SAA1575HL Datasheet, PDF (18/56 Pages) NXP Semiconductors – Global Positioning System GPS baseband processor

Philips Semiconductors
Global Positioning System (GPS)
baseband processor
Product specification
SAA1575HL
7.6 The real-time clock
The Real-Time Clock (RTC) is a functional unit used to
generate time information. Its purpose is to supply
approximate GPS time to the system firmware for the initial
acquisition of satellites (a warm start). The power supply
for the RTC is separate from the rest of the IC, allowing a
low capacity battery to be used to maintain the low power
RTC function.
The timebase for the RTC should be provided by a
dedicated 32.768 kHz crystal which can be omitted if the
RTC is not required. This is divided down by a fixed divider
to provide the 1 Hz timebase used for the rest of the RTC
block. A digital sampling circuit is also included to prevent
digital noise due to the on-chip processor causing
incorrect timekeeping.
The SAA1575HL uses a digital under-sampling system to
ensure that ground bounce does not cause RTC
timekeeping errors. This places a restriction on the ratio of
XTAL1 and XTAL3 frequencies for which the RTC will
operate correctly. This has been optimistic for the case
f_XTAL1 = 30 MHz, f_XTAL3 = 32 kHz and, assuming that the
RTC crystal frequency will always be 32 kHz, will operate
correctly for the entire specified range of system
frequencies.
handbook, full pagewidth_off-chip
C
XTAL
(optional)
C
XTAL3
32 kHz
32 kHz
1 Hz
OSCILLATOR
SAMPLER
PRE-SCALER
REAL-TIME
CLOCK COUNTERS
XTAL4
system clock
MHB468
Fig.9 Real-time clock circuit.
1999 Jun 04
18