P87LPC764FN datasheet(12/60 Pages) PHILIPS

12 / 60 page

Philips Semiconductors

Product data

P87LPC764

Low power, low price, low pin count (20 pin)

microcontroller with 4 kbyte OTP

2003 Sep 03

Internal Reference Voltage

An internal reference voltage generator may supply a default

reference when a single comparator input pin is used. The value of

the internal reference voltage, referred to as Vref, is 1.28 V ±10%.

Comparator Interrupt

Each comparator has an interrupt flag CMFn contained in its

configuration register. This flag is set whenever the comparator

output changes state. The flag may be polled by software or may be

used to generate an interrupt. The interrupt will be generated when

the corresponding enable bit ECn in the IEN1 register is set and the

interrupt system is enabled via the EA bit in the IEN0 register.

Comparators and Power Reduction Modes

Either or both comparators may remain enabled when Power Down

or Idle mode is activated. The comparators will continue to function

in the power reduction mode. If a comparator interrupt is enabled, a

change of the comparator output state will generate an interrupt and

wake up the processor. If the comparator output to a pin is enabled,

the pin should be configured in the push-pull mode in order to obtain

fast switching times while in power down mode. The reason is that

with the oscillator stopped, the temporary strong pull-up that

normally occurs during switching on a quasi-bidirectional port pin

does not take place.

Comparators consume power in Power Down and Idle modes, as

well as in the normal operating mode. This fact should be taken into

account when system power consumption is an issue.

Comparator Configuration Example

The code shown in Figure 5 is an example of initializing one

comparator. Comparator 1 is configured to use the CIN1A and

CMPREF inputs, outputs the comparator result to the CMP1 pin,

and generates an interrupt when the comparator output changes.

The interrupt routine used for the comparator must clear the

interrupt flag (CMF1 in this case) before returning.

SU01189

CmpInit:

mov

PT0AD,#30h

; Disable digital inputs on pins that are used

;

for analog functions: CIN1A, CMPREF.

anl

P0M2,#0cfh

; Disable digital outputs on pins that are used

orl

P0M1,#30h

;

for analog functions: CIN1A, CMPREF.

mov

CMP1,#24h

; Turn on comparator 1 and set up for:

;

– Positive input on CIN1A.

;

– Negative input from CMPREF pin.

;

– Output to CMP1 pin enabled.

call

delay10us

; The comparator has to start up for at

;

least 10 microseconds before use.

anl

CMP1,#0feh

; Clear comparator 1 interrupt flag.

setb

EC1

; Enable the comparator 1 interrupt. The

;

priority is left at the current value.

setb

; Enable the interrupt system (if needed).

ret

; Return to caller.

Figure 5.

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