Initial Release

gpio/dht_sensor/CMakeLists.txt

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+add_executable(dht
+        dht.c
+        )
+
+target_link_libraries(dht pico_stdlib)
+
+pico_add_extra_outputs(dht)
+
+# add url via pico_set_program_url
+example_auto_set_url(dht)
+

gpio/dht_sensor/README.adoc

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+= DHT-11, DHT-22, and AM2302 Sensors
+:xrefstyle: short
+
+The DHT sensors are fairly well known hobbyist sensors for measuring relative humidity and temperature using a capacitive humidity sensor, and a thermistor. While they are slow, one reading every ~2 seconds, they are reliable and good for basic data logging. Communication is based on a custom protocol which uses a single wire for data. 
+
+[NOTE]
+======
+The DHT-11 and DHT-22 sensors are the most common. They use the same protocol but have different characteristics, the DHT-22 has better accuracy, and has a larger sensor range than the DHT-11. The sensor is available from a number of retailers.
+======
+
+== Wiring information
+
+See <<dht-wiring-diagram>> for wiring instructions.
+
+[[dht-wiring-diagram]]
+[pdfwidth=75%]
+.Wiring the DHT-22 temperature sensor to Raspberry Pi Pico, and connecting Pico's UART0 to the Raspberry Pi 4.
+image::pi-and-pico-uart-and-dht-sensor.png[]
+
+NOTE: One of the pins (pin 3) on the DHT sensor will not be connected, it is not used.
+
+You will want to place a 10 kΩ resistor between VCC and the data pin, to act as a medium-strength pull up on the data line. 
+
+Connecting UART0 of Pico to Raspberry Pi as in <<dht-wiring-diagram>> and you should see something similar to <<dht-serial-output-diagram>> in `minicom` when connected to `/dev/serial0` on the Raspberry Pi.
+
+[[dht-serial-output-diagram]]
+[pdfwidth=75%]
+.Serial output over Pico's UART0 in a terminal window.
+image::serial-output.png[]
+
+Connect to `/dev/serial0` by typing,
+
+----
+$ minicom -b 115200 -o -D /dev/serial0
+----
+
+at the command line.
+
+== List of Files
+
+A list of files with descriptions of their function;
+
+CMakeLists.txt:: Make file to incorporate the example in to the examples build tree.
+dht.c:: The example code.
+
+== Bill of Materials
+
+.A list of materials required for the example
+[[dht-22-bom-table]]
+[cols=3]
+|===
+| *Item* | *Quantity* | Details
+| Breadboard | 1 | generic part
+| Raspberry Pi Pico | 1 | http://raspberrypi.org/
+| 10 kΩ resistor | 1 | generic part
+| M/M Jumper wires | 4 | generic part
+| DHT-22 sensor | 1 | generic part
+|===

gpio/dht_sensor/dht.c

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+/**
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ **/
+
+#include <stdio.h>
+#include <math.h>
+#include "pico/stdlib.h"
+#include "hardware/gpio.h"
+
+const uint LED_PIN = PICO_DEFAULT_LED_PIN;
+const uint DHT_PIN = 15;
+const uint MAX_TIMINGS = 85;
+
+typedef struct {
+    float humidity;
+    float temp_celsius;
+} dht_reading;
+
+void read_from_dht(dht_reading *result);
+
+int main() {
+    stdio_init_all();
+    gpio_init(LED_PIN);
+    gpio_init(DHT_PIN);
+    gpio_set_dir(LED_PIN, GPIO_OUT);
+    while (1) {
+        dht_reading reading;
+        read_from_dht(&reading);
+        float fahrenheit = (reading.temp_celsius * 9 / 5) + 32;
+        printf("Humidity = %.1f%%, Temperature = %.1fC (%.1fF)\n",
+               reading.humidity, reading.temp_celsius, fahrenheit);
+
+        sleep_ms(2000);
+    }
+}
+
+void read_from_dht(dht_reading *result) {
+    int data[5] = {0, 0, 0, 0, 0};
+    uint last = 1;
+    uint j = 0;
+
+    gpio_set_dir(DHT_PIN, GPIO_OUT);
+    gpio_put(DHT_PIN, 0);
+    sleep_ms(20);
+    gpio_set_dir(DHT_PIN, GPIO_IN);
+
+    gpio_put(LED_PIN, 1);
+    for (uint i = 0; i < MAX_TIMINGS; i++) {
+        uint count = 0;
+        while (gpio_get(DHT_PIN) == last) {
+            count++;
+            sleep_us(1);
+            if (count == 255) break;
+        }
+        last = gpio_get(DHT_PIN);
+        if (count == 255) break;
+
+        if ((i >= 4) && (i % 2 == 0)) {
+            data[j / 8] <<= 1;
+            if (count > 16) data[j / 8] |= 1;
+            j++;
+        }
+    }
+    gpio_put(LED_PIN, 0);
+
+    if ((j >= 40) && (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF))) {
+        result->humidity = (float) ((data[0] << 8) + data[1]) / 10;
+        if (result->humidity > 100) {
+            result->humidity = data[0];
+        }
+        result->temp_celsius = (float) (((data[2] & 0x7F) << 8) + data[3]) / 10;
+        if (result->temp_celsius > 125) {
+            result->temp_celsius = data[2];
+        }
+        if (data[2] & 0x80) {
+            result->temp_celsius = -result->temp_celsius;
+        }
+    } else {
+        printf("Bad data\n");
+    }
+}