The SAA1575HL is an integrated circuit which implements
a complete baseband function for Global Positioning
System (GPS) receivers. It combines a 16-bit Philips
80C51XA microcontroller, 8 GPS channel correlators and
related peripherals in a single IC. Users can implement a
complete GPS receiver using only the SAA1575HL, the
UAA1570HL front-end Philips IC (or similar), external
memory and a few discrete components.
The IC is aimed at low cost applications. A low power
solution was also used where possible, although this was
of secondary importance to cost. The core of the
SAA1575HL operates at 3 V.
However, for compatibility with current automotive
applications, the periphery is supplied from separate pins
and can be operated between 3 and 5 V, as required.
The function of the SAA1575HL is to read the 1 or 2-bit
sampled IF bitstream from a front-end IC and, under
control of firmware on an external ROM, calculate the full
GPS solution. The results are communicated to a host in
National Maritime Electronics Association (NMEA) format
via a standard serial port. A second serial port can be used
to provide differential GPS information to the processor for
more advance applications. In addition, various other
functions are integrated onto the IC such as a real-time
GPS clock, a power-down/reset controller, timer/counters
and a watchdog timer.
To summarise, the SAA1575HL has the following
functional units:
- 16-bit 80C51XA microcontroller core
- 2 kbytes words on-chip SRAM (16-bit words)
- 8 GPS channel correlators
- 2 UARTs
- 8 general purpose I/O lines
- 3 timer/counters
- 1 real-time clock
- 1 watchdog timer
- 1 power-down/reset controller.
The structure is based on a 16-bit microcontroller core
operating on all other units as memory mapped
peripherals and registers. A 16-bit data bus and a 19-bit
address bus are extended to external pins so that external
data and program memory can be accessed. On-chip
decoder circuits eliminate the need for external glue logic
for external memory access.
Each of the 8 GPS channel correlators includes a carrier
Numerically Controlled Oscillator (NCO), PN code
generator, phase rotator and low-pass filter. They
correlate the local PN sequence with the digitized input
GPS signal and generate the filtered correlation result for
the microcontroller. The firmware provided then generates
a navigation solution and provides standard GPS data
outputs to the user.
The GPS firmware is located in off-chip program memory. It processes the GPS signals from up to 8 satellites and
generates GPS information that can be output to the host processor through one of the two serial ports. Much of
hardware configuration of the SAA1575HL can be controlled by the firmware and so details such as the external bus
timing may change between firmware revisions. For the purpose of this document, the standard Philips firmware has
been assumed (release HD00).
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