
Address Map
The bq4845 provides 16 bytes of clock and control status
registers. Table 1 is a map of the bq4845 registers, and
Table 2 describes the register bits.
Clock Memory Interface
The bq4845 has the same interface for clock/calendar
and control information as standard SRAM. To read and
write to these locations, the user must put the bq4845 in
the proper mode and meet the timing requirements.
Read Mode
The bq4845 is in read mode whenever OE (Output en-
able) is low and CS (chip select) is low. The unique ad-
dress, specified by the 4 address inputs, defines which
one of the 16 clock/calendar bytes is to be accessed. The
bq4845 makes valid data available at the data I/O pins
within tAA (address access time). This occurs after the
last address input signal is stable, and providing the CS
and OE (output enable) access times are met. If the CS
and OE access times are not met, valid data is available
after the latter of chip select access time (tACS) or output
enable access time (tOE).
CS and OE control the state of the eight three-state
data I/O signals. If the outputs are activated before tAA,
4
Ad-
dress
(h)
D7
D6
D5
D4
D3
D2
D1
D0
12-Hour
Range (h)
Register
0
10-second digit
1-second digit
00–59
Seconds
1
ALM1
ALM0
1-second digit
00–59
Seconds alarm
10-second digit
2
0
10-minute digit
1-minute digit
00–59
Minutes
3
ALM1
ALM0
1-minute digit
00–59
Minutes alarm
10-minute digit
4
PM/AM
0
10-hour digit
1-hour digit
01–12 AM/ 81– 92 PM Hours
5
ALM1
ALM0
10-hour digit
1-hour digit
01–12 AM/ 81–92 PM Hours alarm
PM/AM
6
0
10-day digit
1-day digit
01–31
Day
7
ALM1
ALM0
10-day digit
1-day digit
01–31
Day alarm
8
0
Day-of-week digit
01–07
Day-of-week
9
0
10 mo.
1-month digit
01–12
Month
A
10-year digit
1-year digit
00–99
Year
B
*
WD2
WD1
WD0
RS3
RS2
RS1
RS0
Programmable
rates
C
*
AIE
PIE
PWRIE ABE
Interrupt en-
ables
D
*
AF
PF
PWRF BVF
Flags
E
*
UTI
STOP
24/12
DSE
Control
F
*
Unused
Notes:
* = Unused bits; unwritable and read as 0.
0 = should be set to 0 for valid time/calendar range.
Clock calendar data in BCD. Automatic leap year adjustment.
PM/AM = 1 for PM; PM/AM = 0 for AM.
DSE = 1 enables daylight savings adjustment.
24/12 = 1 enables 24-hour data representation; 24/12 = 0 enables 12-hour data representation.
Day-of-Week coded as Sunday = 1 through Saturday = 7.
BVF = 1 for valid battery.
STOP = 1 turns the RTC on; STOP = 0 stops the RTC in back-up mode.
Table 1. bq4845 Clock and Control Register Map
Aug. 1995
bq4845/bq4845Y