SAA1575HL Datasheet, PDF (26/56 Pages) NXP Semiconductors – Global Positioning System GPS baseband processor

Philips Semiconductors
Global Positioning System (GPS)
baseband processor
Product specification
SAA1575HL
7.9.2 RTC CLOCK (XTAL3)
If the on-chip real-time clock is required (as with the
standard Philips firmware), a low frequency clock signal is
required to run the clock. The SAA1575HL is designed so
that a standard 32.768 kHz watch crystal can be used for
this purpose. Since this is much slower than the system
clock, a much lower power is required to run just the
real-time clock, allowing it to be powered from a
low-capacity battery when the main power supply fails.
As with the system clock, there is an on-chip oscillator so
that only a few passive external components are required.
These would be an external crystal accompanied by two
capacitors connected to ground, a series resistor
(optional) and a shunt resistor to ensure start-up under all
conditions.
Optimum values of C and R_P will depend on the crystal
used. However, typical values would be C = 22 pF and
R_P = 1 MΩ.
7.9.3 REFERENCE CLOCK (RCLK)
The reference clock input, RCLK, is used as the source for
the sampling of the IF input signal. A divided-down version
of RCLK is output on the sample clock pin, SCLK, for use
by the front-end IC.
The division ratio of RCLK/SCLK is programmable in
firmware. In the standard Philips firmware this ratio is set
to 3.
handbook, half_op_fa_fg_-ce_hip
C
XTAL RP
(optional)
C
on-chip
XTAL3
OSCILLATOR
RTC clock
XTAL4
MHB473
Fig.14 RTC clock oscillator circuit.
1999 Jun 04
26