AM2315  0.1.4
AM2315 Documentation

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AM2315

Arduino library for an AM2315 I2C temperature and humidity sensor.

Description

AM2315 is a sensor similar to the DHT12 with an I2C interface. Although in theory this could enable multiple sensors on one bus the AM2315 has a fixed address 0x5C so one need to implement a multiplexing strategy to have multiple sensors in practice. See multiplexing below.

The AM2315 can also be read with the https://github.com/RobTillaart/AM232X library as it uses the same protocol. The AM232X library allows to read some internal registers.

Typical parameters

range accuracy repeatability
Temperature -40 - 125 0.5C 0.2
Humidity 0.0 - 99.9 2% 0.1
Sample time 2 seconds

Hardware connection

// AM232X PIN layout AM2315 COLOR
// ============================================
// bottom view DESCRIPTION COLOR
// +---+
// |o | VDD RED
// |o | SDA YELLOW
// |o | GND BLACK
// |o | SCL GREY
// +---+
//
// do not forget pull up resistors between SDA, SCL and VDD.

I2C clock speed

The datasheet states the AM2315 should be used on 100 KHz I2C only. When overclocking I got good readings up to 190 KHz in a test with

  • Arduino UNO
  • very short wires (< 1 meter)
  • not using pull ups.
  • version 0.1.1 of this library
I2C clock timing us Notes
50 KHz 4570 under-clocking works (e.g. long wires)
100 KHz 3276 specs default, robust

| 150 KHz | 2836 | | 160 KHz | 2792 | | 170 KHz | 2750 | 0.5 ms off, interesting for performance. | 180 KHz | 2700 | near critical. DO NOT USE. | 190 KHz | 2672 | near critical. DO NOT USE. | 200 KHz | crash | sensor needs a power cycle reboot. DO NOT USE.

If robustness is mandatory stick to the default of 100 KHz. If performance is mandatory do not go beyond 170 KHz.

Interface

Constructor

  • AM2315(TwoWire *wire = &Wire) constructor, default using Wire (I2C bus), optionally set to Wire0 .. WireN.
  • bool begin(uint8_t dataPin, uint8_t clockPin) begin for ESP32 et al, to set I2C bus pins. Returns true if device address 0x5C is connected.
  • bool begin() initializer for non ESP32 e.g. AVR. Returns true if device address 0x5C is connected.
  • bool isConnected(uint16_t timeout = 3000) returns true if the device address 0x5C is found on I2C bus. As the device can be in sleep modus it will retry for the defined timeout (in micros) with a minimum of 1 try. minimum = 800 us and maximum = 3000 us according to datasheet.

Core

  • int8_t read() read the sensor and store the values internally. It returns the status of the read which should be AM2315_OK == 0.
  • float getHumidity() returns last read humidity + optional offset, or AM2315_INVALID_VALUE == -999 in case of error. This error can be suppressed, see below.
  • float getTemperature() returns last read temperature + optional offset, or AM2315_INVALID_VALUE == -999 in case of error. This error can be suppressed, see below.
  • uint32_t lastRead() returns the timestamp in milliseconds since startup of the last successful read.

Offset

  • void setHumOffset(float offset = 0) set an offset for humidity to calibrate (1st order) the sensor. Default offset = 0, so no parameter will reset the offset.
  • float getHumOffset() return current humidity offset, default 0.
  • void setTempOffset(float offset = 0) set an offset for temperature to calibrate (1st order) the sensor. Default offset = 0, so no parameter will reset the offset.
  • float getTempOffset() return current temperature offset, default 0.

Control

Functions to adjust the communication with the sensor.

  • void setWaitForReading(bool b ) flag to enforce a blocking wait (up to 2 seconds) when read() is called.
  • bool getWaitForReading() returns the above setting.
  • bool wakeUp() function that will try for 3 milliseconds to wake up the sensor. This can be done before an actual read to minimize the read() call.
  • void setSuppressError(bool b) suppress error values of AM2315_INVALID_VALUE == -999 => you need to check the return value of read() instead.

This can be used to keep spikes out of your graphs / logs.

  • bool getSuppressError() returns the above setting.

Error codes

| name | value | notes | |:-------------------------------—|---—:|:---------—| | AM2315_OK | 0 | | AM2315_ERROR_CHECKSUM | -10 | I2C problem. | AM2315_ERROR_CONNECT | -11 | I2C problem. | AM2315_MISSING_BYTES | -12 | I2C problem. | AM2315_WAITING_FOR_READ | -50 | called read() too fast, within 2 seconds. | AM2315_HUMIDITY_OUT_OF_RANGE | -100 | not used by default. | AM2315_TEMPERATURE_OUT_OF_RANGE | -101 | not used by default. | AM2315_INVALID_VALUE | -999 | can be suppressed.

Operation

See examples

In setup() you have to call the begin() to initialize the Wire library and do an initial read() to fill the variables temperature and humidity. To access these values one must use getTemperature() and getHumidity(). Multiple calls will give the same values until read() is called again.

Note that the sensor can go into sleep mode after 3 seconds after last read, so one might need to call wakeUp() before the read().

Multiplexing

Multiplexing the AM232X can be done in several ways. This is not a complete list or tutorial but should get you started.

  1. Control the power line by means of an extra pin (+ transistor). Only switch on the sensor you want to use. Drawback might be time the sensor takes to boot and to be ready for the first measurement.
  2. Use an AND gate between the I2C SCL (clock) line and the I2C SCL pin of the sensors. This way one can enable / disable communication per sensor. This will still need an IO pin per sensor but does not have the "boot time" constraint mentioned above. you may use a PCF8574 to control the AND gates. https://github.com/RobTillaart/PCF8574
  3. Use a TCA9548A I2C Multiplexer, or similar. https://github.com/RobTillaart/TCA9548

Which method fit your application depends on your requirements and constraints.

Future

  • update documentation
  • test more (other platforms)
  • keep in sync with AM2315 class
    • merge in a far future.
  • update unit test
  • add examples

Won't

  • add calls for meta information (no description yet)
    • 0x07 status register
    • 0x08-0x0B user register HIGH LOW HIGH2 LOW2 (use AM232x library to access those)