Add Example for reading a PWM signal with PIO

pio/pio_pwmin.py

@@ -0,0 +1,98 @@
+from machine import Pin, PWM
+from rp2 import PIO, StateMachine, asm_pio
+from time import sleep, ticks_ms, ticks_diff
+
+pwm_out = PWM(Pin(16))
+
+pwm_out.freq(100)
+pwm_out.duty_u16((2**16-1)//2)
+
+@asm_pio()
+def pwmin():
+    pull(block)           # wait for activation
+
+    set(x, 0)               # Set x = 0
+    mov(x, x | (0b01 << 3)) # invert x = Max-Value for 32 bits. (0b01 << 3) sets the invert bit.
+
+    wait(1, pin, 0)         # wait for a full PWM cycle to start measurement
+    wait(0, pin, 0)         # wait for pin to be low
+
+    label("count_low")          
+    jmp(pin, "out_low")         # jump to output if pin is high
+    jmp(x_dec, "count_low")     # jump back to count loop, decrement X
+    label("out_low")
+
+    mov(isr, x)             # move x into ISR
+    push(noblock)             # push into fifo
+
+    label("count_high")
+    jmp(x_dec, "next")      # count down X, jump to next instruction
+    label("next")
+    jmp(pin, "count_high")  # as long as the pin is high, jump back up to continue countdown
+
+    mov(isr, x)             # move x into ISR
+    push(noblock)           # push into fifo
+    irq(0)
+
+base_frq = 100_000_000
+sm = rp2.StateMachine(0, pwmin, freq=base_frq, jmp_pin=Pin(16), in_base=Pin(16))
+sm.active(1)
+
+'''
+W A R N I N G
+
+This example code will hang, if no PWM signal is present,
+e.g. when the PWM is at 0% or 100% duty cycle.
+
+'''
+def readPwm(sm):
+    # Send data to start measurement
+    sm.put(0)
+
+    low = sm.get()
+    total = sm.get()
+
+    # Convert to duration
+    low = 2**32 - 1 - low
+    total = 2**32 - 1 - total
+
+    # Total is in ticks, based on base_frq.
+    # Due to the code, it counts by 1 for every 2 clock cycles
+    period = total / base_frq * 2
+
+    return {
+        "period":period,
+        "duty_low":low/total,
+        "duty":1.0-(low/total),
+        "freq":1/period
+        }
+
+print("PWMIn Selfcheck")
+
+for f in [100, 200, 500,
+          1_000, 2_000, 5_000,
+          10_000, 20_000, 50000,
+          100_000, 200_000, 500_000]:
+    for d in [0.1, 0.25, 0.5, 0.75, 0.9]:
+        # Set new output
+        pwm_out.freq(f)
+        pwm_out.duty_u16(int((2**16-1)*d))
+
+        # Wait a bit
+        sleep(0.5)
+
+        read = readPwm(sm)
+        diff_freq = abs(read["freq"]-f)
+        diff_duty = abs(read["duty"]-d)
+
+        if (diff_freq <= f*0.01) and (diff_duty < 0.01):
+            print("{} Hz / {} duty OK".format(f, d))
+        else:
+            print("{} Hz / {} duty OUTSIDE LIMITS ---------".format(f, d))
+
+        print("\t{:.2f} Hz / {:.2f} duty cycle measured".format(read["freq"], read["duty"]))
+
+        print("\tDiff: {:.2f} Hz".format(diff_freq))
+        print("\tDiff: {:.2%} of freq".format(abs(1.0-read["freq"]/f)))
+        print("\tDiff: {:.2%} duty cycle".format(diff_duty))
+        print("")