I am currently using and trying to understand the OneWire Library
. This library helps to interact with devices on the 1Wire bus by Dallas
Let pin = 2 be the digital pin of Arduino that is used.
The variables bitmask and baseReg (uint8_t) are initialized as
#define PIN_TO_BASEREG(pin) (portInputRegister(digitalPinToPort(pin)))
#define PIN_TO_BITMASK(pin) (digitalPinToBitMask(pin))
bitmask = PIN_TO_BITMASK(pin);
baseReg = PIN_TO_BASEREG(pin);
and setting the pin to low is done by
#define IO_REG_TYPE uint8_t
#define IO_REG_ASM asm("r30")
#define DIRECT_MODE_OUTPUT(base, mask) ((*(base+1)) |= (mask))
#define DIRECT_WRITE_LOW(base, mask) ((*(base+2)) &= ~(mask))
volatile IO_REG_TYPE *reg IO_REG_ASM = baseReg;
DIRECT_MODE_OUTPUT(reg, mask); // drive output low
- bitmask contains the position of the pin in the related registers DDRX, PORTX and PINX where X is A,B,C or D - depending on the pin and AVR used, right?
- What does basereg contain?
- If I do understand the code right, the line starting with "volatile" declares a pointer to an unsigned char called reg that is stored in register 30 of the AVR. But why is that register used? Is the pointer itself or its value stored there?
- How does the DIRECT_WRITE_LOW macro work? Does (base+1) correspond to the DDR and (base+2) to the PORT register?
- Why is DIRECT_MODE_OUTPUT called after DIRECT_WRITE_LOW?
Could you please help me to answer these questions?
Thanks in advance!