Motorola Axiom CMD11E1 Guía de usuario descargar pdf (Pagina 93)

Chapter 9

Analog to Digital Conversion

9.1 Objective

To become familiar with Analog to Digital Conversion.

9.2 Background

In this lab you will learn how to use the built in analog to digital converter. In

an earlier lab you learnt how to read a digital input on one of the port A pins.

The input was either a 1 or a 0 depending on whether the pin voltage was above

or below 2.5 volts. In this lab you will learn how to measure a voltage more

precisely. HC11 has a built in analog to digital convertor that will measure an

analog voltage between a lower limit and a higher limit

and convert it to a an

8-bit value. The converted value would be zero if the voltage that is measured is

equal to the lower limit and the value will be 255 (biggest 8 bit number) when the

voltage is at the upper limit. For voltages in between, the converted value would

be linearly related, with the value rounded down to the nearest integer. Thus the

converted value is given by the formula

D =

255

V − V RL

V RH − V RL

It is important that you do not exceed these limits. In this experiment you will be using

the variable voltage supply that is available in PE7 pin. The voltage in this pin will be within

the acceptable limits. However, if you try to measure any voltage generated by some external

device, it is your responsibility to ensure that you do not exceed these limits.

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Motorola Axiom CMD11E1 Guía de usuario Pagina 93