PCF2123BS NXP [NXP Semiconductors], PCF2123BS Datasheet - Page 22

• Manufacturer Part Number: PCF2123BS
• Description: SPI Real time clock/calendar
• Manufacturer: NXP [NXP Semiconductors]

NXP Semiconductors
PCF2123_1
Product data sheet
9.8.2 Countdown timer function
The duration of both of these timers will be affected by the register Offset_register (see
Section
consistent.
The 8-bit countdown timer at address 0Fh is controlled by the register Timer_clkout at
address 0Eh. The register Timer_clkout selects one of 4 source clock frequencies for the
timer (4.096 kHz, 64 Hz, 1 Hz, or
Table 28.
[1]
[2]
Remark: Note that all timings which are generated from the 32.768 kHz oscillator are
based on the assumption that there is 0 ppm deviation. Deviation in oscillator frequency
will result in deviation in timings. This is not applicable to interface timing.
The timer counts down from a software-loaded 8-bit binary value, n. Loading the counter
with 0 stops the timer. Values from 1 to 255 are valid. When the counter reaches 1, the
countdown timer flag (bit TF) will be set and the counter automatically re-loads and starts
the next timer period. Reading the timer will return the current value of the countdown
counter (see
CTD[1:0]
00
01
10
11
Fig 15. General countdown timer behavior
countdown value, n
countdown counter
timer source clock
When not in use, CTD must be set to
Time periods can be affected by correction pulses.
9.11). Only when the Offset_register has the value 00h will the periods be
In this example it is assumed that the timer flag is cleared before the next countdown period
expires and that the pin INT is set to pulsed mode.
Bits CTD0 and CTD1 for timer frequency selection and countdown timer
durations
Timer source clock
frequency
4.096 kHz
64 Hz
1 Hz
1
60
Figure
INT
TE
TF
Hz
[2]
[2]
xx
xx
15).
[1]
Rev. 01 — 19 November 2008
03
03
Delay
Minimum timer duration
n = 1
244 s
15.625 ms
1 s
60 s
1
1
60
60
Hz for power saving.
Hz) and enables or disables the timer.
02
duration of first timer period after
enable may range from n
01
03
02
n
SPI Real time clock/calendar
Maximum timer duration
n = 255
62.256 ms
3.984 s
255 s
4 h 15 min
01
1 to n + 1
03
PCF2123
© NXP B.V. 2008. All rights reserved.
02
n
01
001aaf906
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03