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.

Note that the 24 bits of the HX711 contains some noise so depending on setup, load etc. only 16 to 20 of the bits are expected significant in practice. This translates roughly to 4 or max 5 significant digits in a single measurement That's why multiple measurements are advised to average and reduce the noise.

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.

HX711_MP

• https://github.com/RobTillaart/HX711_MP

The library HX711_MP is derived from this HX711 library version 0.3.5.

The HX711_MP is not compatible, see readme.md of HX711_MP

The main difference is that the HX711_MP uses a multi-point calibration which replaces the "offset + scale" concept. Functions related to this concept are removed in HX711_MP and replaced by its own set of calibration functions.

This multi-point calibration allows to compensate for non-linear behaviour in the sensor readings.

10 or 80 SPS

The datasheet mentions that the HX711 can run at 80 samples per second SPS. To select this mode connect the RATE pin(15) of the chip to VCC (HIGH). Connecting RATE to GND (LOW) gives 10 SPS.

All breakout boards I tested have RATE connected to GND and offer no pin to control this from the outside.

This library does not provide means to control the RATE yet. If there is a need (issue) I will implement this in the library. For now one can add an IOpin for this and use digitalWrite().

Related

• https://github.com/bogde/HX711
• https://github.com/RobTillaart/weight (conversions kg <> stone etc.)
• https://github.com/RobTillaart/HX711
• https://github.com/RobTillaart/HX711_MP

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

#include "HX711.h"

Base

• HX711() constructor.
• ~HX711()
• void begin(uint8_t dataPin, uint8_t clockPin, bool fastProcessor) sets a fixed gain 128 for now. The fastProcessor option adds a 1 uS delay for each clock half-cycle to keep the time greater than 200 nS.
• 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 of last read.

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 an 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 the times parameter 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.
• bool set_scale(float scale = 1.0) set scale factor which is normally a positive number larger than 50. Depends on load-cell used. Returns false if scale == 0.
• float get_scale() returns set scale factor.
• void set_offset(long offset = 0) idem.
• long get_offset() idem.

Tare & calibration I

Steps to take for calibration

1. clear the scale.
2. call tare() to determine and 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 determine the offset to calibrate the zero (reference) level. See below.
• float get_tare() returns the offset * scale. Note this differs after calls to calibrate_scale(). Use get_offset() to get only the offset.
• bool tare_set() checks if a tare has been set. Assumes offset is not zero, which is true for all load cells tested.
• void calibrate_scale(uint16_t weight, uint8_t times = 10) idem.

Tare & calibration II

A load cell + HX711 module without weight gives a raw value, mostly not equal to zero. The function get_tare() is used to measure this raw value and allows the user to define this value as a zero weight (force) point. This zero point is normally without any load, however it is possible to define a zero point with a "fixed" load e.g. a cup, a dish, even a spring or whatever. This allows the system to automatically subtract the weight of the cup etc.

Warning: The user must be aware that the "fixed" load together with the "variable" load does not exceed the specifications of the load cell.

E.g. a load cell which can handle 1000 grams with a cup of 300 grams should not be calibrated with a weight of more than 700 grams. In fact it is better to calibrate with a weight in the order of 80 to 90% of the maximum load so in this example a weight of 500 to 600 grams.

Furthermore it is also important to do the calibration at the temperature you expect to do the weight measurements. See temperature section below.

Inner formula

Weight = get_scale() x raw + get_tare().

With the two parameters one can interpolate the inner formula. This can be used e.g to make an ideal graph of the conversion. This can be compared with actual values to get an indication of the accuracy of the load cell.

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.

Multiple HX711

Separate lines

Simplest way to control multiple HX711's is to have a separate DOUT and CLK line for every HX711 connected.

Multiplexer

Alternative one could use a multiplexer like the https://github.com/RobTillaart/HC4052 or possibly an https://github.com/RobTillaart/TCA9548. Although to control the multiplexer one need some extra lines and code.

Share CLOCK line

See HX_loadcell_array.ino

Another way to control multiple HX711's is to share the CLK line. This has a few side effects which might be acceptable or not.

Known side effects - page 4 and 5 datasheet.

• The CLK is used to select channel and to select gain for the NEXT sample.
• The CLK is used for power down.
• After wake up after power down all HX711's will reset to channel A and gain 128. WARNING: if one of the objects does a powerDown() or reset() it resets its internal states. The other objects however won't reset their internal state, so a mismatch can occur.

So in short, sharing the CLK line causes all HX711 modules share the same state. This can introduce extra complexity if one uses mixed gains or channels. If all HX711's use the same settings it should work, however extra care is needed for powerDown() and reset().

WARNING: Sharing the data lines is NOT possible as it could cause short circuit.

See https://github.com/RobTillaart/HX711/issues/40

Future

Must

• update documentation HX711
• keep in sync with HX711_MP

Should

• test B channel explicitly.
• test reset and reboot behaviours.
• investigate read()
  • investigate the need of yield after interrupts
  • investigate blocking loop at begin => less yield() calls ?

Could

• test different load cells
• make enum of the MODE's
• add examples
  • example the adding scale
  • void weight_clr(), void weight_add(), float weight_get() - adding scale
• decide pricing keep/not => move to .cpp
• add setRate() and getRate() - sample rate 10/80 SPS
  • optional?

Wont

• 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.

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