I'm trying to port the MFRC522 Arduino library (from GitHub) to the ARM MCUs. My problem is with the PCD_ReadRegister function:
/**
* Reads a number of bytes from the specified register in the MFRC522 chip.
* The interface is described in the datasheet section 8.1.2.
*/
void MFRC522::PCD_ReadRegister( byte reg, ///< The register to read from. One of the PCD_Register enums.
byte count, ///< The number of bytes to read
byte *values, ///< Byte array to store the values in.
byte rxAlign ///< Only bit positions rxAlign..7 in values[0] are updated.
) {
if (count == 0) {
return;
}
//Serial.print(F("Reading ")); Serial.print(count); Serial.println(F(" bytes from register."));
byte address = 0x80 | (reg & 0x7E); // MSB == 1 is for reading. LSB is not used in address. Datasheet section 8.1.2.3.
byte index = 0; // Index in values array.
SPI.beginTransaction(SPISettings(SPI_CLOCK_DIV4, MSBFIRST, SPI_MODE0)); // Set the settings to work with SPI bus
digitalWrite(_chipSelectPin, LOW); // Select slave
count--; // One read is performed outside of the loop
SPI.transfer(address); // Tell MFRC522 which address we want to read
while (index < count) {
if (index == 0 && rxAlign) { // Only update bit positions rxAlign..7 in values[0]
// Create bit mask for bit positions rxAlign..7
byte mask = 0;
for (byte i = rxAlign; i <= 7; i++) {
mask |= (1 << i);
}
// Read value and tell that we want to read the same address again.
byte value = SPI.transfer(address);
// Apply mask to both current value of values[0] and the new data in value.
values[0] = (values[index] & ~mask) | (value & mask);
}
else { // Normal case
values[index] = SPI.transfer(address); // Read value and tell that we want to read the same address again.
}
index++;
}
values[index] = SPI.transfer(0); // Read the final byte. Send 0 to stop reading.
digitalWrite(_chipSelectPin, HIGH); // Release slave again
SPI.endTransaction(); // Stop using the SPI bus
} // End PCD_ReadRegister()
I read the datasheet several times & can't understand what's the function of this parameter!
Based on datasheet for reading a register from rc522:
