I have made a UART controller in VHDL (transmitter, receiver and a FIFO for each component) and I'd like to send/receive 128 bits of data.

Is there anything that prevent me from implementing a 128-bit FIFO, and send/receive 130-bit UART frames ? Or should I send/receive 16 frames of 10 bits (8 bits of data plus 1 bit of start and 1 bit of stop).

  • \$\begingroup\$ What are you sending the data to? \$\endgroup\$ Jun 25, 2018 at 9:23
  • \$\begingroup\$ If you want compatibility with standard UART receivers, send the data as a 160bit frame - essentially 16 standard 10-bit UART packets strung together i.e. 0xxxxxxxx10xxxxxxxx10....xxx1. \$\endgroup\$ Jun 25, 2018 at 9:26
  • \$\begingroup\$ Sorry for being a bit unclear... I implemented the UART for my FPGA Basys3 Board, and I want to communicate with my PC through serial port. \$\endgroup\$
    – Cedric
    Jun 25, 2018 at 9:29
  • \$\begingroup\$ if you want to talk to your PC then it only does 7 or 8 bit characters you're going to be limited by that, a 128-bit uart is not going to help you \$\endgroup\$
    – Taniwha
    Jun 25, 2018 at 9:40
  • \$\begingroup\$ Would it work if I implement a UART controller in C or Python instead of using generic tools like Minicom or Putty ? \$\endgroup\$
    – Cedric
    Jun 25, 2018 at 9:42

1 Answer 1


Start and stop "frame" bits are important in asynchronous communication as they provide a way to sychronise the receiving bit clock with the stream being sent

if you send 128 bits of data the two clocks must agree in frequency to better than 0.4%. Most crystal oscillators are much better than 4000ppm so it could work. but I've not seen such a scheme ever used.

Also the better the clock agreement the better the noise immunity of the system - towards the end of the packet the signal degades as the sampling point drifts towards the edge of the bits.

However most UARTSs send only 8 bits of data between each pair of frame bits. this requires only 5% speed accuracy making communication practical with a cheap ceramic resonator, or even a good RC oscillator.

If you want to communicate with a PC you will need to use a line discipline that it understands, that typically means no more than 8 data bits between the start and stop bits, so transmitting 128 bits of data will require a total of 160 bits on the wire.

  • \$\begingroup\$ Ok thank you for the answer, I will try to send the whole bits in one frame and if I see it's not working I'll come back to the "traditional" protocol. \$\endgroup\$
    – Cedric
    Jun 25, 2018 at 9:39
  • \$\begingroup\$ ordinary PC serial ports can only handle (up-to) 8 bits per frame but you can pack those frames together tightly to produce a packet of several bytes. (or kilobytes....) \$\endgroup\$
    – Jasen
    Jun 25, 2018 at 9:41

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