UART transmit timing.

Alice Dalton

Other Parts Discussed in Thread: MSP430G2955

I am using a MSP430G2955. I am using the UART for serial communications on a bus. The transmit speed is fairly low, 4800 baud. Is there any way to determine when a byte is through transmitting. I have some timing critical functions that must be done immediately following a byte transmission. The interrupt flag UCA0TXIFG is set to indicate that the buffer is ready for another byte, but that does not necessarily correspond with a byte finishing transmitting. Currently I am doing a time delay of approximately 2ms ( (1/4800) X 10 bits ) but this is not always giving me good results.

over 8 years ago

0 Dennis Eichmann over 8 years ago

Guru 74080 points

Do you need transmission in both directions? If not, connect TX to RX and use UCA0RXIFG to detect a complete transmitted byte.

Dennis

0 Clemens Ladisch over 8 years ago

Guru 243220 points

You could wait for UCBUSY to clear.

0 Alice Dalton over 8 years ago in reply to Dennis Eichmann

Intellectual 540 points

Yes unfortunately I do need the receive as well. After every other byte transmitted, I have to do certain functions and then be ready for the response back.

0 Dennis Eichmann over 8 years ago in reply to Alice Dalton

Guru 74080 points

Then use UCBUSY, but you will have to poll it instead of using interrupts.

0 Dennis Eichmann over 8 years ago in reply to Dennis Eichmann

Guru 74080 points

Someone might correct me if I am wrong, but isn't it also set while receiving? Means if a byte comes in while one is transmitted and the transmitted byte has already finished, but the receiving one not yet, it will stay set until complete reception.

0 Brian Boorman over 8 years ago

Guru 25210 points

You could do something along these lines if you have a spare timer available:

1. In IRQ for UCA0TXIFG, if last byte, setup timer CCR and start.
2. In the timer CCR take care of your housekeeping task (if short) or set a flag for background loop.

The time from UCA0TXIFG to last bit transmitted should be relatively static (in terms of clock cycles) and you can pre-compute that.

0 Alice Dalton over 8 years ago in reply to Dennis Eichmann

Intellectual 540 points

That should not be a problem. My unit will be the bus master and nothing else is allowed to answer back until I am finished transmitting.

0 Alice Dalton over 8 years ago in reply to Brian Boorman

Intellectual 540 points

That is how I originally did it. But I was never able to get the timing just right.

0 Brian Boorman over 8 years ago in reply to Alice Dalton

Guru 25210 points

It should work if the UART and the TimerA module both are using the same reference clock. What kind of jitter were you seeing on the timing?

0 Dennis Eichmann over 8 years ago in reply to Alice Dalton

Guru 74080 points

But if there is another TX connected from any other device which is normally connected to your RX, then connecting your own TX to that line as well will lead to two line-driving devices. This causes shorts from Vcc to GND. You would need series resistors to limit the current. Or the other device would need TriState functionality.

0 Tony Philipsson over 8 years ago in reply to Alice Dalton

Guru 12050 points

>wait for UCBUSY to clear. There is none on the MSP430x2xx

Do you know if RX is not being used while you TX?, if so set UCLISTEN bit it will "hardwire" tx-pin to rx-pin that you can do temporary.
So RXIFG flag is set when byte have been sent.

OR:
Enable UC0CLK and wire that to a P2 rising edge IRQ, count 20times for two bytes to have been sent.
Maybe don't even need to wire it, as rising edge port irq may even work while it's a UC0CLK pin.

Can it be set as a uart pixel clk output, not sure. this thread talks about using it as an input.
e2e.ti.com/.../408439

0 Alice Dalton over 8 years ago in reply to Dennis Eichmann

Intellectual 540 points

All devices are connected to level translators and then to the bus. Part of that level translator is series resistors.

0 Alice Dalton over 8 years ago in reply to Dennis Eichmann

Intellectual 540 points

This did the trick. I am now in control of the bus timing. Polling the bit was not a problem, since all other functions were dependant on the byte being transmitted.

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UART transmit timing.