© 1999 Scenix Semiconductor, Inc. All rights reserved. - 8 - www.scenix.com SX18AC  / SX20AC / SX28AC speeds.  Although  the  device  has  an  internal  write-back
section   to   prevent   such   conditions,   it   is   still   recom-
mended that the user program include a NOP instruction
as    a    buffer    between    successive    read-modify-write
instructions performed on I/O pins of the same port.
Also note that reading an I/O port is actually reading the
pins, not the output data latches. That is, if the pin output
driver is enabled and driven high while the pin is held low
externally, the port pin will read low. 3.2    Port Configuration Each port pin offers the following configuration options:
•    data direction
•    input voltage levels (TTL or CMOS)
•    pullup type (pullup resistor enable or disable)
•    Schmitt trigger input (for Port B and Port C only)
Port B offers the additional option to use the port pins for
the Multi-Input Wakeup/Interrupt function and/or the ana-
log comparator function.
Port configuration is performed by writing to a set of con-
trol registers associated with the port. A special-purpose
instruction is used to write these control registers:
•    mov !RA,W (move W to Port A control register)
•    mov !RB,W (move W to Port B control register)
•    mov !RC,W (move W to Port C control register)
Each one of these instructions writes a port control regis-
ter for Port A, Port B, or Port C. There are multiple control
registers for each port. To specify which one you want to
access, you use another register called the MODE regis-
ter.
3.2.1  MODE Register
The MODE register controls access to the port configura-
tion registers. Because  the MODE register is  not mem-
ory-mapped,   it   is   accessed   by   the   following   special-
purpose instructions:
•    mov M, #lit (move literal to MODE register)
•    mov M,W (move W to MODE register)
•    mov W,M (move MODE register to W)
The  value  contained  in  the  MODE  register  determines
which port control register is accessed by the “mov !rx,W”
instruction as indicated in Table 3-3. MODE register val-
ues not listed in the table are reserved for future expan-
sion  and  should  not  be  used.  Therefore,  the  MODE
register should always contain a value from 08h to 0Fh.
Upon reset, the MODE register is initialized to 0Fh, which
enables access to the port direction registers.
After a value is written to the MODE register, that setting
remains  in  effect  until  it  is  changed  by  writing  to  the
MODE  register  again.  For  example,  you  can  write  the
value 0Eh to the MODE register just once, and then write
to each of the three pullup configuration registers using
the three “mov !rx,W” instructions. The following code example shows how to program the
pullup control registers. First  the  MODE  register  is  loaded  with  0Eh  to  select
access  to  the  pullup  control  registers  (PLP_A,  PLP_B,
and PLP_C). Then the MOV !rx,W instructions are used
to  specify  which  port  pins  are  to  be  connected  to  the
internal pullup resistors. Setting a bit to 1 disconnects the
corresponding pullup resistor, and clearing a bit to 0 con-
nects the corresponding pullup resistor.
3.2.2  Port Configuration Registers
The port configuration registers that you control with the
MOV !rx,W instruction operate as described below.
RA, RB, and RC Data Direction Registers (MODE=0Fh)
Each register bit sets the data direction for one port pin.
Set the bit to 1 to make the pin operate as a high-imped-
ance input. Clear the bit to 0 to make the pin operate as
an output.
PLP_A, PLP_B, and PLP_C: Pullup Enable Registers
(MODE=0Eh)
Each  register  bit  determines  whether  an  internal  pullup
resistor is connected to the pin. Set the bit to 1 to discon-
nect the pullup resistor or clear the bit to 0 to connect the
pullup resistor. Table 3-3.  MODE Register and Port Control Register Access MODE Reg.    mov !RA,W mov !RB,W mov !RC,W 08h not used CMP_B not used 09h not used WKPND_B not used 0Ah not used WKED_B not used 0Bh not used WKEN_B not used 0Ch not used ST_B ST_C 0Dh LVL_A LVL_B LVL_C 0Eh PLP_A PLP_B PLP_C 0Fh RA Direction   RB Direction   RC Direction mov    M,#$0E ;MODE=0Eh to access port pullup ;registers mov W,#$03 ;W = 0000 0011 mov !RA,W ;disable pullups for A0 and A1 mov W,#$FF ;W = 1111 1111 mov !RB,W ;disable all pullups for B0-B7 mov W,#$00 ;W = 0000 0000 mov !RC,W ;enable all pullups for C0-C7