Philips Semiconductors
Product data
Low power, low price, low pin count (20 pin)
microcontroller with 4 kbyte OTP
P87LPC764
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 V
ref
, 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.
CmpInit:
mov
anl
orl
mov
PT0AD,#30h
P0M2,#0cfh
P0M1,#30h
CMP1,#24h
call
anl
setb
setb
ret
delay10us
CMP1,#0feh
EC1
EA
;
;
;
;
;
;
;
;
;
;
;
;
;
;
;
Disable digital inputs on pins that are used
for analog functions: CIN1A, CMPREF.
Disable digital outputs on pins that are used
for analog functions: CIN1A, CMPREF.
Turn on comparator 1 and set up for:
– Positive input on CIN1A.
– Negative input from CMPREF pin.
– Output to CMP1 pin enabled.
The comparator has to start up for at
least 10 microseconds before use.
Clear comparator 1 interrupt flag.
Enable the comparator 1 interrupt. The
priority is left at the current value.
Enable the interrupt system (if needed).
Return to caller.
SU01189
Figure 5.
2003 Sep 03
11
PHILIPS [ NXP SEMICONDUCTORS ]
