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SX20AC/SX28AC

For example, suppose all four Port A pins are configured

as outputs and with RA0 and RA1 to be high, and RA2

and RA3 to be low:

The second "mov" instruction in this example writes the

Port A data register (RA), which controls the output levels

of the four Port A pins, RA0 through RA3. Because Port

A has only four I/O pins, only the four least significant bits

of this register are used. The four high-order register bits

are "don't care" bits. Port B and Port C are both eight bits

wide, so the full widths of the RB and RC registers are

used.

When a write is performed to a bit position for a port that

has been configured as an input, a write to the port data

on the pin. If later that pin is configured to operate as an

output, it will reflect the value that has been written to the

data register.
When a read is performed from a bit position for a port,

the operation is actually reading the voltage level on the

pin itself, not necessarily the bit value stored in the port

data register. This is true whether the pin is configured to

operate as an input or an output. Therefore, with the pin

configured to operate as an input, the data register con-

tents have no effect on the value that you read. With the

pin configured to operate as an output, what is read gen-

erally matches what has been written to the register.

Figure 3-2. Port B, Port C Configuration

MODE

RB or RC

PLP_B or PLP_C

LVL_B or LVL_C

0 = Output
1 = Hi-Z Input

0 = Pullup Enable
1 = Pullup Disable

0 = CMOS
1 = TTL

Port B: Input, MIWU, Comparator

Pullup Resistor
(

20k

)

Port B or

Internal Dat

Bus

M
U
X

Mode = 0F

Mode = 0E

Mode = 0D

Mode

ST_B or ST_C

0 = Schmitt Trigger Enable
1 = Schmitt Trigger Disable

Port C: Input Only

TTL Buffer

CMOS Buffer

M
U
X

Port C PIN

Schmitt Trigger Buffer

Direction

RB or RC

Data

mov

W,#$03

;load W with the value 03h

;(bits 0 and 1 high)

mov

$05,W

;write 03h to Port A data

;register