tests/extmod: Add test for machine.UART.IRQ_TXIDLE.

The test checks whether the message created by the IRQ handler appears
about at the end of the data sent by UART.

Supported MCUs resp. boards:
- RP2040
- Teensy 4.x
- Adafruit ItsyBitsy M0
- Adafruit ItsyBitsy M4
- NRF52 (Arduino Nano Connect 33 BLE)

Signed-off-by: Damien George <damien@micropython.org>

tests/extmod/machine_uart_irq_txidle.py

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+# Test machine.UART.IRQ_TXIDLE firing after a transmission.
+# Does not require any external connections.
+
+try:
+    from machine import UART
+
+    UART.IRQ_TXIDLE
+except (ImportError, AttributeError):
+    print("SKIP")
+    raise SystemExit
+
+import time, sys
+
+# Configure pins based on the target.
+if "rp2" in sys.platform:
+    uart_id = 0
+    tx_pin = "GPIO0"
+    rx_pin = "GPIO1"
+    min_window = 1
+elif "samd" in sys.platform and "ItsyBitsy M0" in sys.implementation._machine:
+    uart_id = 0
+    tx_pin = "D1"
+    rx_pin = "D0"
+    # For SAMD delay_ms has to be used for the trailing window, and the
+    # mininmal time is 11 ms to allow for scheduling.
+    min_window = 11
+elif "samd" in sys.platform and "ItsyBitsy M4" in sys.implementation._machine:
+    uart_id = 3
+    tx_pin = "D1"
+    rx_pin = "D0"
+    min_window = 11
+elif "mimxrt" in sys.platform:
+    uart_id = 1
+    tx_pin = None
+    min_window = 0
+elif "nrf" in sys.platform:
+    uart_id = 0
+    tx_pin = None
+    min_window = 0
+else:
+    print("Please add support for this test on this platform.")
+    raise SystemExit
+
+
+def irq(u):
+    print("IRQ_TXIDLE:", u.irq().flags() == u.IRQ_TXIDLE)
+
+
+text = "Hello World" * 20
+
+# Test that the IRQ is called after the write has completed.
+for bits_per_s in (2400, 9600, 115200):
+    if tx_pin is None:
+        uart = UART(uart_id, bits_per_s)
+    else:
+        uart = UART(uart_id, bits_per_s, tx=tx_pin, rx=rx_pin)
+
+    uart.irq(irq, uart.IRQ_TXIDLE)
+
+    # The IRQ_TXIDLE shall trigger after the message has been sent. Thus
+    # the test marks a time window close to the expected of the sending
+    # and the time at which the IRQ should have been fired.
+    # It is just a rough estimation of 10 characters before and
+    # 20 characters after the data's end, unless there is a need to
+    # wait a minimal time, like for SAMD devices.
+
+    bits_per_char = 10  # 1(startbit) + 8(bits) + 1(stopbit) + 0(parity)
+    start_time_us = (len(text) - 10) * bits_per_char * 1_000_000 // bits_per_s
+    window_ms = max(min_window, 20 * bits_per_char * 1_000 // bits_per_s + 1)
+
+    print("write", bits_per_s)
+    uart.write(text)
+    time.sleep_us(start_time_us)
+    print("ready")
+    time.sleep_ms(window_ms)
+    print("done")

tests/extmod/machine_uart_irq_txidle.py.exp

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+write 2400
+ready
+IRQ_TXIDLE: True
+done
+write 9600
+ready
+IRQ_TXIDLE: True
+done
+write 115200
+ready
+IRQ_TXIDLE: True
+done