© 2000 Scenix Semiconductor, Inc. All rights reserved. - 25 - www.scenix.com 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
register is used to enable the comparator, to read the out-
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.