OM11077 NXP Semiconductors, OM11077 Datasheet - Page 657

• Manufacturer Part Number: OM11077
• Description: MODULE DIMM LPC2478 ARM7
• Manufacturer: NXP Semiconductors

Specifications of OM11077

• Accessory Type: Module Card
• For Use With/related Products: ARM-57TS-LPC2478
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 568-4742

NXP Semiconductors
UM10237_4
User manual
6.7.2 Prescaler Integer Register (PREINT - 0xE002 4080)
6.7.3 Prescaler Fraction Register (PREFRAC - 0xE002 4084)
6.7.4 Example of Prescaler Usage
Table 579. Reference Clock Divider registers
This is the integer portion of the prescale value, calculated as:
PREINT = int (PCLK/32768) - 1. The value of PREINT must be greater than or equal to 1.
Table 580: Prescaler Integer register (PREINT - address 0xE002 4080) bit description
This is the fractional portion of the prescale value, and may be calculated as:
PREFRAC = PCLK - ((PREINT + 1) x 32768).
Table 581: Prescaler Integer register (PREFRAC - address 0xE002 4084) bit description
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
Name
PREINT
PREFRAC 15
Bit
12:0
15:13
Bit
14:0
15
Symbol
Prescaler Integer Contains the integer portion of the RTC prescaler value.
-
Symbol
Prescaler
Fraction
-
Size
13
Description
Prescale Value, integer portion
Prescale Value, fractional portion
Rev. 04 — 26 August 2009
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
Description
Reserved, user software should not write ones to reserved
bits. The value read from a reserved bit is not defined.
Description
Contains the integer portion of the RTC prescaler value.
Reserved, user software should not write ones to reserved
bits. The value read from a reserved bit is not defined.
Access
R/W
R/W
UM10237
© NXP B.V. 2009. All rights reserved.
0xE002 4080
0xE002 4084
Address
657 of 792
Reset
Value
0
NA
Reset
Value
0
NA