cases the GPIO20 is cleared well before the last character in the transmit string, as a result last 2 or 3 characters are not received by the RS485 device.

Please let me know any other method which will ensure a total transmission completion and then turn off the GPIO20.

over 6 years ago

0 MEGHANA MANAVAZHI over 6 years ago

TI__Genius 10295 points

hi Soham,

You could use the below registers to check the status of communication and then turn off the GPIO20:

SCICTL2->TXEMPTY : if tx has nothing to send

SCICTL2-> TXRDY : If SCITXBUF is full or not

If using FIFOs:

SCIFFTX-> TXFFIL : Level of FIFO

In case if you wish to use RX as an identifier

SCIRXST-> RXRDY : No new character in SCIRXBUF or not

If using FIFOs: : SCIFFRX->RXFFST : Level of FIFO

Regards.

0 Soham Karyakarte19 over 6 years ago in reply to MEGHANA MANAVAZHI

Prodigy 100 points

Thank you Meghana,

I tried checking the SCICTL2.TXEMPTY status and SCICTL2.TXRDY status.

Both are not working. Resetting the TX pin while checking the status of above bits, resets it well before the last byte. The number of bytes being missed is also not constant.

I found the following post which is similar to my application.

http://e2e.ti.com/support/microcontrollers/c2000/f/171/t/412155?RS485-TX-ENABLE-signal

It seems TI recommends to use a timer and set it according to the time taken for a complete transmission after TXRDY is set.

I wanted to avoid using the main timer or ePWM. So I have setup a 'for' loop with assembly instructions to wait till all the bytes are transmitted and reset the TX enable pin.

Regards

0 MEGHANA MANAVAZHI over 6 years ago in reply to Soham Karyakarte19

TI__Genius 10295 points

hi,

Were you able to make progress?

Regards.

0 Soham Karyakarte19 over 6 years ago in reply to MEGHANA MANAVAZHI

Prodigy 100 points

Hello Meghana,

As I mentioned in the previous post and in reference to the link I have shared, I did not come across any kind of flag which denotes a total end of transmission. Hence through empirical data, I set a 'for' loop with assembly instructions (__NOP) where controller just 'waits' and then turns off the transmission bit on RS485 hardware.

Thank you for following up.

C2000™︎ microcontrollers

C2000 microcontrollers forum

TMS320F28035: Ensuring completion of data transmit before toggling the direction gpio in half duplex communication