Reading the RTC

The RTC exposes data registers with the current time fields and status registers that say how to read them.

Register	Meaning
RTC_SECONDS	seconds
RTC_MINUTES	minutes
RTC_HOURS	hours
RTC_DAY	day of month
RTC_MONTH	month
RTC_YEAR	two-digit year
RTC_REG_A	update-in-progress bit
RTC_REG_B	data-mode bit

Update-in-progress

RTC_REG_A contains the UIP bit. While it is set, the RTC is updating its fields, so reads can be inconsistent.

The safe sequence is: read RTC_REG_A, wait while RTC_UIP is set, read RTC_REG_B, then read the date and time fields. Convert from BCD afterward if register B says the device is in BCD mode. Correctness matters more than saving a few port operations.

BCD

BCD stores decimal digits in nibbles:

Raw value	Meaning
0x09	9
0x16	16
0x42	42

Binary 0x42 is decimal 66, so the conversion must be deliberate. BCD exists for historical compatibility with decimal displays and firmware. Convert it to an ordinary binary integer before the rest of the code uses it.

Output structures

The lab functions receive output pointers:

int rtc_read_date(lcom_rtc_date_t *date);

This lets the function return a status code while filling the caller's structure only on success. Returning an int reports failure; the pointer supplies structured output. Validate the pointer before writing and do not leave a half-updated structure after a failed read.