2023-12-29_rpi-serial-rates.md

2023 12 29

So, spinning this up is pretty simple (on the pi 4) we use the configuration tool (UI version actually shows the options we want, command line version misses one) to set UART and SPI interfaces available, then the test code is just using pyserial:

import serial, time 

baud = 2000000

print(f'bit time should be {1000000/baud}us')

ser = serial.Serial('/dev/serial0', baud)

for i in range(10000):
    ser.write(bytearray([95]))
    time.sleep(0.001)

ser.close() 

Now, I'm trying to figure where the limit is, so, baud values and measured bit times:

Baud Setting	Expected Bit Time	Measured	Match
115200	8.681us	8.703us	0.997
1000000	1.0us	1.024us	0.977
2000000	500ns	512ns	0.977
3000000	333ns	352ns	0.946
4000000	250ns	271ns	0.922
5000000	200ns	223ns	0.923
6000000	166ns	176ns	0.943
10000000	100ns	111ns	0.901

So we can actually get up to ~ 5MBit/sec and (nearly) survive, though baud settings might need to be trimmed on a scope to get-robust. This is a pretty good sign, and it means we can probably make a serviceable link using UART, though we might end up wanting to deploy two. I'll see quickly about the SPI speeds and then try to figure out where to push.