MSP432P401R: UART DMA send and DMA receive at same time

Part Number: MSP432P401R

Using DMA channel 3 to service the receive side of UCA1 in UART mode. Using DMA channel 2 to service the transmit side of UCA1.

7-data, 1-stop, No Parity, 460,800bps.

Randomly (much less than 1%) I skip a character in the DMA receive buffer. Test packets are a repeating pattern of ASCII 0,1,2,3,4,5,6,7,8,9,0,1,2,…9 (makes finding the missing character easy to find).  Packets are terminated by CR  LF.

The serial bus is a shared RS-485 bus and I can monitor it with a serial port on Windows PC. The Serial Logger on the PC doesn’t miss any characters. Hence, error is in the receive of the MSP432.

The missing character error does NOT occur in PING PONG mode. Transmit a packet after every received packer.  The error occurs when performing asynchronous transmit/receive.  In this case the arrival of a packet is random relative to the transmit of a packet and both DMA channels can be active at the same time.

I need way to start the Transmit DMA without first clearing the UART TXIFG and then writing a character to the UART TXBUF. I disable the receive DMA channel, clear the TXIFG, then re-enable the receive DMA channel.  I have to disable/enable the receive DMA channel or I receive an extra character sometimes. (This I understand as you read the IFG register, clear the TXIFG bit, then write the byte back to the IFG register and this can sometimes set the RXIFG that was read but cleared by the DMA controller before writing the byte back to the IFG register.

void StartDmaTx( BYTE* pSrc, WORD size )
{
   MAP_DMA_setChannelTransfer(DMA_CH2_EUSCIA1TX | UDMA_PRI_SELECT,  UDMA_MODE_BASIC,
     &pSrc[1],(void *) MAP_UART_getTransmitBufferAddressForDMA(EUSCI_A1_BASE), size-1);

   MAP_DMA_disableChannel(DMA_CHANNEL_3);
   UCA1IFG &= ~UCTXIFG; // clear TX interrupt flag
   MAP_DMA_enableChannel(DMA_CHANNEL_3);
   HMI_XMIT_ON;
   MAP_DMA_enableChannel(DMA_CHANNEL_2);
   MAP_Interrupt_enableInterrupt(INT_DMA_INT2);

   UCA1TXBUF = *pSrc; //start DMA
   hmiTxState = HMITX_ST_WAIT_DMA_DONE;
}

BOOT code:
  /* Configuring DMA module */
    MAP_DMA_enableModule();
    MAP_DMA_setControlBase(sz_DMAControlTable);

    // channel 2 TX
    MAP_DMA_assignChannel(DMA_CH2_EUSCIA1TX);
    DMA_disableChannelAttribute(DMA_CH2_EUSCIA1TX, UDMA_ATTR_ALTSELECT | UDMA_ATTR_USEBURST | UDMA_ATTR_HIGH_PRIORITY | UDMA_ATTR_REQMASK);
    MAP_DMA_setChannelControl(DMA_CH2_EUSCIA1TX | UDMA_PRI_SELECT, UDMA_SIZE_8 | UDMA_SRC_INC_8 | UDMA_DST_INC_NONE | UDMA_ARB_1);
    MAP_DMA_assignInterrupt(DMA_INT2, DMA_CHANNEL_2);

    // channel 3 RX
    MAP_DMA_assignChannel(DMA_CH3_EUSCIA1RX);
    DMA_disableChannelAttribute(DMA_CH3_EUSCIA1RX, UDMA_ATTR_ALTSELECT | UDMA_ATTR_USEBURST | UDMA_ATTR_REQMASK);
    MAP_DMA_setChannelControl(DMA_CH3_EUSCIA1RX | UDMA_PRI_SELECT, UDMA_SIZE_8 | UDMA_DST_INC_8 | UDMA_SRC_INC_NONE | UDMA_ARB_1);
    MAP_DMA_assignInterrupt(DMA_INT1, DMA_CHANNEL_3);