PCF8564A All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 3 — 26 August 2013 17 of 48
NXP Semiconductors
PCF8564A
Real time clock and calendar
8.8.2 Register Timer
[1] Countdown period in seconds: where TV is the
countdown timer value.
The timer register is an 8-bit binary countdown timer. It is enabled or disabled via the timer
control register. The source clock for the timer is also selected by the timer control
register. Other timer properties such as single or periodic interrupt generation are
controlled via the register Control_2 (address 01h).
For accurate read back of the count down value, the I
2
C-bus clock (SDA) must be
operating at a frequency of at least twice the selected timer clock. Since it is not possible
to freeze the countdown timer counter during read back, it is recommended to read the
register twice and check for consistent results.
8.9 EXT_CLK test mode
The test mode is entered by setting the TEST1 bit of register Control_1 to logic 1. The
CLKOUT pin then becomes an input. The test mode replaces the internal 64 Hz signal
with that applied to the CLKOUT pin. Every 64 positive edges applied to CLKOUT then
generates an increment of one second.
The signal applied to the CLKOUT pin should have a minimum pulse width of 300 ns and
a maximum period of 1000 ns. The 64 Hz clock, now sourced from CLKOUT, is divided
down to 1 Hz by a 2
6
divide chain called a prescaler. The prescaler can be set to a known
state by using the STOP bit. When the STOP bit is set, the prescaler is reset to logic 0.
(STOP must be cleared before the prescaler can operate.)
From a STOP condition, the first 1 second increment will take place after 32 positive
edges on CLKOUT. Thereafter, every 64 positive edges will cause a 1 second increment.
Remark: Entry into EXT_CLK test mode is not synchronized to the internal 64 Hz clock.
When entering the test mode, no assumption as to the state of the prescaler can be made.
8.9.1 Operation example
1. Set EXT_CLK test mode (Bit 7 Control_1 = 1).
2. Set STOP (Bit 5 Control_1 = 1).
3. Clear STOP (Bit 5 Control_1 = 0).
4. Set time registers to desired value.
5. Apply 32 clock pulses to CLKOUT.
Table 25. Timer - timer register (address 0Fh) bit description
Bit Symbol Value Description
7to0 TV[7:0] 0htoFFh
countdown timer value
[1]
Table 26. Timer register bits value range
Bit
7 6 5 4 3 2 1 0
1286432168421
CountdownPeriod
TV
SourceClockFrequency
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