28824 Parallax Inc, 28824 Datasheet - Page 5

• Manufacturer Part Number: 28824
• Description: SERVOPAL SERVO PULSER TIMER
• Manufacturer: Parallax Inc

Specifications of 28824

• Accessory Type: Servo Motor Control
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: BASIC Stamp®
• For Use With: 28803 - KIT BOARD OF EDU W/O PWR SUPP
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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
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:
Here’s a BS2 program fragment that sets the timeout period to 5 minutes:
PULSOUT nInp, 4
PULSOUT nInp, 4
PULSOUT nInp, 3000
And here’s the output waveform:
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
program 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.
© Parallax, Inc. • ServoPAL (2007.10.30)
Alarm
Time per
PULSOUT unit
Maximum
PULSOUT units
/Inp
4 – 30mS
100mS or more
6mS
500mS
3600
Programmed time = Pulse width * 50000 (approximately)
BS1
'Skip Servo 0.
'Skip Servo 1.
'Set timeout to 300 seconds.
100mS
18000
BS2
Reset the ServoPAL.
Kill the servo’s output.
100mS
18000
BS2e
300000mS = 5 min.
BS2sx
45000
40mS
45000
BS2p
40mS
BS2pe
100mS
18000
Page 5 of 9
BS2px
45000
40mS