pcf2127a NXP Semiconductors, pcf2127a Datasheet - Page 12

• Manufacturer Part Number: pcf2127a
• Description: Integrated Rtc, Tcxo And Quartz Crystal
• Manufacturer: NXP Semiconductors

NXP Semiconductors
PCF2127A_1
Product data sheet
8.3.1.1 Temperature measurement
8.3.1 Temperature compensated crystal oscillator
8.3.2 Clock output
8.3 Register CLKOUT_ctl
Table 8.
The frequency of tuning fork quartz crystal oscillators are temperature-dependent. In the
PCF2127A the frequency drift caused by temperature variation is corrected by adjusting
the load capacitance of the crystal oscillator.
The load capacitance is changed by switching between two load capacitance values using
a modulation signal with a programmable duty cycle. Every chip is calibrated in order to
produce, at the measured temperature, the correct duty cycle which compensates for the
frequency drift.
The frequency accuracy can be evaluated by measuring the frequency of the square
wave signal available at the output pin CLKOUT. However, the selection of
f
frequency measurement occurs when f
The PCF2127A has a temperature sensor circuit used to perform the temperature
compensation of the frequency. The temperature is measured immediately after power-on
and then periodically with a period set by the temperature conversion rate TCR[1:0] in the
register CLKOUT_ctl.
Table 9.
[1]
A programmable square wave is available at pin CLKOUT. Operation is controlled by the
COF control bits in register CLKOUT_ctl. Frequencies of 32.768 kHz (default) down to
1 Hz can be generated for use as a system clock, microcontroller clock, input to a charge
pump, or for calibration of the oscillator.
CLKOUT is an open-drain output and enabled at power-on. When disabled, the output is
high-impedance.
The duty cycle of the selected clock is not controlled, however, due to the nature of the
clock generation, all but the 32.768 kHz frequencies will be 50 : 50.
Bit
7 to 6 TCR[1:0]
5 to 3 -
2 to 0 COF[2:0]
TCR[1:0]
00
01
10
11
CLKOUT
Default value.
Symbol
= 32 kHz (default value) leads to inaccurate measurements. The most accurate
CLKOUT_ctl - CLKOUT control register (address 03h) bit description
Temperature measurement period
[1]
Rev. 01 — 21 January 2010
Temperature measurement period
4 min
2 min
1 min
30 seconds
Value
see
-
see
Table 9
Table 10
CLKOUT
Description
temperature measurement period
unused
CLKOUT frequency selection
Integrated RTC, TCXO and quartz crystal
= 1 Hz is selected (see
PCF2127A
Table
© NXP B.V. 2010. All rights reserved.
10).
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