4.14 Optional GPS interface
The GPS was a late addition to the design. It is added to
facilitate several useful features:
Calibration of the 27MHz reference oscillator
Calibration of the 20MHz system clock
For WSPR, discipline of the frequency (drift
correction)
For WSPR, setting and discipline of the real time
clock
For WSPR, setting the Maidenhead Locator square
The optional GPS interface is provided by a 4-pin header at the bottom right of the board,
next to the 3.5mm socket for the keyer. The pinout of the 4-pin header exactly matches the
one on the QRP Labs Ultimate3S QRSS/WSPR transmitter kit, and the QRP Labs QLG1
GPS receiver kit. This makes connecting the radio to the QLG1 GPS kit very easy, if you
wish. However other GPS modules could also be used. The following requirements of the
GPS module must be met:
Positive-going (leading edge) 1 pulse per second signal with a reasonable pulse
width – e.g. some older GPS module have a 1us pulse and this will not be long
enough. The QLG1 GPS receiver provides a 100ms (0.1s) duration pulse, though
shorter pulses would also work.
9600 baud serial data sending standard NMEA 0183 GPS sentences
“TTL” voltage levels i.e. 0V “low” and 5V “high” (or at least, below 1.5V and above
3.5V)
The 1pps and Serial data signals from the GPS are connected via 1K resistors, to the “dit”
and “dah” inputs to the microcontroller. The GPS is therefore wired directly in parallel with
the paddle. The GPS may not be used at the same time as the paddle. This would cause
the transmitter to be keyed by the 1pps and serial data signals. The 1K resistors protect the
GPS outputs in case the paddles inadvertently ground them. Always put the kit into
Practice Mode (see menu 4.7) before connecting a GPS, to avoid accidentally frying
the PA.
The ATmega328P has a USART serial data peripheral. However, the pin assignments were
already made, so far down the design process, at the time the GPS receiver option was
included. Furthermore, the I/O pin availability was already scarce. So, the two GPS signals
share the same I/O as the paddle inputs. A software-emulated USART peripheral module
now reads the GPS serial data, at 9600 baud.
If you connect a GPS and forget to put the kit in practice mode, it will automatically
recognize serial data at the paddle input and temporarily enable practice mode (a P
appears in the display) to product the PA circuit.
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