7
1pps gate is used to implement a frequency counter that measures the ¼ reference oscillator value.
The measured value is fed into the computation of the register values to achieve the desired output
frequency.
The GPS discipline mechanism uses a two-stage process to achieve an accurate output frequency
quickly but with minimum “jitter” once done. The first stage is a coarse adjustment to get close to the
required final value. The subsequent fine-tuning adjustment varies the measured 27MHz value
retained internally, in steps of 0.01Hz so that the jitter from one second to the next is minimised.
Additionally the coarse value is stored in EEPROM so that the next time the kit is switched on, the
output frequency will already be near its correct value, and lock will be faster.
The baud rate setting specifies the speed of the serial data coming from the GPS. At the moment this
serial data is not used by the VFO/Signal Generator kit.
GPS Mode settings:
0 GPS is not used at all. The kit is in free-running mode
. The 27MHz oscillator won’t be
measured and disciplined (compensated) by the GPS.
1 The kit triggers on the falling (negative) edge of the 1pps signal.
2 The kit triggers on the rising (positive) edge of the 1pps signal. This is appropriate for most
GPS modules, where the pulse width is 100ms for example.
Baud Rate setting (default 9,600 baud):
This second setting on the GPS screen determines the baud rate for receiving serial data input from
the GPS module. It must be set to match the data output from the GPS module otherwise no data
can be decoded. You will need to refer to your GPS module documentation to determine the correct
speed.
Any baud rate can be entered here. 9,600 is most common and is the default. The kit should be able
to support 115kbps but higher than this may be subject to inaccuracies and may not function
properly.
The kit uses no parity, 1 stop bit, and 8-bit data. I have never encountered a GPS module that does
not use the same settings, which appear to be very standard.
Line 1
#FQ #EE
The Line 1, Line 2, Line 3 and Line 4 configuration settings determine the display format. Line 1 is the
top row of the LCD, Line 4 is the bottom row. Configuration parameters for lines 3 and 4 are only
visible if you have set the LCD Rows parameter to 4 (see later section). The kit is supplied with a 2-
row, 16-column LCD. However you can connect a larger (4 row) HD44780-compatible display. You
can enter free fixed text (for example, your callsign). You can also include #tags. Each tag is a #
