how to send a break signale via asyncio.StreamWriter function

[skylin008]

Thanks peterhinch to shared the uasyncio impressive project . I had write a LIN bus driver used the asyncio.StreamWriter, the LIN driver has a break signal, how to send the break signal via asyncio.StreamWriter. thanks!

[peterhinch]

I don't think you can do this. As you probably know, break is not a character but a sustained logic 0 on the interface. So you'll have to use uart.sendbreak. I haven't tested this but it may well block for a period.

[peterhinch]

Blocking for 680μs is unlikely to be a problem for an asyncio application. But the question is for how long uart.sendbreak blocks. This may be baudrate and/or platform dependent. You might like to measure it:

from time import ticks_ms, ticks_diff
# Initialise UART to your requirements
t = ticks_ms()
uart.sendbreak()
dt = ticks_diff(ticks_ms(), t)
print(dt)

[robert-hh]

The minimum length for sendbreak is 13 bit times. So 680µs is about 13 bit times at 19200 baud.

[skylin008]

Thanks @peterhinch @robert-hh, I had checked the send break signal via oscilloscope， it only 570uS

[skylin008]

Thanks @peterhinch @robert-hh, I had checked the send break signal via oscilloscope display， it only 570uS

[robert-hh]

Looking at the STM32 manual, it pulls the output low for 10/11+stop bit # time, which is 11 bit, when configured with 1 stop bit. So you could temporarily reconfigure UART to 2 Stop bits + parity, send a break, and then configure back, or configure the UART temporarily to a lower baudrate, send break and them put it back.