27220 Parallax Inc, 27220 Datasheet - Page 168

• Manufacturer Part Number: 27220
• Description: BOOK STAMPWORKS
• Manufacturer: Parallax Inc

Specifications of 27220

• Accessory Type: Booklet
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• For Use With/related Products: StampWorks
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Not applicable / Not applicable

Page 158 · StampWorks
Behind the Scenes
Previous projects have used RCTIME to read resistive components. This is a form of
analog input, but isn’t voltage measurement. For that, the BASIC Stamp needs help
from an external device. The simplest way to measure a variable voltage is with an
analog-to-digital converter.
In this experiment, the National Semiconductor ADC0831 is used to convert a voltage
(0 – 5) to a synchronous serial signal that can be read by the BASIC Stamp with
SHIFTIN. One thing of note about the Read_0831 subroutine is that we specify
nine bits in SHIFTIN, even though the result is only eight bits? Why?
The ADC0831 requires one pulse on the clock line after being activated to do the
voltage conversion. The next eight clock pulses move the data out of the device as
shown in the illustration below:
The first clock pulse (gray) after the CS line goes low causes the ADC0831 to do the
voltage conversion. The MSBPOST mode is used with SHIFTIN as the data bits are
presented MSB first, and after the clock line falls. The POST modes sample the data
line after each clock pulse.
The voltage measurement – which is actually the positive difference between the
Vin+ (pin 2) and Vin- (pin 3) pins – will be a value between 0 and 255 (Vref). In our
first application we have connected Vin- to ground and Vref to Vdd; this gives us a
voltage span of 5.00 volts. Dividing five (volts) by 255, we find that each bit in the
result is equal to 19.6 millivolts. For display purposes, the result is converted to
millivolts by multiplying by 19.6 (result */ $139C).