modelHx711

HX711 converter: 24-bit amplifier and ADC for a gauge bridge, PD_SCK/DOUT serial link

Extends from Internal.Devices.PartialSupplyPin (Supply of a peripheral: optional GND and VCC pins, internal supply rail - GND shown and ideal internal supply VOH by default).

Information

The HX711 (Avia Semiconductor) is the converter of electronic scales: a programmable-gain amplifier followed by a 24-bit analog-to-digital converter, designed to read a gauge bridge directly. This model reproduces its behaviour as seen from the microcontroller, based on the datasheet.

Bridge side

The module supplies the bridge through E+/E− (voltage AVDD) and measures the differential voltage between A+ and A−. The conversion is ratiometric: the result depends on the ratio between the bridge output and its excitation, not on the value of the supply.

code = vIn · gain / vRef · 2²⁴, rounded and clamped to [−2²³, 2²³−1] (full scale: ±AVDD/(2·gain), i.e. ±17 mV at gain 128).

Microcontroller side

Every 1/rate seconds, a new data is ready: DOUT goes to 0. If the previous one has not been read, DOUT first goes back up briefly (tUpdate), so that each new data is announced by a falling edge.
Each rising edge of PD_SCK shifts a bit out on DOUT, most significant first. After the 24 data bits, 1 to 3 extra pulses choose the gain of the next conversion: 25 pulses → 128 (channel A), 26 → 32 (channel B), 27 → 64 (channel A). DOUT then goes back to 1.
PD_SCK held high for more than 60 µs powers the chip down. When it returns to 0, the chip restarts as at power-up: gain 128, and first data after 4 conversions (400 ms at 10 samples/s).

The microcontroller therefore drives PD_SCK bit by bit (bit-banging). Its pin accesses must take simulated time (parameter MCU.gpioOpTime, 5 µs by default): otherwise, the pulses would have a zero duration.

Simplifications

Channel B (inputs B+/B−, gain 32) is not wired: it reads 0 V.
Each conversion is an instantaneous sample of the input (no averaging over the period), with no settling time after a gain change.
The noise (noiseLsb, standard deviation in LSB) is Gaussian and reproducible: the seed fixes the sequence of draws.

The icon shows the gain and the last converted code. The cyan light signals a data ready, the amber light the power-down.

Parameters

Type	Name	Default	Description
Electrical › Supply
Boolean	useSupplyPin (from PartialSupplyPin)	false	Show the VCC supply pin: the peripheral is supplied by the circuit (its high levels follow VCC, its consumption IQ is drawn from VCC) - unchecked: ideal internal supply VOH, nothing to wire
Modelica.Units.SI.Voltage	VOH (from PartialSupplyPin)	Interfaces.VOH	Logic high voltage: ideal internal supply, high level of the outputs and of the pull-ups - with the VCC pin, the voltage of VCC is used instead
Modelica.Units.SI.Current	IQ (from PartialSupplyPin)	1e-3	Current drawn from VCC by the peripheral (quiescent consumption, outputs excluded)
Boolean	useGroundPin (from PartialSupplyPin)	true	Show the GND pin - unchecked: the peripheral is referenced to the simulation ground (0 V), nothing to wire
SI.Voltage	VDropout	0.1	Dropout of the analog regulator of the module: with the VCC pin, AVDD is limited to VCC - VDropout
Conversion
Real	rate	10	Conversion rate (RATE pin of the chip: 10 or 80 samples per second)
SI.Voltage	AVDD	4.3	Excitation voltage of the bridge, E+ output (module supplied with 5 V) - with the VCC pin, at most VCC - VDropout
Real	noiseLsb	0	Conversion noise, standard deviation in LSB (0 = perfect, reproducible measurement)
Integer	seed	711	Noise seed: same seed, same sequence of measurements
Timing
SI.Time	tPowerDown	60e-6	PD_SCK held high longer than this: power-down
SI.Time	tUpdate	10e-6	Time during which DOUT goes back up before each new data, when the previous one has not been read
Integer	settlingConversions	4	Conversions discarded after power-up or wake-up (400 ms at 10 samples/s)
Electrical › Levels
SI.Voltage	VOL	Interfaces.VOL	Low level of DOUT
SI.Voltage	VIH	Interfaces.VIH	Threshold above which an input reads high (PD_SCK)
SI.Voltage	VIL	Interfaces.VIL	Threshold below which an input reads low (PD_SCK)
Electrical › Impedances
SI.Resistance	ROut	Interfaces.ROut	Series resistance of the DOUT output

Connectors

Type	Name	Description
Modelica.Electrical.Analog.Interfaces.NegativePin	GND (from PartialSupplyPin)	Common reference (ground), to be connected to the microcontroller's one - shown when useGroundPin is checked
Modelica.Electrical.Analog.Interfaces.PositivePin	VCC (from PartialSupplyPin)	Supply of the peripheral (e.g. 3V3(OUT) of a Raspberry Pi Pico) - shown when useSupplyPin is checked
Modelica.Electrical.Analog.Interfaces.PositivePin	PD_SCK	Serial clock and power-down control (SCK pin of the module), driven by the microcontroller
Modelica.Electrical.Analog.Interfaces.PositivePin	DOUT	Serial data (DT pin of the module), read by the microcontroller
Modelica.Electrical.Analog.Interfaces.PositivePin	E_plus	Bridge excitation (+)
Modelica.Electrical.Analog.Interfaces.PositivePin	A_plus	Differential input of channel A (+), from S+ of the bridge
Modelica.Electrical.Analog.Interfaces.NegativePin	A_minus	Differential input of channel A (-), from S- of the bridge
Modelica.Electrical.Analog.Interfaces.NegativePin	E_minus	Bridge excitation (-), connected to the module ground

Components

Type	Name	Description
Integer	code	Last conversion result (two's complement, 24 bits)
Integer	gain	Gain of the current conversion (128 or 64 on channel A, 32 on channel B)
Integer	pulses	PD_SCK pulses received since the data became ready
Boolean	ready	Data ready, not read yet: DOUT is at 0
Boolean	poweredDown	Powered down (PD_SCK held high longer than tPowerDown)
SI.Voltage	vIn	Differential voltage of channel A, A+ - A-
SI.Voltage	vRef	Excitation voltage, E+ - E-: reference of the conversion (ratiometric measurement)