PIC18F2680ISOSQTP datasheet(261/482 Pages) MICROCHIP

261 / 482 page

Preliminary

DS39625C-page 259

PIC18F2585/2680/4585/4680

20.2

Comparator Operation

A single comparator is shown in Figure 20-2, 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 20-2 represent

the uncertainty, due to input offsets and response time.

20.3

Comparator Reference

Depending on the comparator operating mode, either

an external or internal voltage reference may be used.

The analog signal present at VIN- is compared to the

signal at VIN+ and the digital output of the comparator

is adjusted accordingly (Figure 20-2).

FIGURE 20-2:

SINGLE COMPARATOR

20.3.1

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).

20.3.2

INTERNAL REFERENCE SIGNAL

The comparator module also allows the selection of an

internally

generated

voltage

reference

from

the

comparator voltage reference module. This module is

described in more detail in Section 21.0 “Comparator

Voltage Reference Module”.

The internal reference is only available in the mode

where four inputs are multiplexed to two comparators

(CM2:CM0 = 110). In this mode, the internal voltage

reference is

applied

the

VIN+

both

comparators.

20.4

Comparator Response Time

Response time is the minimum time, after selecting a

new reference voltage or input source, before the

comparator 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

comparator

outputs.

Otherwise,

the

maximum delay of the comparators should be used

(see Section 27.0 “Electrical Characteristics”).

20.5

Comparator Outputs

The comparator outputs are read through the CMCON

outputs may also be directly output to the RE1 and RE2

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

of the RE1 and RE2 pins will switch and the output of

each pin will be the unsynchronized output of the

comparator.

The

uncertainty

each

the

comparators is related to the input offset voltage and

the

response

time

given

the

specifications.

Figure 20-3 shows the comparator output block

diagram.

The TRISE bits will still function as an output enable/

disable for the RE1 and RE2 pins while in this mode.

The polarity of the comparator outputs can be changed

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

–

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

digital input may cause the input buffer to

consume more current than is specified.

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