P83C266BDR datasheet(44/92 Pages) PHILIPS

44 / 92 page

1999 Mar 10

Philips Semiconductors

Product speciﬁcation

Microcontrollers for PAL/SECAM TV

with OSD and VST

P8xCx66 family

16 ANALOG-TO-DIGITAL CONVERTERS (ADC)

The 3 channel ADC consists of a 4-bit Digital-to-Analog

Converter (DAC), a comparator, an analog channel

selector and control circuitry. The block diagram of the

ADC circuit is shown in Fig.27.

One of these channels can be used to measure the level

of the Automatic Frequency Control signal. This is

achieved by comparing the ADC signal with the output of

a 4 bit DAC using the comparator (accuracy

±1⁄2 LSB).

The ADC inputs ADC0, ADC1 and ADC2 share the same

pins as general I/O port lines P3.0, P3.1 and P3.2,

respectively. Selection of the pin function as either an ADC

input or a general I/O port line is achieved using bits

ADCE0, ADCE1 and ADCE2 in SFR SAD2 (see

Section 16.2).

The conversion time of the ADC is equal to 8 machine

cycles (8

µs at 12 MHz). Some NOP instructions should be

added in between the instruction which changes the

reference voltage or the channel selection and the

instruction which reads the VHI register bit. The ST bit

must be set to a logic 1 at the same time as the reference

voltage or channel selection change, otherwise the VHI

state of the previous comparison will be read out.

The recommended procedure is as follows:

1. Write SFR SAD (X, CH<1:0>, ST = 1, SAD<3:0>)

2. Wait 8 machine cycles (for example 8 NOPs)

3. Read SFR SAD for VHI value.

ST is reset by hardware when it is set to a logic 1 by a

written instruction. When ST is read, the value is always a

logic 0.

Fig.27 ADC block diagram.

handbook, full pagewidth

4-BIT DAC

COMPARATOR

SAD3

SAD2

SAD1

SAD0

ADCE1

ADCE0

ADCE2

CH1

CH0

MGL397

ADC enable selection (SFR address: CAH)

AFC value selection (SFR address: C8H)

ENABLE

SELECTOR

ADC

CHANNEL

SELECTOR

P3.2/ADC2

P3.1/ADC1

P3.0/ADC0

Vref

DERIVATIVE PORT

SELECTOR

EN1

EN0

VHI

(SFR address: C8H)

internal bus

Channel selection

EN2

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