GY-63_MS5611/libraries/HX711

HX711

Arduino library for HX711 24 bit ADC used for load cells and scales.

Description

This HX711 library has an interface which is a superset of a library by Bogde. Some missing functions were added to get more info from the library.

Another important difference is that this library uses floats. The 23 bits mantissa of the IEEE754 float matches the 24 bit ADC very well. Furthermore using floats gave a smaller footprint on the Arduino UNO.

Breaking change 0.3.0

In issue #11 it became clear that the timing of the default shiftIn() function to read the value of the internal ADC was too fast on some processor boards for the HX711. This resulted in missing the first (= sign) bit or the value read could be a factor two higher than it should. If one calibrated the sensor this would be compensated with the factor that is derived in the calibration process.

In 0.3.0 a dedicated shiftIn() function is added into this library that uses hard coded delayMicroseconds to keep the timing of the clock within HX711 datasheet parameters. This should guarantee that the sign bit is always read correctly on all platforms. Drawback is that reading the HX711 takes an extra 50-55 microseconds. How much this affects performance is to be investigated.

Main flow

First action is to call begin(dataPin, clockPin) to make connection to the HX711.

Second step is calibration for which a number of functions exist.

• tare() measures zero point
• set_scale(factor) set a known conversion factor e.g. from EEPROM.
• calibrate_scale(WEIGHT, TIMES) determines the scale factor based upon a known weight e.g. 1 Kg.

Steps to take for calibration

1. clear the scale
2. call tare() to set the zero offset
3. put a known weight on the scale
4. call calibrate_scale(weight)
5. scale is calculated.
6. save the offset and scale for later use e.g. EEPROM.

Interface

Base

• HX711() constructor.
• ~HX711()
• void begin(uint8_t dataPin, uint8_t clockPin) sets a fixed gain 128 for now.
• void reset() set internal state to start condition. Since 0.3.4 reset also does a power down / up cycle.
• bool is_ready() checks if load cell is ready to read.
• void wait_ready(uint32_t ms = 0) wait until ready, check every ms.
• bool wait_ready_retry(uint8_t retries = 3, uint32_t ms = 0) wait max retries.
• bool wait_ready_timeout(uint32_t timeout = 1000, uint32_t ms = 0) wait max timeout milliseconds.
• float read() raw read.
• float read_average(uint8_t times = 10) get average of times raw reads. times = 1 or more.
• float read_median(uint8_t times = 7) get median of multiple raw reads. times = 3..15 - odd numbers preferred.
• float read_medavg(uint8_t times = 7) get average of "middle half" of multiple raw reads. times = 3..15 - odd numbers preferred.
• float read_runavg(uint8_t times = 7, float alpha = 0.5) get running average over times measurements. The weight alpha can be set to any value between 0 and 1, times >= 1.
• uint32_t last_read() returns timestamp in milliseconds.

Gain + channel

Use with care as it is not 100% reliable - see issue #27. (solutions welcome).

Read datasheet before use.

Constants (see .h file)

• HX711_CHANNEL_A_GAIN_128 = 128 This is the default in the constructor.
• HX711_CHANNEL_A_GAIN_64 = 64
• HX711_CHANNEL_B_GAIN_32 = 32 Note fixed gain for channel B.

The selection of channels + gain is in theory straightforward.

• bool set_gain(uint8_t gain = 128, bool forced = false) values: 128 (default), 64 or 32. If one uses an invalid value for the parameter gain, the channel and gain are not changed. If forced == false it will not set the new gain if the library "thinks" it already has the right value. If forced == true, it will explicitly try to set the gain/channel again. This includes a dummy read() so the next "user" read() will give the right info.
• uint8_t get_gain() returns set gain (128, 64 or 32).

By setting the gain to one of the three constants the gain and the channel is selected. The set_gain() does a dummy read if gain has changed (or forced == true) so the next call to read() will return info from the selected channel/gain.

According to the datasheet the gain/channel change may take up to 400ms (table page 3).

Warning 1: if you use set_gain() in your program the HX711 can be in different states. If there is a expected or unexpected reboot of the MCU, this could lead to an unknown state at the reboot of the code. So in such case it is strongly advised to call set_gain() explicitly in setup() so the device is in a known state.

Warning 2: In practice it seems harder to get the channel and gain selection as reliable as the datasheet states it should be. So use with care. (feedback welcome) See discussion #27.

Mode

Get and set the operational mode for get_value() and indirect get_units().

Constants (see .h file)

• HX711_RAW_MODE new in 0.3.3
• HX711_AVERAGE_MODE
• HX711_MEDIAN_MODE
• HX711_MEDAVG_MODE
• HX711_RUNAVG_MODE

In HX711_MEDIAN_MODE and HX711_MEDAVG_MODE mode only 3..15 samples are allowed to keep memory footprint relative low.

• void set_raw_mode() - will cause read() to be called only once!
• void set_average_mode() take the average of n measurements.
• void set_median_mode() take the median of n measurements.
• void set_medavg_mode() take the average of n/2 median measurements.
• void set_runavg_mode() default alpha = 0.5.
• uint8_t get_mode() returns current set mode. Default is HX711_AVERAGE_MODE.

Get values

Get values from the HX711 corrected for offset and scale. Note that in HX711_RAW_MODE times will be ignored => just call read() once.

• float get_value(uint8_t times = 1) read value, corrected for offset.
• float get_units(uint8_t times = 1) read value, converted to proper units.
• void set_scale(float scale = 1.0) set scale factor; scale > 0.
• float get_scale() returns set scale factor.
• void set_offset(long offset = 0) idem.
• long get_offset() idem.

Tare & calibration

Steps to take for calibration

1. clear the scale
2. call tare() to set the zero offset
3. put a known weight on the scale
4. call calibrate_scale(weight)
5. scale is calculated.
6. save the offset and scale for later use e.g. EEPROM.

• void tare(uint8_t times = 10) call tare to calibrate zero level
• float get_tare() idem.
• bool tare_set() checks if a tare has been set.
• void calibrate_scale(uint16_t weight, uint8_t times = 10) idem.

Power management

• void power_down() idem. Explicitly blocks for 64 microseconds. (See Page 5 datasheet).
• void power_up() wakes up the HX711. It should reset the HX711 to defaults but this is not always seen. See discussion issue #27 GitHub. Needs more testing.

Pricing

Some price functions were added to make it easy to use this library for pricing goods or for educational purposes. These functions are under discussion if they will stay in the library. For weight conversion functions see https://github.com/RobTillaart/weight

• float get_price(uint8_t times = 1) idem.
• void set_unit_price(float price = 1.0) idem.
• float get_unit_price() idem.

Notes

Scale values for load cells

These scale values worked pretty well with a set of load cells I have, Use calibrate to find your favourite values.

• 5 KG load cell scale.set_scale(420.52);
• 20 KG load cell scale.set_scale(127.15);

Connections HX711

• A+/A- uses gain of 128 or 64
• B+/B- uses gain of 32

Colour scheme wires of two devices.

HX711 Pin	Colour dev 1	Colour dev 2
E+	red	red
E-	black	black
A-	white	blue
A+	green	white
B-	not connected	not connected
B+	not connected	not connected

Temperature

Load cells do have a temperature related error. (see datasheet load cell) This can be reduced by doing the calibration and take the tare at the operational temperature one uses for the measurements.

Another way to handle this is to add a good temperature sensor (e.g. DS18B20, SHT85) and compensate for the temperature differences in your code.

Operation

See examples

Future

must

• update documentation
• test B channel explicitly.
• test reset and reboot behaviours.

should

• add examples
• optimize the build-in ShiftIn() function to improve performance again.
• investigate read()
  • investigate the need of yield after interrupts
  • investigate blocking loop at begin of read()
• why store the gain as _gain while the iterations m = 1..3 is used most
  • read() less code (changes from explanatory code to vague)
  • very small performance gain.
  • code moves to both get/set_gain() so footprint might rise.

could

• test different load cells
• make enum of the MODE's
• move code to .cpp

the adding scale

• void weight_clr(), void weight_add(), float weight_get() - adding scale
  • might be a nice example