USCI-UART

[jeromeDms]

Hi
I'm using USCI0 as UART, 31250 kb/sec (low speed) on M484KIDAE
Everything works fine when sending single bytes.
Anyway, I'm trying to send multiple bytes in a loop, and only the first one is sent.

The following does not work, it sends only the first byte

for(;;) {
   isUartTxFull = UUART_GET_TX_FULL(UUART0);
   if(isUartTxFull == 0) {
        UUART0->TXDAT = myData8b;
   }
}

The following works, it sends all bytes, but waiting buffer empty is not applicable in my app.

for(;;) {
   isUartTxEmpty = UUART_IS_TX_EMPTY(UUART0);
   if(isUartTxEmpty == 1) {
        UUART0->TXDAT = myData8b;
   }
}

UUART_GET_TX_FULL() returns 0 but adding one more byte in the buffer does nothing.

I found In the TRM of M480 the following on page 1547 :

5. Transmit and receive data.

• Write transmit data register UUART_TXDAT to transmit data.
• Wait until TXSTIF(UUART_PROTSTS[1]) is set and then user can write the next data in
  UUART_TXDAT.

I tried testing the TXSTIF before writing the next byte but it does not help.
Also, the USCI UART examples in the BSP are not testing this bit.

Thanks for any help you can provide.

[ychuang3]

The BSP sample code uses a baud rate of 115200, and continuous transmission of 1KB works without issues.
In reality, the USCI UART has only a one-byte TX FIFO and a two-byte RX FIFO.
Therefore, waiting for the TX FIFO FULL flag to clear is functionally the same as waiting for the TX FIFO EMPTY flag—except that the EMPTY flag is set slightly later.
Since waiting for TX FIFO EMPTY works fine, we recommend adopting this approach.

[jeromeDms]

Thanks for your response.
I'm observing a similar issue with the M484KIDAE when using UART.
Please note I do not see this issue while running the same project on a M482LGCAE.

The UART is supposed to have 16 bytes TX FIFO and I found the UART_FIFOSTS_TXFULL_Msk is not set properly.
If I add a test to UART_FIFOSTS_TXOVIF_Msk in my loop I can see this bit is set, but UART_FIFOSTS_TXFULL_Msk is not.

for(;;) {
   isUartTxFull = UART_GET_TX_FULL(UART0);
   if(isUartTxFull == 0) {
        UART0->TXDAT = myData8b;
   }
   else break;
}

I'm not using interrupts for the TX part, and I'd like to be able to send 16 bytes.
It seems the UART_FIFOSTS_TXFULL_Msk takes some time before being set ?
What would be the proper way to do ?

Thanks

[ychuang3]

Please note that UART0 is different from UUART0.
UART0 and UUART0 are different IPs. UUART0 belongs to the USCI UART IP, while UART0 is part of the standard UART IP.
These two IPs have different designs: the UART IP has a TX FIFO of 16 bytes, whereas the UUART only has a 1-byte TX buffer.
Please refer to the UART and USCI chapters in the TRM for more details.

[jeromeDms]

Yes I'm aware of that.
My previous message mentions I found UART_FIFOSTS_TXFULL_Msk is not set properly on UART, not on UUART.
I'm observing this UART issue with the M484KIDAE when using UART, again I'm not talking about UUART.
Please note I do not see this issue while running the same project on a M482LGCAE.

[mjchen1]

At present, we are still unclear about the cause of the issue you mentioned. Perhaps you can consider using UART_FIFOSTS[21:16] (TXPTR) instead of relying on the TX FULL flag.

[jeromeDms]

Thanks
Yes that will make the job.
Please note I'm using M482 as well in another product and I do not observe this issue.