Introduction.
We have completed the development of two timer circuits which may be
used with a PC Com
Port or with a Basic Stamp or similar. They may be also be used in
standalone mode with the
result being displayed on a serial LCD.
Typical applications include SONAR ranging or any similar application
where a process is
initiated and one desires to know the amount of time until a signal is
returned. With a SONAR, a
pulse is launched and one measures the amount of time to receive the echo.
With a Pinewood
Derby, one might cause a solenoid to release the racer and measure the
amount of time until the
racer breaks an optical path at the finish.
The inspiration for these timers was my frustration in attempting to
interface a Poloroid 6500
Sonar Ranging device with a PC. A PC does not resolve time to sufficient
accuracy and I soon
discovered I needed a great deal of external timing circuitry and in fact,
spent many weeks with
various external oscillator, counter and latch arrangements. It worked,
but by the time it was
completed there was a great deal of circuitry. There just had to be a
better way!
(Please note that these timers may be used as an interface between a PC
or Stamp and a Polaroid
6500 SONAR. The SONAR package is available from Ben Wirz at .
Details.
The new designs use the Microchip PIC12C508 in an 8-pin DIP package
plus a three terminal
ceramic resonator. They may be interfaced directly with such processors
as the Basic Stamp or
with a PC using a MAX232 level converter.
When input /GO is brought to a logic zero, output Q_RISE is brought to
a logic one. This might
be used to launch a pulse using the Polaroid 6500 SONAR. The processor
then times until
receipt of a positive logic level on input ECHO. The time is then
serially transmitted to the PC
(or Stamp) at 9600 Baud.
When interfacing with a PC, the process may be initiated by bringing
the PC serial TX_D low (a
start bit). This may be done by sending any character to the timer. When
interfacing with a
Stamp, this may be done by momentarily bringing a lead low.
Upon completion of the timing, the amount of elapsed time is then
returned to the PC or Stamp at
9600 8N1 in decimal format.
Two Models.
Two different models are available.
TIMER_1 has a resolution of 10 usecs and would be suitable for SONAR
applications. The data
is returned as the number of msecs followed by a newline character.
Examples.
0.37
5.99
66.78
324.23
The maximum time is 655.35 msecs. If an "echo" signal is not found
within this time, a value of
655.35 is returned.
TIMER_2 has a resolution of 1 msec and would be more suitable for
Pinewood Derby
Applications. The data is returned as the number of seconds followed by a
newline character.
Examples;
0.720
5.998
63.335
The maximum time is 65.535 seconds. If the response from the system is
not received within this
time, a value of 65.535 is returned.
Clock Inaccuracy.
The PIC's internal 4.0 MHz oscillator proved to be inadequate. An
initial six percent error plus
additional error due to variations in temperature and voltage supply is
not sufficiently accurate for
serial communication, let alone the accuracy of the timing measurement.
Thus, both of these use an external 4.0 MHz ceramic resonator having an
accuracy of 0.5 percent.
A 4.0 MHz TTL clock oscillator may also be used for greater accuracy.
However, recognize that in a given application, you are using the same
resonator at close to the
same voltage supply and the same temperature. That is, the clock
frequency is repeatable and
thus the error may be "corrected" in your software using a scaling factor.
For example, in using
your SONAR you may find you are getting readings of 36.2 inches when in
fact, the target is 36.0
inches. Knowing this, you can then correct your readings by multiplying
by 36.2/36.0.
Local LCD Output.
The unit may also be interfaced with our PIC-n-LCD serial 20X4 LCD to
provide a local display.
Thus, with each reading the data is sent to the PC or Stamp via the
MAX232. In addition, the
value is also displayed on the PIC-n-LCD.
This permits the user to employ these timers in a standalone mode
without a PC or Stamp. Thus
the /GO signal might be applied by momentarily depressing a pushbutton.
The PIC then brings
PIC output Q_RISE high which initiates the process. The elapsed time may
then be observed on
the LCD display.
Pricing.
We have packaged this as a TIMER_KIT. This kit is intended for
evaluation and includes one
TIMER_1 and one TIMER_2 PIC. It also includes one 4.0 MHz ceramic
resonator, one
MAX232 plus the four associated capacitors, our logic probe, and a
resistor and a capacitor for
"simulating" a SONAR plus a small supply of 22 AWG wire suitable for
breadboarding on a
solderless breadboard. The package also includes documentation including
assembly, sample
applications and sample software. The TIMER_KIT is $16.00.
Additional TIMER_1 and TIMER_2 PICs may be purchased for $3.50 each.
Note that this
includes only the programmed PIC.
4.0 MHz ceramic resonators are available for $0.75, 4.0 MHz TTL clock
oscillators for $2.00 and MAX232s with the four 1.0 uFd capacitors for
$2.00.
Return Policy.
As with all of our manuals and kits, these items may be returned for
full credit if they don't meet
your needs or expectations.