Manual
the internal batteries or the external power source is available, an alternative path is provided
via D1 and DC_IN.
Q7-A provides a level translated active low flag to the processor when external power is
available thus allowing mode switching and the benefits of full power GPS to be realised.
SIRF_TX and SIRF_RX are the main UART outputs (non-inverted TTL, 57600,8N1) carrying
the NMEA data.
Bluetooth transceiver/processor and flash memory.
U3 is the BlueCore 2 single chip Bluetooth system incorporating 2.4GHz transmit/receive and
baseband functions.
Data originating from the GPS is presented to BC_RX, the BC2 UART input, for processing
and eventual transmission as RF. Timing for all processor and RF synthesis is provided by
the 16MHz crystal Y1.
When transmitting, the outgoing balanced RF is first matched and converted to 50 ohm
single ended by inductors L2, L3 and balun Y2. Filter Y3 provides harmonic and spurious
output rejection before signals are passed to the chip antenna. Note that DC is provided to
the push-pull output section of the BC2 via the balun.
In receive, the reverse order of operation applies – signals from the 50 ohm chip antenna are
first filtered (out of of band signals) then translated to a balanced input, under control of the
internal T/R switch.
U5 is a 4Mbit 1.8V flash memory that directly interfaces with the BC2.
Q13-A and Q13-B buffer and translate the PIO ports to provide LED indicators for low battery
(DS1, Red) and Bluetooth status (DS2, Blue).
Power supplies and logic translation.
3 x AAA cells provide the main (int) power and this supply goes via the reverse battery
protection device, Q5, to the 3V switching regulator, U13. Low battery status is monitored by
U9 which outputs an active low to both the SiRF and BC2 processors, via D5, when level
drops below 3.2V.
J3 is the external 5V DC input socket. D2 provides reverse polarity protection while U11, a
precision shunt regulator, conducts if the supply goes over 5.7V, saturating Q16 and shutting
off Q17. This open circuit over voltage protection serves to protect U13 but is not intended
to duplicate the full automotive load dump already provided in the 12V to 5V cigarette
adaptor.
When the On/Off switch, S3, is pressed, Q9 turns on and provides an enable to U13, the 3V
switchmode regulator. This provides power to U7, which outputs 1.8V to the BC2. The BC2
then outputs a logic high to PWR_ON, switching on Q8-A and effectively latching the power
switch on.
If the BC2 is already powered on then pushing the switch S3 again will turn on Q8-B,
providing the BC2 with a logic high on PWR_OFF and initiating the power down sequence.
Q18 and Q19 allow the BC2 to control the 3V GPS supply, saving power when the Bluetooth
link is lost or yet to be acquired.