SX20AC75 datasheet(9/52 Pages) ETC

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SX18AC/SX20AC/SX28AC/SX18AC75/SX20AC75/SX28AC75

3.1.1 Read-Modify-Write Considerations

Caution must be exercised when performing two succes-

sive read-modify-write instructions (SETB or CLRB oper-

ations) on I/O port pin. Input data used for an instruction

must be valid

during the time the instruction is executed,

and the output result from an instruction is valid only

after

that instruction completes its operation. Unexpected

results from successive read-modify-write operations on

I/O pins can occur when the device is running at high

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

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

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.

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

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