User manual for I²C-bus RTC demo board OM13515
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© NXP B.V. 2014. All rights reserved.
5.3.2 Reading the clock
Reading the clock (2 minutes after writing)
I
2
C-bus S START condition
Slave address 1010 0010 R/
W = 1, read mode
Register address 0000 0100 address pointer to Seconds
register
I
2
C-bus Sr repeated start condition
Slave address 1010 0011 read mode
Read register 4 e.g. 56 seconds, (clock
integrity ok OS = 0)
Minutes e.g. 47 (Minutes)
Hours e.g. 23 (PM, 03h)
Days e.g. 15 (15
th
)
Weekdays e.g. 01 (Monday)
Month e.g. 12 (December)
Year e.g. 14 (20)14
I
2
C-bus P STOP condition
6. RTC tuning
6.1 Frequency tuning
The 32 kHz quartzes are typically sold with a tolerance at room temperature of either
±10 ppm or ±20 ppm. 11.5 ppm corresponds to 1 s/day.
The quartzes require a characteristic load capacity of either 7 pF or 12.5 pF. Oscillators
utilizing 7 pF quartzes feature slightly lower power consumption, where the quartzes of
12.5 pF have largest production quantities. Program the CAP_SEL bit in register
Control_1 accordingly. The tracks between quartz and RTC represent also some
parasitic capacitances and must be kept short.
The PCF85063 has a tuning facility where above tolerances can be compensated.
Tuning procedure:
• Measure the 32xxx Hz (f) signal at the CLKOUT pin.
• The offset is calculated in ppm as
Δf
[ppm]
= 10
6
× (f - 32768) / 32768
• Consult the offset table in the data sheet. Take the correction value and write it into
the register 02h.
