ATtiny13 – reading temperature and humidity from DHT11

This article shows how to make ATtiny13 to read temperature and humidity from DHT11 sensor and log this information using a Software UART. What you need is connect DHT11 DATA-pin to PB0 and RX-pin of USB to serial adapter to PB3 of ATtiny13. You also need to connect sensor to power supply (3.3V or 5V). Well, with a little code modification, the Attiny13 would print this information on display (i.e. TM1637 or MAX7219). The complete code is on GitHub, here.

Required Parts

• ATtiny13  – i.e. MBAVR-1 (Minimalist Development Board)
• DHT11 sensor
• USB to serial adapter

Software

This code is written in C and can be compiled using the avr-gcc. All information about how to compile this project is here.

/**
 * Copyright (c) 2019, Łukasz Marcin Podkalicki <lpodkalicki@gmail.com>
 * ATtiny13/030
 * Read data from DHT11 sensor and log temperature and humidity
 * using software uart (19200/8N1).
 *
 * Settings:
 *  FUSE_L=0x6A
 *  FUSE_H=0xFF
 *  F_CPU=1200000
 */

#include <avr/io.h>
#include <util/delay.h>
#include "uart.h"

#define DHT_PIN            PB0

#define DHT_ERR_OK         (0)
#define DHT_ERR_TIMEOUT    (-1)

#define DHT_PIN_INPUT()    (DDRB &= ~_BV(DHT_PIN))
#define DHT_PIN_OUTPUT()   (DDRB |= _BV(DHT_PIN))
#define DHT_PIN_LOW()      (PORTB &= ~_BV(DHT_PIN))
#define DHT_PIN_HIGH()     (PORTB |= _BV(DHT_PIN))
#define DHT_PIN_READ()     (PINB & _BV(DHT_PIN))
#define DHT_TIMEOUT        (10)

static void dht_init(void);
int8_t dht_read(uint8_t *temperature, uint8_t *humidity);

int
main(void)
{
    uint8_t temperature, humidity;

    /* setup */
    dht_init();

    /* loop */
    while (1) {
        dht_read(&temperature, &humidity);
        uart_puts("T:");
        uart_putu(temperature);
        uart_puts("st.C, H:");
        uart_putu(humidity);
        uart_puts("%\n");
        _delay_ms(1000);
    }
}

void
dht_init(void)
{

    DHT_PIN_INPUT();
    DHT_PIN_HIGH();
}

static int8_t
dht_await_state(uint8_t state)
{
    uint8_t counter = 0;
    while ((!DHT_PIN_READ() == state) && (++counter < DHT_TIMEOUT)) { _delay_us(1); };
    return counter;
}

int8_t
dht_read(uint8_t *temperature, uint8_t *humidity)
{
    uint8_t i, j, data[5] = {0, 0, 0, 0, 0};

    /* send start sequence */
    DHT_PIN_OUTPUT();
    DHT_PIN_LOW();
    _delay_ms(20);
    DHT_PIN_INPUT();
    DHT_PIN_HIGH();

    /* read response sequence */
    if (dht_await_state(0) < 0 || dht_await_state(1) < 0 || dht_await_state(0) < 0) {
        return DHT_ERR_TIMEOUT;
    }

    /* read data */
    for (i = 0; i < 5; ++i) {
        for (j = 0; j < 8; ++j) {
            data[i] <<= 1;
            data[i] |= !!(dht_await_state(1) > 0 && dht_await_state(0) > 1);
        }
    }

    *temperature = data[2];
    *humidity = data[0];

    return DHT_ERR_OK;
}