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Uart module turn-on time

Al Bowers
Prodigy 70 points
Other Parts Discussed in Thread: MSP430F149

We use an older uP MSP430F149, that has two Uart modules.

We are in a very tight byte by byte transfer operation.

We need to know how long the Uart takes to turn on and "stabilize"...

From issuing the "SWRST" command, when can we expect bytes to flow out (or into). Possibly could be a setup time.

I can find no references in either Hardware or Software manuals.

Thanks, Al

  • 0
    Guru 74080 points
    I didn't ever see an information like this as well (which does not mean there is none about it), but do you have problems with a "too slow" UART module after clearing the reset-bit? Or is it just for interest?

    Dennis
  • 0 in reply to Dennis Eichmann
    Prodigy 70 points
    Hi, Dennis - we are trying to design a protocol transfer device, and are making sure of our error budget.


    We have 130us to go from transmit to receive, and of course, the firmware takes some of this time...


    We run pretty slowly, and are concerned that the UART turn-on is tied to the DCO (or MCLK).


    Thanks, Al
  • 0 in reply to Al Bowers
    Guru 74080 points

    Ah, OK, I see. But why switching from transmit to receive in UART mode? It is asynchronous and can do both at the same time independent from each other since there is no clock.

  • 0 in reply to Dennis Eichmann
    Prodigy 70 points
    Yeah, I know this, but some stubborn old guys keep insisting we need to try and get this setup time, since they feel UART on then off is lower power consumption...
  • 0 in reply to Al Bowers
    Guru 74080 points
    Which is correct, of course ;) hope someone from inside TI can help you with that.

    Dennis
  • 0 in reply to Al Bowers
    Guru 58965 points
    The UART is driven by a clock (UCLK, ACLK, or SMCLK). This clock is divide into the bit-clock. It takes up to 12.999 bit-clocks to send/receive a "character". The 12 includes start bit, up to 8 data-bits, optional parity-bit, and up to 2 stop-bits. The 0.999 part signifies the wait for the arrival of beginning of bit-clock cycle. And there is a pipe-line of 2 "characters" end-to-end. (I am not sure if there is an additional small fraction.) The data in the pipe-line is invisible to the software ("latency"). On top of this, there is the interrupt latency. Are these what you are budgeting? What is the 170us you refer to?

    The total latency is the time interval between (a) the transmitting software sends a "character" to TXBUF, and (b) the receiving software gets that "character" from RXBUF.
  • 0 in reply to Al Bowers
    Guru 46710 points

    >We have 130us to go from transmit to receive
    >UART on then off is lower power consumption...

    Sorry, I don't get it. Time critical low power asynhronous half duplex communication device? :) Like rs485 where not only TX/RX switching needed but also powering down UART module to supposedly save power? Do you run into actual timing problems or just asking? Do you put CPU into LPM? Which one? UART shall be quickly transmitting after wake-up from LPM? - So much unclear details about what actually you want to achieve. Could be helpful if you explain UART, TX/RX switching part of application in more details, also reason of need to shut down such a hilariously simple (thus low power) peripheral as UART of msp430.

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