BQ4285E Datasheet, PDF (5/32 Pages) Texas Instruments – Enhanced RTC With NVRAM Control

bq4285E/L
Programming the RTC
The time-of-day, alarm, and calendar bytes can be written
in either the BCD or binary format (see Table 2).
These steps may be followed to program the time, alarm,
and calendar:
1. Modify the contents of register B:
a. Write a 1 to the UTI bit to prevent trans-
fers between RTC bytes and user buffer.
b. Write the appropriate value to the data for-
mat (DF) bit to select BCD or binary format
for all time, alarm, and calendar bytes.
c. Write the appropriate value to the hour
format (HF) bit.
2. Write new values to all the time, alarm, and
calendar locations.
3. Clear the UTI bit to allow update transfers.
On the next update cycle, the RTC updates all 10 bytes
in the selected format.
Square-Wave Output
The bq4285E/L divides the 32.768kHz oscillator fre-
quency to produce the 1 Hz update frequency for the
clock and calendar. Thirteen taps from the frequency di-
vider are fed to a 16:1 multiplexer circuit. The output of
this mux is fed to the SQW output and periodic inter-
rupt generation circuitry. The four least-significant bits
of register A, RS0–RS3, select among the 13 taps (see
Table 3). The square-wave output is enabled by writing
a 1 to the square-wave enable bit (SQWE) in register B.
A 32.768kHz output may be selected by setting
OSC2–OSC0 in register A to 011 while SQWE = 1 and
32KE = 1.
Table 2. Time, Alarm, and Calendar Formats
Address
0
1
2
3
4
5
6
7
8
9
RTC Bytes
Seconds
Seconds alarm
Minutes
Minutes alarm
Hours, 12-hour format
Hours, 24-hour format
Hours alarm, 12-hour format
Hours alarm, 24-hour format
Day of week (1=Sunday)
Day of month
Month
Year
Decimal
0–59
0–59
0–59
0–59
1–12
0–23
1–12
0–23
1–7
1–31
1–12
0–99
Range
Binary
00H–3BH
00H–3BH
00H–3BH
00H–3BH
01H–OCH AM;
81H–8CH PM
00H–17H
01H–OCH AM;
81H–8CH PM
00H–17H
01H–07H
01H–1FH
01H–0CH
00H–63H
Binary-Coded
Decimal
00H–59H
00H–59H
00H–59H
00H–59H
01H–12H AM;
81H–92H PM
00H–23H
01H–12H AM;
81H–92H PM
00H–23H
01H–07H
01H–31H
01H–12H
00H–99H
5