add a quadrature encoder pio example (#126)

Co-authored-by: Paulo Marques <pm@quant-insight.com>

pio/quadrature_encoder/quadrature_encoder.c

@@ -0,0 +1,59 @@
+#include <stdio.h>
+#include "pico/stdlib.h"
+#include "hardware/pio.h"
+#include "hardware/timer.h"
+
+#include "quadrature_encoder.pio.h"
+
+//
+// ---- quadrature encoder interface example
+//
+// the PIO program reads phase A/B of a quadrature encoder and increments or
+// decrements an internal counter to keep the current absolute step count
+// updated. At any point, the main code can query the current count by using
+// the quadrature_encoder_*_count functions. The counter is kept in a full
+// 32 bit register that just wraps around. Two's complement arithmetic means
+// that it can be interpreted as a 32 bit signed or unsigned value and it will
+// work anyway.
+//
+// As an example, a two wheel robot being controlled at 100Hz, can use two
+// state machines to read the two encoders and in the main control loop it can
+// simply ask for the current encoder counts to get the absolute step count. It
+// can also subtract the values from the last sample to check how many steps
+// each wheel as done since the last sample period.
+//
+// One advantage of this approach is that it requires zero CPU time to keep the
+// encoder count updated and because of that it supports very high step rates.
+//
+
+
+int main()
+{
+	int new_value, delta, old_value = 0;
+
+	// Base pin to connect the A phase of the encoder.
+	// The B phase must be connected to the next pin
+	const uint PIN_AB = 10;
+
+	stdio_init_all();
+
+	PIO pio = pio0;
+	const uint sm = 0;
+
+	uint offset = pio_add_program(pio, &quadrature_encoder_program);
+	quadrature_encoder_program_init(pio, sm, offset, PIN_AB, 0);
+
+	while (1) {
+		// note: thanks to two's complement arithmetic delta will always
+		// be correct even when new_value wraps around MAXINT / MININT
+		new_value = quadrature_encoder_get_count(pio, sm);
+		delta = new_value - old_value;
+		old_value = new_value;
+
+		printf("position %8d, delta %6d\n", new_value, delta);
+		sleep_ms(100);
+	}
+
+	return 0;
+}
+