Andy Pomfret
Simple library for the DHT11 temperature and humidity sensor. Forked from an existing Mbed DHT11 project.
Simple DHT11 temperature and humidity library.
Example usage
#include "mbed.h"
#include "DHT11.h"
DHT11 dht(D8); // Change pin name here if required
main()
{
printf("T:%d, H:%d\r\n", dht.readTemperature(), dht.readHumidity());
}
The sensor may be read as often as desired, but temperature and humidity values are cached and will only be updated if they are more than 2 seconds old. This is the underlying sensor update rate.
Please note that this project has been modified only enough to make it work for its intended purpose. Various parts of this project still need work, and the source code should not be seen as an example of best practice.
Diff: DHT11.cpp
- Revision:
- 0:4d4c5ea17d86
- Child:
- 1:95b80cc3f676
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/DHT11.cpp Wed Sep 10 15:14:31 2014 +0000
@@ -0,0 +1,186 @@
+/** @file
+ Library for the use of the DHT11, a temperature and humidity sensor
+ Shigenori Inoue, September 10, 2014
+ */
+
+#include "DHT11.h"
+
+// Constructor
+DHT11::DHT11(PinName pin) : io(pin, PIN_INPUT, OpenDrain, 1), io_irq(pin)
+{
+ io_irq.rise(this, &DHT11::pos_edge);
+ io_irq.fall(this, &DHT11::neg_edge);
+ io_irq.disable_irq();
+ init();
+}
+
+// Destructor
+DHT11::~DHT11(void) {}
+
+// Reading the data bits from DHT11
+int DHT11::readData()
+{
+ // Initialize
+ eod = false;
+ err = OK;
+ data = 0;
+ cnt = 0;
+ wdt = 0;
+
+ // Checking the measurement frequency
+ if (t.read_ms() < 2000 & first_time == false) {
+ t.reset();
+ return TOO_FAST_READ;
+ }
+
+ // Checking the data bus
+ if (io == 0) {
+ io.input();
+ return BUS_BUSY;
+ }
+
+ // Sending start signal
+ io.output();
+ t.reset();
+ t.start();
+ do {
+ io = 0;
+ } while (t.read_ms() < 18);
+ io.input();
+ io = 1;
+
+ // Waiting for the start of the response signal
+ t.reset();
+ t.start();
+ do {
+ if (t.read_us() > 100) {
+ io.input();
+ io = 1;
+ return NOT_PRESENT;
+ }
+ } while (io == 1);
+
+ // Wainting for the start of the ready signal
+ t.reset();
+ t.start();
+ do {
+ if (t.read_us() > 100) {
+ io.input();
+ io = 1;
+ return NOT_READY;
+ }
+ } while (io == 0);
+
+ // Wainting for the end of the ready signal
+ t.reset();
+ t.start();
+ do {} while (io == 1);
+
+ // Starting the pulse width sensing
+
+ io_irq.enable_irq();
+
+ do {
+ wait_us(100);
+ if (wdt > 50) {
+ return WATCHDOG_ERR;
+ }
+ wdt++;
+ } while (eod == false);
+
+ // Calculating the check sum
+ chksum = ((data & 0xff00000000) >> 32)
+ + ((data & 0x00ff000000) >> 24)
+ + ((data & 0x0000ff0000) >> 16)
+ + ((data & 0x000000ff00) >> 8);
+ if (chksum != (data & 0x00000000ff)) {
+ io.input();
+ io = 1;
+ return CHKSUM_ERR;
+ } else {
+ t.reset();
+ first_time = false;
+ return OK;
+ }
+}
+
+// Extracting humidity data from the received data
+int DHT11::readHumidity()
+{
+ if (err == OK) {
+ return (data & 0xff00000000) >> 32;
+ } else {
+ return 0xffffffff;
+ }
+}
+
+// Extracting temperature data from the received data
+int DHT11::readTemperature()
+{
+ if (err == OK) {
+ return (data & 0x0000ff0000) >> 16;
+ } else {
+ return 0xffffffff;
+ }
+}
+
+// Initialization of variables
+void DHT11::init(void)
+{
+ t_pulse_us = 0;
+ first_time = true;
+ data = 0;
+ chksum = 0;
+ cnt = 0;
+ wdt = 0;
+ err = OK;
+ eod = false;
+}
+
+void DHT11::pos_edge(void)
+{
+ // Disabling the interruptions
+ io_irq.disable_irq();
+
+ // Initializing the Timer
+ t.reset();
+
+ // Enabling the interruptions
+ io_irq.enable_irq();
+}
+
+void DHT11::neg_edge(void)
+{
+ // Disabling the interruptions
+ io_irq.disable_irq();
+
+ // Reading the length of
+ t_pulse_us = t.read_us();
+
+ // Detecting 0 if the pulse width ranges from 26 us to 28 us
+ if (20 <= t_pulse_us && t_pulse_us <= 30) {
+ // Shifting the data buffer and not adding 1 (because this bit is zero)
+ data = data << 1;
+
+ // Counting up the bits
+ cnt++;
+ }
+
+ // Detecting 1 if the pulse width ranges from 68 us to 72 us
+ else if (60 <= t_pulse_us && t_pulse_us <= 80) {
+ // Shifting the data buffer and adding 1 (because this bit is one)
+ data = data << 1;
+ data++;
+
+ // Counting up the bits
+ cnt++;
+ }
+
+ // Detecting the end of Data
+ if (cnt < 40) {
+ // Enabling the interruptions
+ io_irq.enable_irq();
+ } else {
+ eod = true;
+ }
+}
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