Configuration of the 52840 UART peripheral for 7 bit transmission.

RE: Configuration of the 52840 UART peripheral for 7 bit transmission.

hmolesworth — Mon, 03 May 2021 20:22:17 GMT

.. test the Rx parity bit first, using the same table:

static const uint8_t EvenParityTable[128] =
    {0x00,     // 00   0000 0000  0
     0x80,     // 01   0000 0001  1
     0x80,     // 02   0000 0010  1
     0x00,     // 03   0000 0011  0
     0x80,     // 04   0000 0100  1
     0x00,     // 05   0000 0101  0
     0x00,     // 06   0000 0110  0
     0x80,     // 07   0000 0111  1
     0x80,     // 08   0000 1000  1
     0x00,     // 09   0000 1001  0
     0x00,     // 0A   0000 1010  0
     0x80,     // 0B   0000 1011  1
     0x00,     // 0C   0000 1100  0
     0x80,     // 0D   0000 1101  1
     0x80,     // 0E   0000 1110  1
     0x00,     // 0F   0000 1111  0
     ... etc
TxChar must lie in range 0x00:0x7F (7-bits)

   // Send bytes
   TxChar |= EvenParityTable[TxChar];
   
   // Receive bytes
   RxChar = RawChar & 0x7F;
   // Check if parity different
   if ((RawChar & 0x80) ^ EvenParityTable[RxChar])
   {
      panic, parity failed
   }

RE: Configuration of the 52840 UART peripheral for 7 bit transmission.

ccasebeer — Mon, 03 May 2021 20:11:18 GMT

I can see how this would work on the TX.

Would it work on the RX?

I could always just go null out the 8'th bit (the read in parity)....

I wonder if the RX'ed stop would be messed.

Edit. Now that I think about it. It would work. I just need to zero out the read in parity bit.

Greatly! appreciated.

RE: Configuration of the 52840 UART peripheral for 7 bit transmission.

hmolesworth — Mon, 03 May 2021 19:25:48 GMT

You would have to work in 8-bit no parity mode, then spoof the 8th bit as the required even parity for every single byte before transmitting the buffer. That can be done with a 128-byte lookup table which has a mask of the 8th parity bit which would be or'd with the data byte, so very fast.

static const uint8_t EvenParityTable[128] =
    {0x00,     // 00   0000 0000  0
     0x80,     // 01   0000 0001  1
     0x80,     // 02   0000 0010  1
     0x00,     // 03   0000 0011  0
     0x80,     // 04   0000 0100  1
     0x00,     // 05   0000 0101  0
     0x00,     // 06   0000 0110  0
     0x80,     // 07   0000 0111  1
     ...
     
TxChar must lie in range 0x00:0x7F (7-bits)

   TxChar |= EvenParityTable[TxChar];