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Tech Support: (888) 997-8267 ServoPAL (#28824): Servo Pulser and Timer
General Description
The ServoPAL is a tiny module that plugs in between your BASIC Stamp and two servo motors to pulse the motors so your PBASIC program doesn’t have to. In addition, it provides an “alarm clock” function to perform timing in the background while the BASIC Stamp is busy with other tasks.
Features
• Plugs in between servo headers and servos: no wiring necessary.
• Simplifies PBASIC programming for both standard and continuous-rotation servos.
• Pulses two servos continuously based on single pulses received from the BASIC Stamp.
• Provides an alarm output (200mS to 30 min delay), which can be set by a single pulse.
• All interfacing is done by pulsing: no serial protocols to learn.
• Runs from the servo’s power (up to 6.5VDC): no additional power source needed.
• Compact size: stackable side-to-side with additional units on 0.1” servo headers.
What’s Included
ServoPAL module
What You Need to Provide
• BASIC Stamp and carrier board (such as the BOE).
• One or two servo motors.
Installation
Installation of the ServoPAL is a simple:
1. Unplug the servo connectors from the servo headers.
2. Plug the ServoPAL into the servo headers. (See diagram below.)
3. Plug the servo connectors into the ServoPAL.
Gnd Gnd Vdd
Vdd /Inp
Alarm Gnd Gnd
Vdd Vdd Servo 0Servo 1
Top View                                  Servo Header                      Everything Connected
Below is a photo of the ServoPAL installed on a BOE-Bot’s P12/P13 servo header. The wheel servo
motor cables are plugged into the ServoPAL. In this case, the /Inp  input pin is P12, the Alarm  output pin is P13, the right servo is Servo 0, and the left servo is Servo 1.
Hardware Interface and Initialization
Interface to the ServoPAL is realized through its /Inp  input and Alarm  “output”. When the ServoPAL powers up, both lines are configured as normally-high inputs, pulled up to a nominal +5V through internal 20K to 50K resistances. Commands to the ServoPAL are sent as negative pulses to the /Inp  pin. If the alarm feature is not used, the BASIC Stamp can use the Alarm  pin for any other purpose. If this is done, however, you must be careful not to trigger an alarm inadvertently, or else a bus conflict may occur.
The ServoPAL runs autonomously on power obtained from the servo headers. Therefore, it does not reset when the BASIC Stamp resets, and continues to send out pulses during and after reset if it was sending them out before. But it can  be reset, nonetheless, by sending a pulse of 100mS duration to the /Inp  pin. Here’s the recommended initialization sequence for the ServoPAL in PBASIC:
'{$STAMP BS2}
'{$PBASIC  2.5}
nInp PIN12'Define the input pin.
Alarm PIN13'Define the alarm pin.
Restart:
INPUT nInp'Make sure nInp isn't being driven.
DO UNTIL nInp'Wait for ServoPAL to power up.
LOOP
LOW nInp'Set pin to an output and hold it low
PAUSE100'for100mS.
timeout of 5000ms exceeded
HIGH nInp'Raise the pin.
This sequence accomplishes a couple things:
1. In systems, like the BOE-Bot, where the servo power is switched on after the BASIC Stamp
powers up, it will wait for the ServoPAL to be turned on. It does this by setting the nInp line to an inpu
t and waiting for that line to go high, signifying not only that that the ServoPAL has powered up, but that it has come out of reset and engaged its internal pull-ups.
2. In case the ServoPAL was already running, it then sends a long reset pulse to terminate any
servo pulses, to abort any pending alarm, and to ready the ServoPAL for further commands. Programming Servo Pulses
Servo pulses are programmed in much the same way you’d send pulses directly to a servo motor, using the PBASIC PULSOUT command. The only differences are that the pulses are negative-going instead of positive and that both servos are programmed from the same pin. Here’s an example that will cause the Servo 0 output to begin pulsing with 1.5mS pulses. These pulses are automatically repeated every 10-15mS without further intervention from the BASIC Stamp.
PULSOUT nInp,750
Notice that this is identical to the statement that would have been used, had the servo been connected directly to the BASIC Stamp. Once the initialization code is executed, your program is primed for negative-going pulses, and you don’t need to do anything special to send them. Here’s what the waveforms look like that result from the above example:
/Inp
Servo 0
But what about Servo 1? There’s only one pin, so how does it get programmed? That’s easy: just send another pulse within 1mS of the first one, and that one will be assigned to Servo 1:
PULSOUT nInp,750
PULSOUT nInp,1000
The preceding code will program Servo 0 with a pulse width of 1.5mS and Servo 1 with a pulse width of 2.0mS. Here’s what the waveforms will look like:
/Inp
Servo 0
Servo 1
To terminate a sequence of pulses on /Inp, just wait at least 2mS before sending another pulse. This
signals the ServoPAL that the next pulse is intended for Servo 0.
Now, what if you want to program only Servo 1 and leave Servo 0 alone? That’s easy, as well. Just send a very short pulse (4 – 100µS) for Servo 0 first, then a pulse for Servo 1. The ServoPAL will interpret the short pulse as indicating that you don’t want to change Servo 0’s pulse stream, if one has already been programmed. Here’s an example:
PULSOUT nInp,2
PULSOUT nInp,500
Here’s what the resulting waveforms look like:
/Inp
Servo 0
Servo 1
And finally, what if you want to terminate a servo’s pulse stream? The ServoPAL will only send pulses to a servo that are between approximately 0.5 and 2.5mS. If it encounters a request for pulses well outside this range, it will cease sending pulses to the affected servo. Here’s how you can use this to stop sending pulses to a servo:
PULSOUT nInp,2000'Send a4mS pulse to Servo0.
Here’s the result:
/Inp
Servo 0
Servo 1
To summarize, here’s a table showing the various pulse durations and what they mean:
4 - 100µS Skip this servo. (Don’t change its output.)
0.5 - 2.5mS Program this pulse width into the affected servo.
4 – 30mS Kill the servo’s output.
100mS or more Reset the ServoPAL.
Pulse widths outside the individual ranges shown may have unpredictable effects.
Programming the Alarm Function
In addition to servo control, the ServoPAL has an alarm clock (timer) function. You can set an alarm to
occur anywhere from 200mS to 30 minutes after it’s set (triggered). For programming purposes, the timer is treated by the ServoPAL as a third servo: i.e. after sending pulses for Servo 0 and Servo 1, you
send a pulse for the timer. As soon as such a pulse is received, the Alarm output is pulled low. It will remain low until the programmed time elapses, whereupon it will again float and be pulled high by means
of the internal pullup resistor.
The relation between the pulse width sent by the BASIC Stamp and the actual time programmed into the
timer is
Programmed time = Pulse width * 50000 (approximately)
So a pulse width of 4µS yields a timeout of 200mS, and a pulse width of 36000µS yields a timeout of 30
minutes. In terms of PULSOUT units, here’s the timing for various BASIC Stamps:
BS1 BS2 BS2e BS2sx BS2p BS2pe BS2px Time per
PULSOUT unit 500mS 100mS 100mS 40mS 40mS 100mS 40mS Maximum
PULSOUT units 3600 18000 18000 45000 45000 18000 45000 Here’s a BS2 program fragment that sets the timeout period to 5 minutes:
PULSOUT nInp,4'Skip Servo0.
PULSOUT nInp,4'Skip Servo  1.
PULSOUT nInp,3000'Set timeout to300seconds.
And here’s the output waveform:
/Inp
Alarm 6mS
300000mS = 5 min.
One additional thing to note about the timer is that it’s retriggerable. What this means is that if you set it
before a previous setting has timed out, it will start over with the new timeout value. This feature makes the ServoPAL useful as a watchdog timer for the BS2p, BS2pe, and BS2px, which support automatic
polling. As long as you keep resetting the watchdog timer, the Alarm output will stay low. But if your p
rogram hangs up somewhere, the timer will eventually time out and the rising edge on Alarm can be used to (re)start the program specified by POLLRUN.
Simple Example Program
Here’s a simple-minded BS2 program that moves a BOE-Bot forward for 5 seconds, then in reverse for five seconds, using the Alarm output as a timer:
'=========================================================================
'
'ServoPAL_Simple_Demo.bs2
'Demonstrate ServoPAL capabilites on a BOE-Bot
'Parallax,Inc.
'support@parallax
'2007.05.01
'2007.10.29
'
'{$STAMP BS2}
'{$PBASIC  2.5}
'
'=========================================================================
'-----[Program Description]---------------------------------------------
'This simple demo of the ServoPAL,when used with a BOE-Bot,will move the
'robot forward for five seconds,then in reverse for five seconds.
'-----[I/O Definitions]-------------------------------------------------
nInp PIN12'Define the input pin.
Alarm PIN13'Define the alarm pin.
'-----[Initialization]--------------------------------------------------
INPUT nInp'Make sure nInp isn't being driven.
DO:LOOP UNTIL nInp'Wait for ServoPAL to power up.
LOW nInp'Set pin to an output and hold it low
PAUSE100'for100mS to reset ServoPAL.
HIGH nInp'Raise the pin.
PAUSE100
'-----[Program Code]----------------------------------------------------
PULSOUT nInp,500'Program right servo for full forward.
PULSOUT nInp,1000'Program left servo for full forward.
PULSOUT nInp,50'Program alarm for5seconds.
DO:LOOP UNTIL Alarm'Wait for Alarm.
PULSOUT nInp,1000'Program right servo for full reverse.
PULSOUT nInp,500'Program left servo fro full reverse.
PULSOUT nInp,50'Program alarm for5seconds.
DO:LOOP UNTIL Alarm'Wait for Alarm.
PULSOUT nInp,2000'Turn right servo off.
PULSOUT nInp,2000'Turn left servo off.
DO:LOOP
IR Roaming Program
The next example is an IR roaming program for the BOE-Bot. To use this program, you should conne
ct the IR emitters and sensors as shown in Parallax’s Robotics with the Boe-Bot, figure 7-4 (copied below).

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