P87LPC779FDH datasheet(16/74 Pages) PHILIPS

16 / 74 page

Philips Semiconductors

P87LPC779

CMOS single-chip 8-bit microcontroller

Product data

Rev. 02 — 03 May 2004

16 of 74

9397 750 13213

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.

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