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Prevent ints from corrupting the read sequence #62

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Prevent ints from corrupting the read sequence #62

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40 changes: 29 additions & 11 deletions HX711.cpp
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
#include <Arduino.h>
#include <util/atomic.h> // For ATOMIC_BLOCK(ATOMIC_RESTORESTATE) macro
#include <HX711.h>

#if ARDUINO_VERSION <= 106
Expand Down Expand Up @@ -59,17 +60,34 @@ long HX711::read() {
uint8_t data[3] = { 0 };
uint8_t filler = 0x00;

// pulse the clock pin 24 times to read the data
data[2] = shiftIn(DOUT, PD_SCK, MSBFIRST);
data[1] = shiftIn(DOUT, PD_SCK, MSBFIRST);
data[0] = shiftIn(DOUT, PD_SCK, MSBFIRST);

// set the channel and the gain factor for the next reading using the clock pin
for (unsigned int i = 0; i < GAIN; i++) {
digitalWrite(PD_SCK, HIGH);
digitalWrite(PD_SCK, LOW);
}

// Protect the read sequence from system interrupts. If an interrupt occurs during
// the time the PD_SCK signal is high it will stretch the length of the clock pulse.
// If the total pulse time exceeds 60 uSec this will cause the HX711 to enter
// power down mode during the middle of the read sequence. While the device will
// wake up when PD_SCK goes low again, the reset starts a new conversion cycle which
// forces DOUT high until that cycle is completed.
//
// The result is that all subsequent bits read by shiftIn() will read back as 1,
// corrupting the value returned by read(). The ATOMIC_BLOCK macro disables
// interrupts during the sequence and then restores the interrupt mask to its previous
// state after the sequence completes, insuring that the entire read-and-gain-set
// sequence is not interrupted. The macro has a few minor advantages over bracketing
// the sequence between NoInterrupts() and interrupts() calls.

ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {

// pulse the clock pin 24 times to read the data
data[2] = shiftIn(DOUT, PD_SCK, MSBFIRST);
data[1] = shiftIn(DOUT, PD_SCK, MSBFIRST);
data[0] = shiftIn(DOUT, PD_SCK, MSBFIRST);

// set the channel and the gain factor for the next reading using the clock pin
for (unsigned int i = 0; i < GAIN; i++) {
digitalWrite(PD_SCK, HIGH);
digitalWrite(PD_SCK, LOW);
}
}

// Replicate the most significant bit to pad out a 32-bit signed integer
if (data[2] & 0x80) {
filler = 0xFF;
Expand Down