UART.read() times-out twice as fast on latest version compared to earlier versions

[jamesquilty]

I've just upgraded one of my ESP32-WROVER devices from 1.14 to 1.26.1 and noted that UART.read() began returning incomplete data, i.e. the remainder of the data was present in the serial buffer after UART.read() returned.

I performed a rough experiment to estimate what I understand would be the inter-character timeout timeout_char by reading a serial buffer containing either 0 bytes or pre-populated with 9 bytes. The elapsed time should be dominated by the timeout value. Sure enough, there's a big difference between 1.14 and 1.26.1:

Buffer	1.14	1.26.1
0 bytes	450 us	200 us
9 bytes	500 us	220 us

This would appear to be the cause of the problem and suggests a bug in the MicroPython UART class.

I'm open to suggestions on tracking-down the problem in the source code and working around the problem - preferably without implementing the UART read() and readline() method functionality in my MicroPython code.

Technical Detail

The UART object is constructed with:

serial = machine.UART(1, 115200, bits=8, parity=None, stop=1, tx=tx_pin, rx=rx_pin)

And, while I've tried setting timeout_char, it shows similar unexpected behaviour to timeout which I previously reported. My test function to be called on the REPL is:

def timeread():
    start_ticks = time.ticks_us()
    resp = serial.read()
    print("time to read {1} bytes: {0:d} us".format(time.ticks_diff(time.ticks_us(), start_ticks), len(resp) if resp else 0))

[robert-hh]

It's the opposite: a bug related to timeout was fixed in 1.26. The UART constructor has two settings for timeout. timeout=xx for the timeout before the first byte is received, and timeout_char=xx for the inter-byte timeout. In the previous version, the initial timeout was always applied. Now, you may have to set both timeout and timeout_char according to your needs.

[jamesquilty]

@robert-hh thanks for the quick response! As soon as we saw this problem, we tried setting timeout_char (but not timeout) with the test script I posted in #9979 and obtained the similarly unexpected and confusing results (below) which appear to indicate that setting timeout_char has no effect on read times.

I looked at the 1.26.0 and 1.25.0 release notes to try to find the commit and PR to which you refer and found 155fa94 (#17135) and 2c2f0b2 (#17265) but neither seem to contain bugfixes for UART timeout or timeout_char on all port or the ESP32 port, at least not in those words. I'm not entirely sure which file's commit history would have these changes.

If it's easy for you to find, could you please provide the PR and/or commits which made the changes to mentioned?

---

Test Results

>>> test_UART_timeout(timeout_char=1)
serial.read() elapsed time: 1 ms
Modem response: b'AT\r\r\nOK\r\n'
>>> test_UART_timeout(timeout_char=2)
serial.read() elapsed time: 1 ms
Modem response: b'AT\r\r\nOK\r\n'
>>> test_UART_timeout(timeout_char=4)
serial.read() elapsed time: 1 ms
Modem response: b'AT\r\r\nOK\r\n'
>>> test_UART_timeout(timeout_char=10)
serial.read() elapsed time: 1 ms
Modem response: b'AT\r\r\nOK\r\n'

[robert-hh]

The change happened with this PR: #17938

I just re-tested the behavior with the script below (and the result). The behavior looks fine.
Edit: Note that the timeout settings unit is ms.

from machine import Pin, UART
import time
import sys

# Add a loopback between rx and tx

# uart = UART(0, 38400, tx=Pin(0), rx=Pin(1), cts=Pin(2), rts=Pin(3), flow=3, txbuf=32, timeout=200, timeout_char=50) # RP2
uart = UART(1, 115200, tx=Pin(4), rx=Pin(5), timeout=200, timeout_char=50) # ESP32
# uart = UART(1, 38400, timeout=200, timeout_char=50) # Teensy 4.1
# uart = UART(baudrate=38400, timeout=200, timeout_char=50) # SAMD51
# uart = UART(4, 38400, timeout=200, timeout_char=50) # PyBoard V1.1, PYBD_SF6
# uart = UART(0, 38400, timeout=200, timeout_char=50) # NRF
# uart = UART(1, 9600, pins=(Pin("GP23"), Pin("GP24"))) # CC3200, Pins GP23, GP24
print(uart)


for msg in ("", "123", "12345"):
    t1 = time.ticks_ms()
    uart.write(msg)
    uart.flush()
    rx = uart.read(5)
    t2 = time.ticks_ms()
    print('uart.read() msg="%s"  timeout=%u ms, rx=%s' % ( msg, time.ticks_diff(t2, t1), rx))
    if uart.any() > 0:
        uart.read()

print()

b = bytearray(5)
for msg in ("", "123", "12345"):
    t1 = time.ticks_ms()
    uart.write(msg)
    uart.flush()
    rx = uart.readinto(b)
    t2 = time.ticks_ms()
    print("uart.readinto(bytearray(5)) msg='%s' timeout=%u ms, rx=b'%s'" % ( msg, time.ticks_diff(t2, t1), b.strip(b"\x00").decode()))
    if uart.any() > 0:
        uart.read()

UART(1, baudrate=115201, bits=8, parity=None, stop=1, tx=4, rx=5, rts=-1, cts=-1, txbuf=256, rxbuf=256, timeout=200, timeout_char=50, irq=0)
uart.read() msg=""  timeout=200 ms, rx=None
uart.read() msg="123"  timeout=49 ms, rx=b'123'
uart.read() msg="12345"  timeout=5 ms, rx=b'12345'

uart.readinto(bytearray(5)) msg='' timeout=194 ms, rx=b''
uart.readinto(bytearray(5)) msg='123' timeout=49 ms, rx=b'123'
uart.readinto(bytearray(5)) msg='12345' timeout=5 ms, rx=b'12345'

[jamesquilty]

Robert, I'm very grateful for the time you've spent writing a detailed reply! We'll check this first thing on Monday on our ESP32. Have a great weekend!

[rogernock]

@robert-hh Hi, I just stumbled across this discussion and thought I'd try it out on the RP2 with 1.26.1
Unfortunately it is showing the the original problem of timeout and timeout_char being added together.

Using your example I get this result.

UART(0, baudrate=115200, bits=8, parity=None, stop=1, tx=0, rx=1, txbuf=256, rxbuf=256, timeout=200, timeout_char=50, invert=None, irq=0)
uart.read() msg=""  timeout=201 ms, rx=None
uart.read() msg="123"  timeout=253 ms, rx=b'123'
uart.read() msg="12345"  timeout=1 ms, rx=b'12345'

uart.readinto(bytearray(5)) msg='' timeout=201 ms, rx=b''
uart.readinto(bytearray(5)) msg='123' timeout=253 ms, rx=b'123'
uart.readinto(bytearray(5)) msg='12345' timeout=1 ms, rx=b'12345'

Do you know if the RP2 and other ports will be corrected too?

[robert-hh]

I know that it has been corrected. But it may have been after v1.26.1. You could try the latest v1.27 preview.

[rogernock]

That was it! using MicroPython v1.27.0-preview.463.g22cff2f6f5 on 2025-11-27; Raspberry Pi Pico with RP2040 gives..

UART(0, baudrate=115200, bits=8, parity=None, stop=1, tx=0, rx=1, txbuf=256, rxbuf=256, timeout=200, timeout_char=50, invert=None, irq=0)
uart.read() msg=""  timeout=201 ms, rx=None
uart.read() msg="123"  timeout=52 ms, rx=b'123'
uart.read() msg="12345"  timeout=1 ms, rx=b'12345'

uart.readinto(bytearray(5)) msg='' timeout=201 ms, rx=b''
uart.readinto(bytearray(5)) msg='123' timeout=52 ms, rx=b'123'
uart.readinto(bytearray(5)) msg='12345' timeout=1 ms, rx=b'12345'

Thank you