cancel
Showing results for 
Search instead for 
Did you mean: 
Highlighted
Advisor
Advisor
310 Views
Registered: ‎10-10-2014

AXI UART Lite v2.0 IP PG142 does not mention how to calculate the baud-rate error

If I'm correct, PG142 does not mention how to calculate the actual baud-rate error, nor does the custom IP dialog display this. Is there any way we can check the baud-rate error vs AXI input clock? Any formula for this?

** kudo if the answer was helpful. Accept as solution if your question is answered **
0 Kudos
2 Replies
Highlighted
Moderator
Moderator
193 Views
Registered: ‎04-09-2019

Re: AXI UART Lite v2.0 IP PG142 does not mention how to calculate the baud-rate error

Hi,

You can find in the IP

Regards,

Venu

0 Kudos
Highlighted
170 Views
Registered: ‎01-22-2015

Re: AXI UART Lite v2.0 IP PG142 does not mention how to calculate the baud-rate error

Hi Ronny,

The following post says the UARTlite baud rate error is limited to 3% in the newer versions of Vivado.

https://forums.xilinx.com/t5/Processor-System-Design/AXI-Uartlite-baud-rate-tolerance/td-p/821197

It is sometimes fun to guess a little about what's going on under the hood.

From playing with the AXI UARTlite (2.0) wizard, it appears that the IP is using 16x oversampling to create the UART output waveform.  So, from the AXI CLK, CLKA, frequency that you specify in the wizard, the IP must generate an oversampling clock, CLKOS, that is 16x the specified Baud Rate.  Further, it appears that CLKOS is created by simple integer division of CLKA.  So, the baud rate error depends directly on the accuracy of CLKOS.  

For example, if CLKA=100MHz and baud=460800 bits-per-second (bps) then [100,000,000/(16*460800))=13.56.  So, CLKOS would be created by dividing CLKA by 14 and the resulting error in CLKOS (and in baud) is 3.12%.  Indeed, the wizard does not allow us to select baud=460800 when CLKA=100MHz because the resulting baud error is over the allowed 3%.

However, if CLKA=73.728MHz then this divides evenly into almost all the allowed baud rates times 16x.  In fact, when CLKA=73.728MHz, the wizard allows us to select baud=921600 bps.

Mark

Tags (1)
0 Kudos