Rtc External 32 Khz Oscillator Component Selection; User Manual - Philips LPC2101 User Manual

Philips Semiconductors
Volume 1
For example, if PREFRAC bit 14 is a one (representing the fraction 1/2), then half of the
cycles counted by the 13-bit counter need to be longer. When there is a 1 in the LSB of
the Fraction Counter, the logic causes every alternate count (whenever the LSB of the
Fraction Counter=1) to be extended by one PCLK, evenly distributing the pulse widths.
Similarly, a one in PREFRAC bit 13 (representing the fraction 1/4) will cause every fourth
cycle (whenever the two LSBs of the Fraction Counter=10) counted by the 13-bit counter
to be longer.
Table 200: Prescaler cases where the Integer Counter reload value is incremented
Fraction Counter
--- ---- ---- ---1
--- ---- ---- --10
--- ---- ---- -100
--- ---- ---- 1000
--- ---- ---1 0000
--- ---- --10 0000
--- ---- -100 0000
--- ---- 1000 0000
--- ---1 0000 0000
--- --10 0000 0000
--- -100 0000 0000
--- 1000 0000 0000
--1 0000 0000 0000
-10 0000 0000 0000
100 0000 0000 0000

17.7 RTC external 32 kHz oscillator component selection

The RTC external oscillator circuit is shown in
integrated on chip, only a crystal, the capacitances C
externally to the microcontroller.

User manual

PREFRAC Bit
14 13 12 11 10 9
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Rev. 01 — 12 January 2006
8 7 6 5
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Figure 59. Since the feedback resistance is
and C need to be connected
X1 X2
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
UM10161
Chapter 17: RTC
4 3 2 1 0
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