Reading the HX711 using SPI

README.md

Author: robert-hh, @robert-hh

Source: https://forum.micropython.org/viewtopic.php?f=16&t=2678&start=10#p29735

For interest, I also made a driver using SPI. In that case, MOSI is used as the clock signal. The SPI clock signal is not used. It was made for a LoPy, so it will not immediately run on other MicroPython hardware, YMMV. The interesting parts are the data structures for clock and the lookup table for fast transformation.

The reason for using SPI was, that the ESP32 - just like the ESP8266 - is not good at fast toggling and reading, eventually not matching the timing requirements of the HX711. Using SPI ensures fast timing.

hx711_spi.py

class HX711:
    """
    Use SPI for ensuring appropriate timing when reading the HX711.

    The ESP32 - just like the ESP8266 - is not good at fast toggling and reading
    of general i/o ports, eventually not matching the timing requirements of 
    the HX711. Using SPI ensures fast timing. 

    Source: https://forum.micropython.org/viewtopic.php?f=16&t=2678&start=10#p29735
    """

    def __init__(self, dout, pd_sck, spi_clk, gain=128):

        self.pSCK = Pin(pd_sck , mode=Pin.OUT)
        self.pOUT = Pin(dout, mode=Pin.IN, pull=Pin.PULL_DOWN)
        self.spi = SPI(0, mode=SPI.MASTER, baudrate=1000000, polarity=0, phase=0, pins=(spi_clk, pd_sck, dout))
        self.pSCK(0)

        self.clock_25 = b'\xaa\xaa\xaa\xaa\xaa\xaa\x80'
        self.clock_26 = b'\xaa\xaa\xaa\xaa\xaa\xaa\xa0'
        self.clock_27 = b'\xaa\xaa\xaa\xaa\xaa\xaa\xa8'
        self.clock = self.clock_25
        self.lookup = (b'\x00\x01\x00\x00\x02\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                       b'\x04\x05\x00\x00\x06\x07\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                       b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                       b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                       b'\x08\x09\x00\x00\x0a\x0b\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
                       b'\x0c\x0d\x00\x00\x0e\x0f')
        self.in_data = bytearray(7)

        self.set_gain(gain);

    def set_gain(self, gain):
        if gain is 128:
            self.clock = self.clock_25
        elif gain is 64:
            self.clock = self.clock_27
        elif gain is 32:
            self.clock = self.clock_26
        self.read()

    def read(self):
        # wait for the device to get ready
        while self.pOUT() != 0:
            idle()

        # get the data and set channel and gain
        self.spi.write_readinto(self.clock, self.in_data)

        # pack the data into a single value
        result = 0
        for _ in range (6):
            result = (result << 4) + self.lookup[self.in_data[_] & 0x55]

        # return sign corrected result
        return result - ((result & 0x800000) << 1)