rxuart: Receive-only 8N1 UART

based on an example in the RP2040 datasheet.

Tested at a blazing 9600 baud!

examples/rxuart.py

@@ -0,0 +1,59 @@
+# SPDX-FileCopyrightText: 2021 Jeff Epler, written for Adafruit Industries
+#
+# SPDX-License-Identifier: MIT
+
+import rp2pio
+import adafruit_pioasm
+
+code = adafruit_pioasm.assemble(
+    """
+.program uart_rx_mini
+
+; Minimum viable 8n1 UART receiver. Wait for the start bit, then sample 8 bits
+; with the correct timing.
+; IN pin 0 is mapped to the GPIO used as UART RX.
+; Autopush must be enabled, with a threshold of 8.
+
+    wait 0 pin 0        ; Wait for start bit
+    set x, 7 [10]       ; Preload bit counter, delay until eye of first data bit
+bitloop:                ; Loop 8 times
+    in pins, 1          ; Sample data
+    jmp x-- bitloop [6] ; Each iteration is 8 cycles
+
+"""
+)
+
+
+class RXUART:
+    def __init__(self, pin, baudrate=9600):
+        self.pio = rp2pio.StateMachine(
+            code,
+            first_in_pin=pin,
+            frequency=8 * baudrate,
+            auto_push=True,
+            push_threshold=8,
+        )
+
+    @property
+    def timeout(self):
+        return 0
+
+    @property
+    def baudrate(self):
+        return self.pio.frequency // 8
+
+    @baudrate.setter
+    def baudrate(self, frequency):
+        self.pio.frequency = freqency * 8
+
+    @property
+    def in_waiting(self):
+        return self.pio.in_waiting
+
+    def read(self, n):
+        b = bytearray(n)
+        n = self.pio.readinto(b)
+        return b[:n]
+
+    def readinto(self, buf):
+        return self.pio.readinto(n)