tests/extmod/machine_uart_tx.py: Add a test for timing of UART.flush().

Currently only runs on rp2 but could be extended to run on other targets.

Signed-off-by: robert-hh <robert@hammelrath.com>

tests/extmod/machine_uart_tx.py

@@ -0,0 +1,34 @@
+# Test machine.UART transmission.
+# Does not require any external connections.
+
+try:
+    from machine import UART
+except ImportError:
+    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"
+else:
+    print("SKIP")
+    raise SystemExit
+
+# Test that write+flush takes the expected amount of time to execute.
+for bits_per_s in (2400, 9600, 115200):
+    text = "Hello World"
+    uart = UART(uart_id, bits_per_s, bits=8, parity=None, stop=1, tx=tx_pin, rx=rx_pin)
+
+    start_us = time.ticks_us()
+    uart.write(text)
+    uart.flush()
+    duration_us = time.ticks_diff(time.ticks_us(), start_us)
+
+    # 1(startbit) + 8(bits) + 1(stopbit) + 0(parity)
+    bits_per_char = 10
+    expect_us = (len(text)) * bits_per_char * 1_000_000 // bits_per_s
+    print(bits_per_s, duration_us <= expect_us)

tests/extmod/machine_uart_tx.py.exp

@@ -0,0 +1,3 @@
+2400 True
+9600 True
+115200 True