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The Complete Listing

That's all we need to do and the final program, complete with some minor tidying up can be seen below:

#include <bcm2835.h>
#include <stdio.h>
#include <sched.h>
#include <sys/mman.h>

uint8_t getByte(int b, int buf[]);
void GetDHT22data(uint8_t pin);

int main(int argc, char** argv) {
    const struct sched_param priority = {1};
    sched_setscheduler(0, SCHED_FIFO, &priority);
    mlockall(MCL_CURRENT | MCL_FUTURE);
    if (!bcm2835_init())
        return 1;
    bcm2835_gpio_fsel(RPI_GPIO_P1_07, BCM2835_GPIO_FSEL_INPT);

   return 0;

void GetDHT22data(uint8_t pin) {
    bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_OUTP);
    bcm2835_gpio_write(pin, LOW);
    bcm2835_gpio_fsel(pin, BCM2835_GPIO_FSEL_INPT);
    int i;
    for (i = 1; i < 2000; i++) {
        if (bcm2835_gpio_lev(pin) == 0)break;
    uint64_t t;
    int buf[41];
    int j;
    for (j = 0; j < 41; j++) {
        for (i = 1; i < 2000; i++) {
            if (bcm2835_gpio_lev(pin) == 1)break;
        t = bcm2835_st_read();
        for (i = 1; i < 2000; i++) {
            if (bcm2835_gpio_lev(pin) == 0) break;
        buf[j] = (bcm2835_st_read() - t) > 50;

    int byte1 = getByte(1, buf);
    int byte2 = getByte(2, buf);
    int byte3 = getByte(3, buf);
    int byte4 = getByte(4, buf);
    int byte5 = getByte(5, buf);

    printf("Checksum %d %d \n\r", byte5,
            (byte1 + byte2 + byte3 + byte4) & 0xFF);
    float humidity = (float) (byte1 << 8 | byte2) / 10.0;
    printf("Humidity= %f \n\r", humidity);
    float temperature;
    int neg = byte3 & 0x80;
    byte3 = byte3 & 0x7F;
    temperature = (float) (byte3 << 8 | byte4) / 10.0;
    if (neg > 0)temperature = -temperature;
    printf("Temperature= %f \n\r", temperature);

uint8_t getByte(int b, int buf[]) {
    int i;
    uint8_t result = 0;
    b = (b - 1)*8 + 1;
    for (i = b; i <= b + 7; i++) {
        result = result << 1;
        result = result | buf[i];
    return result;

Where Next

The DHT11/22 is a useful device but when it comes to low cost temperature measurement the DS18D20 is a very popular choice and it is more accurate than the DHT11/22. It is also an example of a standard 1-wire device which gives us access to a range of sensors that work in the same way. This is the subject of later chapters.

What is of importance next is getting the Pi on the IoT - after all it is the "Internet" of Things. So how do we connect our c programs with the outside world? The good news is that in most cases you don't need a web server, it is much easier than this. 




Now On Sale!

You can now buy a print or ebook edition of Raspberry Pi IoT in C from Amazon.


For Errata and Listings Visit: IO Press



This our ebook on using the Raspberry Pi to implement IoT devices using the C programming language. The full contents can be seen below. Notice this is a first draft and a work in progress. 

Chapter List

  1. Introducing Pi (paper book only)

  2. Getting Started With NetBeans In this chapter we look at why C is a good language to work in when you are creating programs for the IoT and how to get started using NetBeans. Of course this is where Hello C World makes an appearance.

  3. First Steps With The GPIO
    The bcm2835C library is the easiest way to get in touch with the Pi's GPIO lines. In this chapter we take a look at the basic operations involved in using the GPIO lines with an emphasis on output. How fast can you change a GPIO line, how do you generate pulses of a given duration and how can you change multiple lines in sync with each other? 

  4. GPIO The SYSFS Way
    There is a Linux-based approach to working with GPIO lines and serial buses that is worth knowing about because it provides an alternative to using the bcm2835 library. Sometimes you need this because you are working in a language for which direct access to memory isn't available. It is also the only way to make interrupts available in a C program.

  5. Input and Interrupts
    There is no doubt that input is more difficult than output. When you need to drive a line high or low you are in command of when it happens but input is in the hands of the outside world. If your program isn't ready to read the input or if it reads it at the wrong time then things just don't work. What is worse is that you have no idea what your program was doing relative to the event you are trying to capture - welcome to the world of input.

  6. Memory Mapped I/O
    The bcm2835 library uses direct memory access to the GPIO and other peripherals. In this chapter we look at how this works. You don't need to know this but if you need to modify the library or access features that the library doesn't expose this is the way to go. 

  7. Near Realtime Linux
    You can write real time programs using standard Linux as long as you know how to control scheduling. In fact it turns out to be relatively easy and it enables the Raspberry Pi to do things you might not think it capable of. There are also some surprising differences between the one and quad core Pis that make you think again about real time Linux programming.

  8. PWM
    One way around the problem of getting a fast response from a microcontroller is to move the problem away from the processor. In the case of the Pi's processor there are some builtin devices that can use GPIO lines to implement protocols without the CPU being involved. In this chapter we take a close look at pulse width modulation PWM including, sound, driving LEDs and servos.

  9. I2C Temperature Measurement
    The I2C bus is one of the most useful ways of connecting moderately sophisticated sensors and peripherals to the any processor. The only problem is that it can seem like a nightmare confusion of hardware, low level interaction and high level software. There are few general introductions to the subject because at first sight every I2C device is different, but here we present one.

  10. A Custom Protocol - The DHT11/22
    In this chapter we make use of all of the ideas introduced in earlier chapters to create a raw interface with the low cost DHT11/22 temperature and humidity sensor. It is an exercise in implementing a custom protocol directly in C. 

  11. One Wire Bus Basics
    The Raspberry Pi is fast enough to be used to directly interface to 1-Wire bus without the need for drivers. The advantages of programming our own 1-wire bus protocol is that it doesn't depend on the uncertainties of a Linux driver.

  12. iButtons
    If you haven't discovered iButtons then you are going to find of lots of uses for them. At its simples an iButton is an electronic key providing a unique coce stored in its ROM which can be used to unlock or simply record the presence of a particular button. What is good news is that they are easy to interface to a Pi. 

  13. The DS18B20
    Using the software developed in previous chapters we show how to connect and use the very popular DS18B20 temperature sensor without the need for external drivers. 

  14. The Multidrop 1-wire bus
    Some times it it just easier from the point of view of hardware to connect a set of 1-wire devices to the same GPIO line but this makes the software more complex. Find out how to discover what devices are present on a multi-drop bus and how to select the one you want to work with.

  15. SPI Bus
    The SPI bus can be something of a problem because it doesn't have a well defined standard that every device conforms to. Even so if you only want to work with one specific device it is usually easy to find a configuration that works - as long as you understand what the possibilities are. 

  16. SPI MCP3008/4 AtoD  (paper book only)

  17. Serial (paper book only)

  18. Getting On The Web - After All It Is The IoT (paper book only)

  19. WiFi (paper book only)



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