[Dprglist] STM32 software uart

[deltagraph at aol.com]

 David,
It has been some time since I have written a software UART, (ack 15 years) but I do remember a handy diagnostic technique that I employed which is to generate a brief pulse on a GPIO pin when sampling each data bit and also a pulse when recognizing a start bit which I typically performed with an edge triggered interrupt. This pulse was used to trigger and oscilloscope and the sample pulses were overlaid with the data stream on the scope display. I never sampled more that one time within a bit time on anything I ever wrote, but of course that might be an option.

I assume some of your devices have a low data rate, e.g. 9600 baud. Higher baud rates can be tough to deal with.  I can't remember if I ever used a dedicated timer, but that might prove very effective in being able to accurately time pulses as long as you can get around logging counts within a single bit time of your data stream.  I would think polling would fail before timer input capture as far as problems with interrupt latency...
Of course for transmission using a timer is real handy since with output compare register and interrupts you can synthesize "exact" bit timings provided your timer has the needed prescaler. I guess that is the nice thing about modern 32 bit MCUs, real nice ability to program the frequency you want. 

Ron Grant



 
 
-----Original Message-----
From: David Anderson via DPRGlist <dprglist at lists.dprg.org>
To: dprglist at lists.dprg.org
Sent: Wed, May 6, 2020 10:46 pm
Subject: [Dprglist] STM32 software uart

Hello DPRG,

As discussed in Tueday's RBNO, I'm trying to port my jBot code to the 
STM32F411RE and have run out of UARTS.  I need 5 and the board only 
supports 3 in hardware (USART1, USART2, USART6).  So I was looking at 
adding another CPU, a teensy, and using SPI or I2C for comm between the 
two boards.

I found several software uarts but all were for sale.  Turns out I 
didn't look hard enough.  There is a free one available from ST Micros 
it self:  stsw-stm3156.  It requires an available timer, which I think 
I  have, and a couple of free GPIO pins.  I've downloaded the sofware 
and docs and am cautiously optimistic...

dpa


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