SX20AC75 datasheet(25/52 Pages) ETC

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SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75

11.0

COMPARATOR

The device contains an on-chip differential comparator.

Ports RB0-RB2 support the comparator. Ports RB1 and

RB2 are the comparator negative and positive inputs,

respectively, while Port RB0 serves as the comparator

output pin. To use these pins in conjunction with the com-

parator, the user program must configure Ports RB1 and

RB2 as inputs and Port RB0 as an output. The CMP_B

put of the comparator internally, and to enable the output

of the comparator to the comparator output pin.

The comparator enable bits are set to “1” upon reset,

thus disabling the comparator. To avoid drawing addi-

tional current during the power down mode, the compara-

tor should be disabled before entering the power down

mode. Here is an example of how to setup the compara-

tor and read the CMP_B register.

The final “mov” instruction in this example performs an

exchange of data between the working register (W) and

the CMP_B register. This exchange occurs only with Port

B accesses. Otherwise, the “mov” instruction does not

perform an exchange, but only moves data from the

source to the destination.

Figure 11-1 shows the comparator block diagram.

CMP_B - Comparator Enable/Status Register

mov

M,#$08

;set MODE register to access

;CMP_B

mov

W,#$00

;clear W

mov

!RB,W

;enable comparator and its

;output

...

;delay after enabling

;comparator for response

mov

M,#$08

;set MODE register to access

;CMP_B

mov

W,#$00

;clear W

mov

!RB,W

;enable comparator and its

;output and also read CMP_B

;(exchange W and CMB_B)

and

W,#$01

;set/clear Z bit based on

;comparator result

snb

$03.2

;test Z bit in STATUS reg

;(0 => RB2<RB1)

jmp

rb2_hi

;jump only if RB2>RB1

...

CMP_EN

CMP_OE

Reserved

CMP_RES

Bit 7

Bit 6

Bits 5–1

Bit 0

CMP_RES

Comparator result: 1 for RB2>RB1 or 0

for RB2<RB1. Comparator must be en-

abled (CMP_EN = 0) to read the result.

The result can be read whether or not the

CMP_OE bit is cleared.

CMP_OE

When cleared to 0, enables the compar-

ator output to the RB0 pin.

CMP_EN

When cleared to 0, enables the compar-

ator.

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