Cant send UART messages at specific baud rates with FT232H board

Question

I have a Adafruit FT232H Breakout board, which has ft232hq chip that supports up to 12 Mbit/second Baud rate communication.

I want to send data at 8.250.000 bit/s using my PC.

Using PUTTY terminal app; I managed to create accurate communication signal at 4,8 and 12 Mbit/s (I checked at logic analyzer data)

But when I tried my goal speed 8.250.000 Bit/s The accurate signal was not generated

How can I achieve this baud rate?

Also I want to mention; Apps like Minicom,stty didn't work at any higher rate than 4 Mbit/s

Python Serial libraries also didn't work at any rate higher than 8Mbit/s

Thanks in advance

Answer 1

From https://ftdichip.com/wp-content/uploads/2020/07/DS_FT232H.pdf page 19:

Baud Rate Generators – The Baud Rate Generators provides an x16 or an x10 clock input to the UART’s from a 120MHz reference clock and consists of a 14 bit pre-scaler and 4 register bits which provide fine tuning of the baud rate (used to divide by a number plus a fraction). This determines the Baud Rate of the UART which is programmable from 183 baud to 12 Mbaud. See FTDI application note AN_120 on the FTDI website for more details.

AN_120 ( https://www.ftdichip.com/Documents/AppNotes/AN_120_Aliasing_VCP_Baud_Rates.pdf ) states under "4.4 Aliasing the FT232H, FT2232H and FT4232H for Baud Rates up to 12MBaud":

Note: Divisor = 1 and Divisor = 0 are special cases. A divisor of 0 will give 12MBaud, and a divisor of 1 will give 8MBaud. Sub-integer divisors are not allowed if the main divisor (n) is either 0 or 1.

TL;DR: 8 Mbit/s is the closest you will get.

Why do you even need that strange baud rate?