RCTIME - BASIC Stamp Command Reference
Page 258 • BASIC Stamp Programming Manual 2.0b • www.parallaxinc.com
Charge time = 4 x 220 x (0.1 x 10
-6
) = 88 x 10
-6
So it takes only 88 µs for the cap to charge/discharge, meaning that the 1
ms charge/discharge time of the example is plenty.
A final note about Figure 5.29: You may be wondering why the 220
Ω
resistor is necessary at all. Consider what would happen if resistor R in
Figure 5.29a were a pot, and were adjusted to 0
Ω. When the I/O pin went
high to discharge the cap, it would see a short direct to ground. The 220
Ω
series resistor would limit the short circuit current to 5V/220
Ω = 23 mA
and protect the BASIC Stamp from damage. (Actual current would be
quite a bit less due to internal resistance of the pin’s output driver, but you
get the idea.)
Demo Program (RCTIME1.bs2)
' This program shows the standard use of the RCTIME instruction measuring an RC
' charge/discharge time. Use the circuit in the RCTIME description (in the manual)
' with R = 10 k pot and C = 0.1 µf. Connect the circuit to pin 7 and run the program.
' Adjust the pot and watch the value shown on the Debug screen change.
'{$STAMP BS2} 'STAMP directive (specifies a BS2)
Result VAR WORD 'Word variable to hold result.
Again:
HIGH 7 'Discharge the cap
PAUSE 1 'for 1 ms.
RCTIME 7, 1, Result 'Measure RC charge time.
DEBUG CLS, DEC Result 'Show value on screen.
GOTO Again
2
2
2
NOTE: This is written for the BS2
but can be used for the BS2e,
BS2sx and BS2p also. Locate the
proper source code file or modify
the STAMP directive before
downloading to the BS2e, BS2sx or
BS2p.
N
OTES ABOUT 220 Ω RESISTER IN
THE
CIRCUITS.
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