LPC2131/2132/2138 User Manual - mct.net

Philips Semiconductors Preliminary User Manual
ARM-based Microcontroller
Example of Prescaler Usage
In a simplistic case, the pclk frequency is 65.537 kHz. So:
PREINT = int (pclk / 32768) - 1 = 1 and PREFRAC = pclk - ((PREINT +1) x 32768) = 1
LPC2131/2132/2138
With this prescaler setting, exactly 32,768 clocks per second will be provided to the RTC by counting 2 pclks 32,767 times, and
3 pclks once.
In a more realistic case, the pclk frequency is 10 MHz. Then,
PREINT = int (pclk / 32768) - 1 = 304 and PREFRAC = pclk - ((PREINT +1) x 32768) = 5,760.
In this case, 5,760 of the prescaler output clocks will be 306 (305+1) pclks long, the rest will be 305 pclks long.
In a similar manner, any pclk rate greater than 65.536 kHz (as long as it is an even number of cycles per second) may be turned
into a 32 kHz reference clock for the RTC. The only caveat is that if PREFRAC does not contain a zero, then not all of the 32,768
per second clocks are of the same length. Some of the clocks are one pclk longer than others. While the longer pulses are
distributed as evenly as possible among the remaining pulses, this "jitter" could possibly be of concern in an application that
wishes to observe the contents of the Clock Tick Counter (CTC) directly.
Clk
13-bit Integer Counter
(Down Counter)
13
Reload
Underflow
13-bit Reload Integer Register
(PREINT)
Extend
Reload
To Clock Tick
Counter
15-bit Fraction Counter
Combinatorial Logic
15-bit Fraction Register
(PREFRAC)
13 VPB Bus
15
Figure 52: RTC Prescaler block diagram
Real Time Clock 217 November 22, 2004
15
15
Clk
pclk
(VPB Clock)