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MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

Chapter 3

Analog-to-Digital Converter (ADC)

3.1 Introduction

This section describes the analog-to-digital converter (ADC). The ADC is an 8-bit, 4-channel analog-to-

digital converter. The ADC module is only available on the MC68HC908QY2, MC68HC908QT2,

MC68HC908QY4, and MC68HC908QT4.

3.2 Features

Features of the ADC module include:

•

4 channels with multiplexed input

•

Linear successive approximation with monotonicity

•

8-bit resolution

•

Single or continuous conversion

•

Conversion complete flag or conversion complete interrupt

•

Selectable ADC clock frequency

3.3 Functional Description

Four ADC channels are available for sampling external sources at pins PTA0, PTA1, PTA4, and PTA5.

An analog multiplexer allows the single ADC converter to select one of the four ADC channels as an ADC

voltage input (ADCVIN). ADCVIN is converted by the successive approximation register-based counters.

The ADC resolution is eight bits. When the conversion is completed, ADC puts the result in the ADC data

Figure 3-2 shows a block diagram of the ADC.

3.3.1 ADC Port I/O Pins

PTA0, PTA1, PTA4, and PTA5 are general-purpose I/O pins that are shared with the ADC channels. The

channel select bits (ADC status and control register (ADSCR), $003C), define which ADC channel/port

pin will be used as the input signal. The ADC overrides the port I/O logic by forcing that pin as input to the

ADC. The remaining ADC channels/port pins are controlled by the port I/O logic and can be used as

general-purpose I/O. Writes to the port register or data direction register (DDR) will not have any affect

on the port pin that is selected by the ADC. Read of a port pin which is in use by the ADC will return a 0

if the corresponding DDR bit is at 0. If the DDR bit is at 1, the value in the port data latch is read.

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