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//! An [`embedded-hal`] driver for the world's lousiest humidity/temperature //! sensors, the ubiquitous DHT11 (as seen in every beginners' Arduino kit and //! high-school science fair project ever) and its slightly more expensive //! cousin, the DHT22/AM2302. //! //! [`embedded-hal`]: https://crates.io/crates/embedded-hal #![no_std] use core::marker::PhantomData; use embedded_hal::{blocking::delay, digital::v2 as digital}; pub mod kind; use self::kind::DhtKind; /// A DHT11 sensor. /// /// These things are literally everywhere — you've definitely seen one and /// probably own several. /// /// A DHT11 is a small, blue rectangle 15.5mm x 12mm on its face and 5.5mm wide. /// The DHT11 has a 1Hz sampling rate, meaning it can be read up to once every /// second. It supposedly works with 3.3V to 5V power and IO voltage — sometimes /// they have been known to not work when supplied 3.3v. It needs a 10 KOhm pullup /// resistor across the VCC and DATA pins, which most sellers will stick in the /// bag with it. /// /// The DHT11 works between 20-80% relative humidity with 5% accuracy, and 0-50C /// with +- 2C accuracy. Which is to say...it's not a very good sensor. The /// primary advantage is that it is dirt cheap and about as common, which is why /// they can be found in every beginner's electronics kit. pub type Dht11<P, T> = Dht<P, T, kind::Dht11>; /// A DHT22 (or AM2302, its wired variant) sensor. /// /// This is the "luxury" version of the DHT series — DigiKey sells them for $10, /// which is twice as expensive as the DHT11. The extra $5 gets you relative /// humidity readings from 0-100% with 2-5% accuracy, and temperature readings /// from -40-80C with += 0.5C accuracy. This means that unless you actually /// don't care about measuring things, it's worth _significantly_ more than /// buying two DHT11s. However, it has an 0.5Hz sampling rate, meaning it can /// only be read once every two seconds. This is fine, because the numbers it /// gives you are actually meaningful, unlike the blue piece of garbage. /// /// It has a white housing and is a bit larger than the DHT11, so all your /// friends will instantly be able to tell you're a big spender. It also needs /// 3.3V to 5V. The AM2302 is exactly the same sensor, but with 3 2cm leads /// rather than pins, and a fancy hole in the case so you can screw it onto /// something. Whether or not this is worth another $5 is up to you, but it does /// have the advantage of not having to remember which of the 4 pins these /// things have "goes nowhere and does nothing". /// /// Unlike the DHT11, these often (but not always!) have a 10K pullup resistor /// inside the housing. Unfortunately, there's no real way to tell whether or /// not you're one of the lucky ones, so you should probably add one anyway. /// Welcome to the wonderful world of cheap electronics components from China! pub type Dht22<P, T> = Dht<P, T, kind::Dht22>; /// A generic DHT-series sensor. /// /// Currently, this supports the DHT11 and DHT22/AM2302. #[derive(Debug)] pub struct Dht<P, T, K> { pin: P, timer: T, _kind: PhantomData<K>, } /// A DHT sensor combined temperature and relative humidity reading. #[derive(Debug, Clone)] pub struct Reading<K> { rh_integral: u8, rh_decimal: u8, t_integral: u8, t_decimal: u8, _kind: PhantomData<fn(K)>, } #[derive(Eq, PartialEq, Debug)] pub struct Error<I>(ErrorKind<I>); #[derive(Eq, PartialEq, Debug)] enum ErrorKind<I> { Io(I), Checksum { expected: u8, actual: u8 }, Timeout, } #[derive(Copy, Clone, Debug)] struct Pulse { lo: u8, hi: u8, } impl<P, T, K, E> Dht<P, T, K> where P: digital::InputPin<Error = E> + digital::OutputPin<Error = E>, K: DhtKind, { /// Returns a new DHT sensor. pub fn new(pin: P, timer: T) -> Self { Self { pin, timer, _kind: PhantomData, } } } impl<P, T, K, E> Dht<P, T, K> where P: digital::InputPin<Error = E> + digital::OutputPin<Error = E>, T: delay::DelayUs<u16> + delay::DelayMs<u16>, K: DhtKind, { #[inline(always)] // timing-critical fn read_pulse_us(&mut self, high: bool) -> Result<u8, ErrorKind<E>> { for len in 0..=core::u8::MAX { if self.pin.is_high()? != high { return Ok(len); } self.timer.delay_us(1); } Err(ErrorKind::Timeout) } fn start_signal_blocking(&mut self) -> Result<(), ErrorKind<E>> { // set pin high for 1 ms to pull up. self.pin.set_high()?; self.timer.delay_ms(1); // send start signal self.pin.set_low()?; self.timer.delay_us(K::START_DELAY_US); // end start signal self.pin.set_high()?; self.timer.delay_us(40); // Wait for an ~80ms low pulse, followed by an ~80ms high pulse. self.read_pulse_us(false)?; self.read_pulse_us(true)?; Ok(()) } /// Read from the DHT sensor using blocking delays. /// /// Note that this is timing-critical, and should be run with interrupts disabled. pub fn read_blocking(&mut self) -> Result<Reading<K>, Error<E>> { self.start_signal_blocking().map_err(ErrorKind::from)?; // The sensor will now send us 40 bits of data. For each bit, the sensor // will assert the line low for 50 microseconds as a delimiter, and then // will assert the line high for a variable-length pulse to encode the // bit. If the high pulse is 70 us long, then the bit is 1, and if it is // 28 us, then the bit is a 0. // // Because timing is sloppy, we will read each bit by comparing the // length of the initial low pulse with the length of the following high // pulse. If it was longer than the 50us low pulse, then it's closer to // 70us, and if it was shorter, than it is closer to 28 us. let mut pulses = [Pulse { lo: 0, hi: 0 }; 40]; // Read each bit from the sensor now. We'll convert the raw pulses into // bytes in a subsequent step, to avoid doing that work in the // timing-critical loop. for pulse in &mut pulses[..] { pulse.lo = self.read_pulse_us(false)?; pulse.hi = self.read_pulse_us(true)?; } Ok(Reading::from_pulses(&pulses)?) } } impl<K: DhtKind> Reading<K> { fn from_pulses<E>(pulses: &[Pulse; 40]) -> Result<Self, ErrorKind<E>> { let mut bytes = [0u8; 5]; // The last byte sent by the sensor is a checksum, which should be the // low byte of the 16-bit sum of the first four data bytes. let mut chksum: u16 = 0; for (i, pulses) in pulses.chunks(8).enumerate() { let byte = &mut bytes[i]; // If the high pulse is longer than the leading low pulse, the bit // is a 1, otherwise, it's a 0. for Pulse { lo, hi } in pulses { *byte <<= 1; if hi > lo { *byte |= 1; } } // If this isn't the last byte, then add it to the checksum. if i < 4 { chksum += i as u16; } } // Does the checksum match? let expected = bytes[4]; let actual = chksum as u8; if actual != expected { return Err(ErrorKind::Checksum { actual, expected }); } Ok(Self { rh_integral: bytes[0], rh_decimal: bytes[1], t_integral: bytes[2], t_decimal: bytes[3], _kind: PhantomData, }) } /// Returns the temperature in Celcius. pub fn temp_celcius(self) -> f32 { K::temp_celcius(self.t_integral, self.t_decimal) } /// Returns the temperature in Fahrenheit. pub fn temp_fahrenheit(self) -> f32 { celcius_to_fahrenheit(self.temp_celcius()) } /// Returns the temperature in Fahrenheit. pub fn humidity_percent(self) -> f32 { K::humidity_percent(self.rh_integral, self.rh_decimal) } } impl<E> From<E> for ErrorKind<E> { fn from(e: E) -> Self { ErrorKind::Io(e) } } // === impl Error === impl<E> From<ErrorKind<E>> for Error<E> { fn from(e: ErrorKind<E>) -> Self { Self(e) } } impl<E> Error<E> { /// Returns `true` if a read from the sensor timed out. pub fn is_timeout(&self) -> bool { match self.0 { ErrorKind::Timeout => true, _ => false, } } /// Returns `true` if an IO error occurred while reading from or writing to /// the sensor's data pin. pub fn is_io(&self) -> bool { match self.0 { ErrorKind::Io(_) => true, _ => false, } } /// Returns `true` if the reading from the sensor had a bad checksum. pub fn is_checksum(&self) -> bool { match self.0 { ErrorKind::Checksum { .. } => true, _ => false, } } /// If the error was caused by an underlying pin IO error, returns it. pub fn into_io(self) -> Option<E> { match self.0 { ErrorKind::Io(io) => Some(io), _ => None, } } } fn celcius_to_fahrenheit(c: f32) -> f32 { c * 1.8 + 32.0 }