bq3285
RS0–RS3 - Frequency Select
Power-Down/Power-Up Cycle
7
-
6
-
5
-
4
-
3
2
1
0
The bq3285 continuously monitors VCC for out-of-
tolerance. During a power failure, when VCC falls below
VPFD (4.17V typical), the bq3285 write-protects the clock
and storage registers. When VCC is below VBC (3V typi-
cal), the power source is switched to BC. RTC operation
and storage data are sustained by a valid backup energy
RS3 RS2 RS1 RS0
These bits select one of the 13 frequencies for the SQW out-
put and the periodic interrupt rate, as shown in Table 3.
OS0–OS2 - Oscillator Control
source. When VCC is above VBC, the power source is VCC
Write-protection continues for tCSR time after VCC rises
above VPFD
.
7
-
6
5
4
3
-
2
-
1
-
0
-
.
OS2 OS1 OS0
Control/Status Registers
These three bits control the state of the oscillator and
divider stages. A pattern of 010 enables RTC operation
by turning on the oscillator and enabling the frequency
divider. A pattern of 11X turns the oscillator on, but
keeps the frequency divider disabled. When 010 is writ-
ten, the RTC begins its first update after 500ms.
The four control/status registers of the bq3285 are acces-
sible regardless of the status of the update cycle (see Ta-
ble 4).
Register A
UIP - Update Cycle Status
Register A Bits
7
6
5
4
3
2
1
0
7
6
-
5
-
4
-
3
-
2
-
1
-
0
-
UIP OS2 OS1 OS0
RS3
RS2 RS1 RS0
UIP
Register A programs:
This read-only bit is set prior to the update cycle. When
UIP equals 1, an RTC update cycle may be in progress.
UIP is cleared at the end of each update cycle. This bit
is also cleared when the update transfer inhibit (UTI)
bit in register B is 1.
n
n
The frequency of the square-wave and the periodic
event rate.
Oscillator operation.
Register A provides:
n
Status of the update cycle.
Table 4. Control/Status Registers
Bit Name and State on Reset
Loc.
Reg. (Hex) Read Write 7 (MSB)
6
5
4
3
2
1
0 (LSB)
A
B
C
D
0A
0B
0C
0D
Yes Yes1 UIP na OS2 na OS1 na OS0 na RS3 na RS2 na RS1 na RS0 na
Yes
Yes
Yes
Yes UTI na PIE
0
0
0
AIE
AF
-
0
0
0
UIE
UF
-
0
0
0
SQWE
0
0
0
DF na HF na DSE na
No INTF
0
PF
-
-
-
-
-
0
0
-
-
0
0
-
-
0
0
No VRT na
Notes:
1. Except bit 7.
2. na = not affected
Jan. 1999 E
8
TI [ TEXAS INSTRUMENTS ]
