/*
* The interrupt handler, enabled after the request pulse, and on
* falling (low to high) transition. This allows lower interrupt
* frequency than on change mode (which would be required for pin
* change interrupts. First pulse is the device response and a
* one(1) bit is encoded as a long pulse (54 + 80 = 134 us), a
* zero(0) bit as a short pulse (54 + 24 = 78 us). Sequence ends
* with a low pulse (54 us) which allows falling/rising detection.
*/
void
DHT::on_interrupt(uint16_t arg)
{
UNUSED(arg);
// Calculate the pulse width and check against thresholds
uint16_t stop = RTT::micros();
uint16_t us = (stop - m_start);
bool valid = false;
// Check the initial response pulse
if (m_state == RESPONSE) {
if (us < BIT_THRESHOLD) goto exception;
m_state = SAMPLING;
m_start = stop;
m_bits = 0;
m_ix = 0;
return;
}
// Sanity check the pulse length
if (us < LOW_THRESHOLD || us > HIGH_THRESHOLD) goto exception;
m_start = stop;
// Sample was valid, collect bit and check for more
m_value = (m_value << 1) + (us > BIT_THRESHOLD);
m_bits += 1;
if (m_bits != CHARBITS) return;
// Next byte ready
m_data.as_byte[m_ix++] = m_value;
m_bits = 0;
valid = (m_ix == DATA_MAX);
if (valid) goto completed;
return;
// Invalid sample reject sequence
exception:
m_ix = 0;
// Sequence completed
completed:
m_state = COMPLETED;
disable();
if (valid) adjust_data();
on_sample_completed(valid);
}
void
DHT::on_event(uint8_t type, uint16_t value)
{
UNUSED(type);
UNUSED(value);
switch (m_state) {
case IDLE:
// Issue a request; pull down for more than 18 ms
m_state = REQUEST;
set_mode(OUTPUT_MODE);
clear();
Watchdog::attach(this, 32);
break;
case REQUEST:
// Request pulse completed; pull up for 40 us and collect
// data as a sequence of on rising mode interrupts
m_state = RESPONSE;
m_start = 0;
detach();
set();
set_mode(INPUT_MODE);
DELAY(40);
enable();
break;
case COMPLETED:
// Data reading was completed; validate data and check sum
// Wait before issueing the next request
uint8_t invalid = 1;
if (m_ix == DATA_MAX) {
uint8_t sum = 0;
for (uint8_t i = 0; i < DATA_LAST; i++)
sum += m_data.as_byte[i];
if (sum == m_data.chksum) {
invalid = 0;
adjust_data();
}
on_sample_completed();
}
m_errors += invalid;
m_state = IDLE;
Watchdog::attach(this, m_period);
break;
}
}
