Nxp Semiconductors UM10237 ユーザーズマニュアル

UM10237_2

© NXP B.V. 2008. All rights reserved.

PREINT = int (PCLK/32768) - 1. The value of PREINT must be greater than or equal to 1.

 

This is the fractional portion of the prescale value, and may be calculated as:

PREFRAC = PCLK - ((PREINT + 1) x 32768).

 

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

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(

12:0

Prescaler Integer

Contains the integer portion of the RTC prescaler value.

0

15:13

-

Reserved, user software should not write ones to reserved 
bits. The value read from a reserved bit is not defined.

NA

14:0

Prescaler 
Fraction

Contains the integer portion of the RTC prescaler value.

0

15

-

Reserved, user software should not write ones to reserved 
bits. The value read from a reserved bit is not defined.

NA