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<h1 class="mt-4 mb-3">ESP32 Project Four
<br>
<small>Read the DHT22 Temperature and Humidity meter</small>
</h1>
<hr>
<h3 class="mb-3">Overview</h3>
<p>
<div class="alert alert-info">The datasheet for the DHT22 can be download <a href="DHT22.pdf">here</a></div>
<p>
The DHT22 handle times that are in order of ten microsecond, but that time granularity cannot be achieved by the RTOS, so a hardware timer must be used.
For more information, please see the <a href="esp-idf/docs/api-reference/peripherals/timer.html" target=”_blank” >Timer API Reference</a>
<br>
The ESP32 has two groups of two hardware timers, and to be able to use them, you need:
<ol>
<li>To include the <code>driver/timer.h</code> header</li>
<li>
Call <code>timer_init</code> routine to initialize the hardware timer. See <a href="esp-idf/docs/api-reference/peripherals/timer.html#_CPPv210timer_init13timer_group_t11timer_idx_tPK14timer_config_t" target=”_blank” >timer_init reference</a> for more details.
<br>
The routine signature is <code>esp_err_t timer_init(timer_group_t group_num, timer_idx_t timer_num, const timer_config_t *config)</code> and the data types associated are
<pre><code>
typedef enum {
TIMER_GROUP_0, /*!<Hw timer group 0*/
TIMER_GROUP_1, /*!<Hw timer group 1*/
TIMER_GROUP_MAX,
} timer_group_t;
typedef enum {
TIMER_0, /*!<Select timer0 of GROUPx*/
TIMER_1, /*!<Select timer1 of GROUPx*/
TIMER_MAX,
} timer_idx_t;
typedef enum {
TIMER_INTR_LEVEL, /*!< Interrupt mode: level mode*/
//TIMER_INTR_EDGE, /*!< Interrupt mode: edge mode, Not supported Now*/
TIMER_INTR_MAX
} timer_intr_mode_t;
typedef enum {
TIMER_COUNT_DOWN, /*!< Descending Count from cnt.high|cnt.low*/
TIMER_COUNT_UP, /*!< Ascending Count from Zero*/
TIMER_COUNT_MAX
} timer_count_dir_t;
typedef struct {
bool alarm_en; /*!< Timer alarm enable */
bool counter_en; /*!< Counter enable */
timer_intr_mode_t intr_type; /*!< Interrupt mode */
timer_count_dir_t counter_dir; /*!< Counter direction */
bool auto_reload; /*!< Timer auto-reload */
uint16_t divider; /*!< Counter clock divider*/
} timer_config_t;</code></pre>
<p>
Note that the clock frequency for the hardware timer is 80MHz, so a divider of 8 or 80 gives you a granularity of 0.1 or 1us, which will be appropriate for these purposes.
</p>
</li>
<li>Call <code>timer_pause</code> to stop the timer. See <a href="esp-idf/docs/api-reference/peripherals/timer.html#_CPPv211timer_pause13timer_group_t11timer_idx_t" target="_blank">timer_pause reference</a> for more information.</li>
<li>Call <code>timer_set_counter_value</code> to reset the timer counter. See <a href="esp-idf/docs/api-reference/peripherals/timer.html#_CPPv223timer_set_counter_value13timer_group_t11timer_idx_t8uint64_t" target="_blank">timer_set_counter_value reference</a> for more information.</li>
<li>Call <code>timer_start</code> to start the time. See <a href="esp-idf/docs/api-reference/peripherals/timer.html#_CPPv211timer_start13timer_group_t11timer_idx_t" target="_blank">timer_start reference</a> for more information.</li>
<li>Call <code>timer_get_counter_value</code> to read timer counter. See <a href="esp-idf/docs/api-reference/peripherals/timer.html#_CPPv223timer_get_counter_value13timer_group_t11timer_idx_tP8uint64_t" target="_blank">timer_get_counter_value reference</a> for more information.</li>
</ol>
</p>
<p>
To send information from the ISR, a message queue can be used. For more information, please see the <a href="freertos/a00018.html" target=”_blank” >FreeRTOS Queue Management Reference</a>.
<br>
To use the Queue Messages you need:
<ol>
<li>To include the <code>freertos/queue.h</code> header</li>
<li>Define a <code>QueueHandle_t</code> object. </li>
<li>
Call <code>xQueueCreate</code> routine to create the queue. See <a href="freertos/a00116.html" target=”_blank” >xQueueCreate reference</a> for more details.
<br>
The routine signature is <code>QueueHandle_t xQueueCreate( UBaseType_t uxQueueLength, UBaseType_t uxItemSize )</code>, where the first parameter is for the expected queue length, and the second one is for the size of each element.
</li>
<li>Call <code>xQueueSendFromISR</code> to queue data form the ISR. See <a href="freertos/a00119.html" target="_blank">xQueueSendFromISR reference</a> for more information.</li>
<li>Call <code>xQueueReset</code> to reset the queue. See <a href="freertos/a00018.html#xQueueReset" target="_blank">xQueueReset reference</a> for more information.</li>
<li>Call <code>xQueueReceive</code> to receive the message from the working task. See <a href="freertos/a00118.html" target="_blank">xQueueReceive reference</a> for more information.</li>
</ol>
</p>
<p>
For this exercise, it is going to be useful what we saw in previous exercises regarding GPIOs managed by interrupts and synchronization with semaphores.
</p>
<p>
In order to read the sensor, you will need to follow these steps.
<ol>
<li>Set the pin as an output</li>
<li>Keep the pin down for at least 500us</li>
<li>Keep the pin up for 20-40us</li>
<li>Set the pin as an input</li>
<li>The sensor will keep the signal down for 80us</li>
<li>The sensor will keep the signal up for 80us</li>
<li>Now you will receive a pulse train of 80 bits where
<ol type="A">
<li>
a zero bit is when you have a signal with:
<ol type="a">
<li>50us down </li>
<li>~30us up</li>
</ol>
</li>
<li>
a one bit is when you have a signal with:
<ol type="a">
<li>50us down </li>
<li>70us up</li>
</ol>
</li>
</ol>
where the first bit received will be the MSB of the first byte.
</li>
</ol>
</p>
<p>
If the received bytes are named b1 b2 b3 b4 and b5, the information received will be:
<ol>
<li>The humidity in tenths of percentage is ( (b1<<8) | b2 ) </li>
<li>The temperature in tenths of celsius degrees is ( ((b3&0x7F)<<8) | b4 ) and the MSB of b3 is the sign</li>
<li>b5 is the checksum given by the sum of the previous bytes</li>
</ol>
</p>
<hr>
<h3 class="mb-3">Step 1<br><small>Read the DHT22 sensor output</small></h3>
<p>
With the above information and the previous exercises as reference, program an example that reads the sensor output.
</p>
<div class="alert alert-info">If you need help, take a look at <a href="http://server:8081/PerformanceIOT/esp-dht22">http://server:8081/PerformanceIOT/esp-dht22</a></div>
<hr>
<div class="alert alert-success">
You should be able to read the sensor outputs. If so, congrats!
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