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P87LPC779

CMOS single-chip 8-bit 80C51 microcontroller with 128-byte data RAM, 8 kB OTP

Philips Electronics 

Philips Semiconductors

P87LPC779

CMOS single-chip 8-bit microcontroller

9397 750 13213

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When an A/D conversion is started, Power-down or Idle mode must be activated

within two machine cycles in order to have the most accurate A/D result. These two

machine cycles are counted at the CPU clock rate. When using the A/D with either

Power-down or Idle mode, care must be taken to insure that the CPU is not restarted

by another interrupt until the A/D conversion is complete. The possible causes of

wake-up are different in Power-down and Idle modes.

A/D accuracy is also affected by noise generated elsewhere in the application, power

supply noise, and power supply regulation. Since the P87LPC779 power pins are

also used as the A/D reference and supply, the power supply has a very direct affect

on the accuracy of A/D readings. Using the A/D without Power-down mode while the

clock is divided through the use of CLKR or DIVM has an adverse effect on A/D

accuracy.

8.4.2 Code examples for the A/D

The first piece of sample code shows an example of port configuration for use with

the A/D. This example sets up the pins so that all four A/D channels may be used.

Port configuration for analog functions is described in Section 8.2 “Analog functions”

on page 12.

; Set up port pins for A/D conversion, without affecting other pins.

mov PT0AD,#78h ; Disable digital inputs on A/D input pins.

anl P0M2,#87h ; Disable digital outputs on A/D input pins.

orl P0M1,#78h ; Disable digital outputs on A/D input pins.

Following is an example of using the A/D with interrupts. The routine ADStart begins

an A/D conversion using the A/D channel number supplied in the accumulator. The

channel number is not checked for validity. The A/D must previously have been

enabled with sufficient time to allow for stabilization.

The interrupt handler routine reads the conversion value and returns it in memory

address ADResult. The interrupt should be enabled prior to starting the conversion.

; Start A/D conversion.

ADStart:

orl ADCON,A ; Add in the new channel number.

setb ADCS

; Start an A/D conversion.

; orl PCON,#01h ; The CPU could be put into Idle mode here.

; orl PCON,#02h ; The CPU could be put into Power-down mode here if

RCCLK = 1.

ret

; A/D interrupt handler.

ADInt:

push ACC

; Save accumulator.

mov A,DAC0

; Get A/D result, by reading DAC0 SFR

mov ADResult,A ; and save it in memory.

clr ADCI

; Clear the A/D completion flag.

anl ADCON,#0fch ; Clear the A/D channel number.

pop ACC

; Restore accumulator.

ret

Following is an example of using the A/D with polling. An A/D conversion is started

using the channel number supplied in the accumulator. The channel number is not

checked for validity. The A/D must previously have been enabled with sufficient time

to allow for stabilization. The conversion result is returned in the accumulator.

Rev. 02 — 03 May 2004

© Koninklijke Philips Electronics N.V. 2004. All rights reserved.

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