Philips LPC2101 - RTC External 32 Khz Oscillator Component Selection

© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
User manual Rev. 01 — 12 January 2006 223
Philips Semiconductors
UM10161
Volume 1 Chapter 17: RTC
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.
17.7 RTC external 32 kHz oscillator component selection
The RTC external oscillator circuit is shown in Figure 59. Since the feedback resistance is
integrated on chip, only a crystal, the capacitances C
X1
and C
X2
need to be connected
externally to the microcontroller.
Table 200: Prescaler cases where the Integer Counter reload value is incremented
Fraction Counter PREFRAC Bit
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
--- ---- ---- ---1
1--------------
--- ---- ---- --10 -1-------------
--- ---- ---- -100 --1------------
--- ---- ---- 1000 ---1-----------
--- ---- ---1 0000
----1----------
--- ---- --10 0000
-----1---------
--- ---- -100 0000
------1--------
--- ---- 1000 0000
-------1-------
--- ---1 0000 0000
--------1------
--- --10 0000 0000
---------1-----
--- -100 0000 0000
----------1----
--- 1000 0000 0000
-----------1---
--1 0000 0000 0000
------------1--
-10 0000 0000 0000
-------------1-
100 0000 0000 0000
--------------1