UM10211 - Standard ICs

NXP Semiconductors
UM10211
Chapter 26: LPC2300 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.
Table 480. 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
9. Battery RAM
The Battery RAM is a 2 kbyte static RAM residing on the APB bus. The address range is
0xE008 4000 to 0xE008 47FF. The Battery RAM is powered from the VBAT pin along with
the RTC, both of which exist in a power domain that is isolated from the rest of the chip.
This allows them to operate while the main chip power has been removed.
10. RTC external 32 kHz oscillator component selection
The RTC external oscillator circuit is shown in Figure 26–104. 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.
UM10211_1
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User manual Rev. 01 — 27 March 2007 511 of 613