August
2004, Issue 169
Ham
Radio Repeater Locator
GPS
DATA
Most
small GPS receivers have an output in NMEA-0183 format,
which is an asynchronous serial datastream of ASCII
characters sent at 4,800 bps (start bit, 8 data bits,
1 stop bit). The signal is nominally 0 V for a mark
condition and 5 V for a space condition. Connecting
a terminal or terminal emulator can monitor the GPS
output. Most computers with an RS-232 input will work
correctly even though these levels are not RS-232 standard.
I
used a Garmin GPSMAP 295 for this project, but almost
any GPS receiver should work. It may be necessary to
look at the output format of your receiver and modify
the parsing routines for different formats. The routine
to parse the GPS’s latitude and longitude output was
written for easy modification.
CIRCUIT
SPECIFICS
The
system is intended for mobile use, so power comes from
your vehicle’s 12-V battery. A 5-V regulator supplies
the LCD module, and a 3.3-V regulator (driven by the
output of the 5-V regulator) is used to drive the Zilog
microcomputer. Because the microcomputer outputs are
5-V tolerant, they can be used to drive the display
directly.
A
10-MHz crystal supplies the system clock. The crystal
frequency is not important, but if a different crystal
is used, the UART clock division must be changed accordingly.
A
single transistor interfaces the GPS NMEA-0183 output,
which varies between 0 and 5 V, to the 3.3-V UART input
of the microcontroller. The transistor also provides
the necessary phase inversion.
A
normally open switch is used to signal the microcontroller
to select the next station for frequency and tone display.
Another switch is used to toggle the sticky bit for
the selected station.
As
an aid to debugging, and to let you know that all is
well, a red LED is connected to one of the microcontroller’s
I/O lines. It is often useful to have such an indicator
during the initial stages of hardware and software development.
As I said earlier, the LED blinks once per second.
An
interface circuit is provided so the HRRL can automatically
program your ham radio transceiver. The one shown in
Figure 3 is designed for use with the Icom 746-PRO “CIV”
bus. A different circuit may be needed for other transceiver
brands.
As
you can see in Photo 2, there are three ministereo audio
connectors on the back of the HRRL that are used for
serial interfaces: the Zilog debug interface, the output
to program the 2-m radio (CIV), and a connector for
a serial interface to a PC (not currently used). A future
version of the program will use this interface for programming
the database. (The current version requires a recompile
of the software to update the database followed by a
download of the new code through the debug port.)
|
(Click
here to enlarge)
|
Photo
2—Moving from left to right, the rear panel has
wires to 12 VDC (to a cigarette lighter socket),
the power switch, three ministereo jacks that connect
to a PC serial port for programming, a PC serial
port running a terminal emulator (currently not
used), and the Icom IC-746PRO programming bus. The
wires on the right go to the GPS receiver. |
SOFTWARE
OPERATION
The
system is interrupt-driven because it is necessary to
be able to take care of inputs that come asynchronously.
A background process takes care of the major function—calculating
the distance to all the stations in the database.
There
are two interrupt routines: GPS character received and
timer 10-ms interrupt. The background routine cycles
through the database of repeaters and calculates the
distance to each one. A global variable gives the current
latitude and longitude values. Because this is can be
changed by the GPS receiver, the background disables
interrupts before copying the latitude and longitude
information to a local variable, which is then used
throughout the main loop.
The
GPS receiver sends characters at 4,800 bps. A line of
characters giving latitude, longitude, and time is sent
twice per second.
A
state machine guides the parsing of the GPS data with
a simple string defining the parse operations. The state
of the parse is an index into the parse string. When
a new character comes along, it is handled according
to the definition in the parse string. This scheme makes
it extremely easy to change the parsing according to
your needs. For example, it’s easy to add a checksum
calculation. Information about signal strength, date,
and altitude can be extracted from the GPS datastream.
A
timer interrupt occurs every 10 ms. A number of different
things happen in response to a timer interrupt: a counter
is incremented to determine if valid GPS information
hasn’t arrived for several seconds; the station-select
switch is polled to see if you want to display information
for a different station, and switch debounce is also
handled; the sticky-bit switch is serviced; the LED
is turned on and off at a 1-s rate; and a new character
is sent to the LCD if it needs service.
Commonly
used LCD modules don’t have a signal to indicate that
they’re ready for the next character to be displayed.
They will not accept characters at the rate that the
microcontroller could send them. Therefore, it’s necessary
either to poll the display to see if it’s ready for
another character or wait for a specified time (often
approximately 40 µs) before sending the next character.
In
order to avoid the loss of CPU cycles required by the
polling method, the software uses the delay approach.
A new character is sent every 10 ms to the LCD (if there
are characters remaining to be sent) in response to
a timer interrupt. This is much slower than possible,
but it’s faster than what’s necessary for this application.
A
buffer of the LCD characters is kept in the lcdBuf array.
Although it isn’t strictly necessary to buffer the LCD
information, the microcontroller is blessed with plenty
of RAM. This approach makes the code clean and straightforward.
Whenever
a routine finishes writing to the buffer, it increments
a line-to-be-written byte (ltbw) that indicates to the
interrupt routine that the corresponding line of characters
should be written to the display.
