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Sheet目录3822

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2003 Microchip Technology Inc.

DS39582B-page 137

PIC16F87XA

12.2

Comparator Operation

A single comparator is shown in Figure 12-2 along with

the relationship between the analog input levels and

the digital output. When the analog input at VIN+ is less

than the analog input VIN-, the output of the comparator

is a digital low level. When the analog input at VIN+ is

greater than the analog input VIN-, the output of the

comparator is a digital high level. The shaded areas of

the output of the comparator in Figure 12-2 represent

the uncertainty due to input offsets and response time.

12.3

Comparator Reference

An external or internal reference signal may be used

depending on the comparator operating mode. The

analog signal present at VIN- is compared to the signal

at VIN+ and the digital output of the comparator is

adjusted accordingly (Figure 12-2).

FIGURE 12-2:

SINGLE COMPARATOR

12.3.1

EXTERNAL REFERENCE SIGNAL

When external voltage references are used, the

comparator module can be configured to have the com-

parators operate from the same or different reference

sources. However, threshold detector applications may

require the same reference. The reference signal must

be between VSS and VDD and can be applied to either

pin of the comparator(s).

12.3.2

INTERNAL REFERENCE SIGNAL

The comparator module also allows the selection of an

internally generated voltage reference for the compara-

tors. Section 13.0 “Comparator Voltage Reference

Module” contains a detailed description of the Compar-

ator Voltage Reference module that provides this signal.

The internal reference signal is used when comparators

are in mode, CM<2:0> = 110 (Figure 12-1). In this

mode, the internal voltage reference is applied to the

VIN+ pin of both comparators.

12.4

Comparator Response Time

Response time is the minimum time, after selecting a

new reference voltage or input source, before the com-

parator output has a valid level. If the internal reference

is changed, the maximum delay of the internal voltage

reference must be considered when using the compar-

ator outputs. Otherwise, the maximum delay of the

comparators should be used (Section 17.0 “Electrical

Characteristics”).

12.5

Comparator Outputs

The comparator outputs are read through the CMCON

outputs may also be directly output to the RA4 and RA5

I/O pins. When enabled, multiplexors in the output path

of the RA4 and RA5 pins will switch and the output of

each pin will be the unsynchronized output of the com-

parator. The uncertainty of each of the comparators is

related to the input offset voltage and the response time

given in the specifications. Figure 12-3 shows the

comparator output block diagram.

The TRISA bits will still function as an output enable/

disable for the RA4 and RA5 pins while in this mode.

The polarity of the comparator outputs can be changed

using the C2INV and C1INV bits (CMCON<4:5>).

–

VIN+

VIN-

Output

VIN–

VIN+

Output

VIN+

VIN-

Note 1: When reading the Port register, all pins

configured as analog inputs will read as a

‘0’. Pins configured as digital inputs will

convert an analog input according to the

Schmitt Trigger input specification.

2: Analog levels on any pin defined as a dig-

ital input may cause the input buffer to

consume more current than is specified.

3: RA4 is an open collector I/O pin. When

used as an output, a pull-up resistor is

required.

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