28128 Parallax Inc, 28128 Datasheet - Page 152

• Manufacturer Part Number: 28128
• Description: KIT BASIC ANALOG/DIGI PARTS ONLY
• Manufacturer: Parallax Inc

Specifications of 28128

• Accessory Type: ADC, DAC
• Product: Microcontroller Accessories
• For Use With/related Products: BASIC Stamp® 2 and Board of Education
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 142 · Basic Analog and Digital
In Chapter #7, the amount of time it takes a capacitor to charge in an RC circuit with a
step input was introduced. The same principle applies here. A step will be applied to the
circuit, and the amount of time it takes the capacitor to charge will be measured. Here's
how it works.
The BASIC Stamp must be programmed to set P15 (the I/O pin we'll be using) high for a
few milliseconds to let the voltage across the capacitor approach 0 volts. The reason the
voltage across the capacitor approaches 0 volts is because Vdd is 5 volts at the positive
terminal of the capacitor, and P15 is set to 5 volts at negative terminal. So, the voltage
across the capacitor is 5-5 volts = 0 volts.
As soon as the voltage across the capacitor is close enough to 0 volts, P15 is set to
function as an input. Immediately, the BASIC Stamp starts counting in 2 microsecond
increments. As soon as P15 becomes an input, it doesn't have an output voltage of 5 volts
any more. P15 behaves like an open circuit as far as the RC circuit is concerned.
All of the sudden, the circuit is different. It's a resistor and a capacitor in series. The
capacitor starts with no charge, but the circuit has 5 volts across it (Vdd – Vss), so the
capacitor starts to charge up. As the capacitor starts charging, the voltage at P15 starts
falling. The voltage at P15 will fall until it crosses the BASIC Stamp's I/O pin threshold
voltage. It takes a certain amount of time for the voltage at P15 to decay from 5 volts, to
the 1.4 volt threshold voltage of P15.
The BASIC Stamp can be used to measure the amount of time it takes the output to drop
from 5 to 1.4 volts, and this measurement can be used to determine an unknown
capacitance or resistance.
The equation for the voltage at P15 is given by:
−
t
×
=
×
R
C
V
V
e
P
15
dd
This equation is widely used in many fields, and it's called the exponential decay
equation. In our case, it's an equation of exponentially decaying voltage. Figure 8-2
shows the decays of three RC circuits that differ only in resistance. Each of the voltage
outputs takes a different amount of time to decay from 5 volts to 1.4 volts.