Complete Listing

import rp2
from machine import Pin
@rp2.asm_pio(set_init=(rp2.PIO.OUT_LOW,rp2.PIO.OUT_LOW),
         autopush=True, in_shiftdir=rp2.PIO.SHIFT_LEFT)
def dht22():
    wrap_target()
    label("again")
    pull(block)
    set(pins, 0)
    mov(x, osr)
    label("loop1")
    jmp(x_dec, "loop1")
    set(pindirs, 0)
    wait(1, pin, 0)
    wait(0, pin, 0)
    wait(1, pin, 0)
    wait(0, pin, 0)
    set(y, 31)
    label("bits")
    wait(1, pin, 0) [25]
    in_(pins, 1)
    wait(0, pin, 0)
    jmp(y_dec, "bits")
      
    set(y, 7)
    label("check")
    wait(1, pin, 0)[25]
    set(pins,2)
    set(pins,0)
    in_(pins, 1)
    wait(0, pin, 0)
    jmp(y_dec, "check")
    push(block)
    wrap()
class DHT22():
    def __init__(self, gpio):
        self.sm = rp2.StateMachine(0, dht22, freq=490196,
           in_base=Pin(gpio), set_base=Pin(gpio),
                                       jmp_pin=Pin(gpio))
        self.sm.active(1)
    def getReading(self):
        self.sm.put(500)
        data=0
        data = self.sm.get()
        byte1 = (data >> 24 & 0xFF)
        byte2 = (data >> 16 & 0xFF)
        byte3 = (data >> 8 & 0xFF)
        byte4 = (data & 0xFF)
        checksum = self.sm.get() & 0xFF
        self.checksum =
           (checksum == (byte1+byte2+byte3+byte4) & 0xFF)
        self.humidity = ((byte1 << 8) | byte2) / 10.0
        neg = byte3 & 0x80
        byte3 = byte3 & 0x7F
        self.temperature = (byte3 << 8 | byte4) / 10.0
        if neg > 0:
            self.temperature = -self.temperature
dht = DHT22(2)
dht.getReading()
print("Checksum", dht.checksum)
print("Humidity= ", dht.humidity)
print("Temperature=", dht.temperature)

Summary

• The DHT22 is a low-cost temperature and humidity sensor.

• It uses a custom single wire bus which is not compatible with the 1-Wire bus.

• Its asynchronous protocol is easy to implement directly in user space.

• A very simple checksum is used to detect errors.

• It is possible to implement the protocol as defined in the datasheet using a PIO by using counting loops to time each pulse.

• A better use of the PIO is to notice that the protocol can be decoded by testing the state of the line a fixed time after the rising edge.

Programming the Raspberry Pi Pico/W In MicroPython Third Edition

By Harry Fairhead & Mike James

Buy from Amazon.

Contents

• Preface
• Chapter 1 The Raspberry Pi Pico – Before We Begin
• Chapter 2 Getting Started
• Chapter 3 Getting Started With The GPIO
• Chapter 4 Simple Output
• Chapter 5 Some Electronics
• Chapter 6 Simple Input
           Extract: Simple Input ****
• Chapter 7 Advanced Input – Events and Interrupts
• Chapter 8 Pulse Width Modulation
           Extract: PWM  ****
• Chapter 9 Controlling Motors And Servos
           Extract: DC Motors ****
• Chapter 10 Getting Started With The SPI Bus
• Chapter 11 A-To-D and The SPI Bus  ****
• Chapter 12 Using The I2C Bus
• Chapter 13 Using The PIO   
• Chapter 14 The DHT22 Sensor Implementing A Custom Protocol
           Extract: A PIO Driver For The DHT22   ****
• Chapter 15 The 1‑Wire Bus And The DS1820
• Chapter 16 The Serial Port
• Chapter 17 Using The Pico W - WiFi
           Extract: HTTP Client  ****     
• Chapter 18 Sockets
           Extract: Sockets ****
• Chapter 19 Asyncio And Servers
           Extract: Asyncio Client  ***NEW!!!
• Chapter 20 Advanced Hardware
• Chapter 21 Direct To The Hardware
           Extract: Direct To The Hardware ****

Also of interest:

Raspberry Pico File System

**** articles from the second edition not yet updated.

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