examples/rp2: Add pio_uart_rx.py example.

This was adapted from the `pio/uart_rx` example from the `pico-examples`
repository:
https://github.com/raspberrypi/pico-examples/blob/master/pio/uart_rx/uart_rx.pio

It demonstrates the `jmp_pin` feature in action.
Signed-off-by: Tim Radvan <tim@tjvr.org>

examples/rp2/pio_uart_rx.py

+# Example using PIO to create a UART RX interface.
+#
+# To make it work you'll need a wire connecting GPIO4 and GPIO3.
+#
+# Demonstrates:
+#   - PIO shifting in data on a pin
+#   - PIO jmp(pin) instruction
+#   - PIO irq handler
+#   - using the second core via _thread
+
+import _thread
+from machine import Pin, UART
+from rp2 import PIO, StateMachine, asm_pio
+
+UART_BAUD = 9600
+
+HARD_UART_TX_PIN = Pin(4, Pin.OUT)
+PIO_RX_PIN = Pin(3, Pin.IN, Pin.PULL_UP)
+
+
+@asm_pio(
+    autopush=True,
+    push_thresh=8,
+    in_shiftdir=rp2.PIO.SHIFT_RIGHT,
+)
+def uart_rx_mini():
+    # fmt: off
+    # Wait for start bit
+    wait(0, pin, 0)
+    # Preload bit counter, delay until eye of first data bit
+    set(x, 7)                 [10]
+    # Loop 8 times
+    label("bitloop")
+    # Sample data
+    in_(pins, 1)
+    # Each iteration is 8 cycles
+    jmp(x_dec, "bitloop")     [6]
+    # fmt: on
+
+
+@asm_pio(
+    in_shiftdir=rp2.PIO.SHIFT_RIGHT,
+)
+def uart_rx():
+    # fmt: off
+    label("start")
+    # Stall until start bit is asserted
+    wait(0, pin, 0)
+    # Preload bit counter, then delay until halfway through
+    # the first data bit (12 cycles incl wait, set).
+    set(x, 7)                 [10]
+    label("bitloop")
+    # Shift data bit into ISR
+    in_(pins, 1)
+    # Loop 8 times, each loop iteration is 8 cycles
+    jmp(x_dec, "bitloop")     [6]
+    # Check stop bit (should be high)
+    jmp(pin, "good_stop")
+    # Either a framing error or a break. Set a sticky flag
+    # and wait for line to return to idle state.
+    irq(block, 4)
+    wait(1, pin, 0)
+    # Don't push data if we didn't see good framing.
+    jmp("start")
+    # No delay before returning to start; a little slack is
+    # important in case the TX clock is slightly too fast.
+    label("good_stop")
+    push(block)
+    # fmt: on
+
+
+# The handler for a UART break detected by the PIO.
+def handler(sm):
+    print("break", time.ticks_ms(), end=" ")
+
+
+# Function for core1 to execute to write to the given UART.
+def core1_task(uart, text):
+    uart.write(text)
+
+
+# Set up the hard UART we're going to use to print characters.
+uart = UART(1, UART_BAUD, tx=HARD_UART_TX_PIN)
+
+for pio_prog in ("uart_rx_mini", "uart_rx"):
+    # Set up the state machine we're going to use to receive the characters.
+    sm = StateMachine(
+        0,
+        globals()[pio_prog],
+        freq=8 * UART_BAUD,
+        in_base=PIO_RX_PIN,  # For WAIT, IN
+        jmp_pin=PIO_RX_PIN,  # For JMP
+    )
+    sm.irq(handler)
+    sm.active(1)
+
+    # Tell core 1 to print some text to UART 1
+    text = "Hello, world from PIO, using {}!".format(pio_prog)
+    _thread.start_new_thread(core1_task, (uart, text))
+
+    # Echo characters received from PIO to the console.
+    for i in range(len(text)):
+        print(chr(sm.get() >> 24), end="")
+    print()