Parallax BASIC Stamp 2e Programming Manual page 260

RCTIME - BASIC Stamp Command Reference
Charge time = 4 x 220 x (0.1 x 10
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}
Result VAR WORD
Again:
HIGH 7
PAUSE 1
RCTIME 7, 1, Result
DEBUG CLS, DEC Result
GOTO Again
Page 258 • BASIC Stamp Programming Manual 2.0b • www.parallaxinc.com
-6 -6
) = 88 x 10
'STAMP directive (specifies a BS2)
'Word variable to hold result.
'Discharge the cap
'for 1 ms.
'Measure RC charge time.
'Show value on screen.
220 Ω
N
OTES ABOUT RESISTER IN
RC .
THE CIRCUITS
2 2 2
2
e p
sx
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.