Bitwise operations

Registers are compact. One byte may contain several independent fields:

Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0
------+-------+-------+-------+-------+-------+-------+-------
 UIP  |  DV2  |  DV1  |  DV0  |  RS3  |  RS2  |  RS1  |  RS0

UIP is bit 7. To isolate it:

uint8_t uip = reg_a & 0x80;

To build the mask without hardcoding:

uint8_t mask = (uint8_t)(1u << 7);

Read values

Use & to test whether a bit is present. A mask with one bit set asks a yes-or-no question about that position. In C, any non-zero result is true. A helper such as bit_is_set should therefore document whether it returns the raw mask or a normalized boolean.

if (value & BIT(7)) {
  /* bit 7 is set */
}

BIT(n) is defined in the public SDK header as (1u << (n)).

Construct write values

Use | to set bits and & ~mask to clear them. These operations change one field while preserving its neighbors.

value |= BIT(2);
value &= (uint8_t)~BIT(2);

This lab mostly reads the RTC. Later labs use the same idea for timer control words, UART line control, and mouse or keyboard commands.

When debugging, write the value in binary beside the test. Most mistakes are position mistakes, not arithmetic mistakes. Bit 0 is the least significant bit.

TIP

Build masks from bit positions, then name them. Named masks document the device protocol and reduce off-by-one errors.