SX28ACSLASHDP datasheet(11/51 Pages) PARALLAX

11 / 51 page

Parallax SX20AC/SX28AC

www.parallax.com

Page 11 of 51

Rev 1.6 11/20/2006

3.1.1. Read-Modify-Write Considerations

Caution must be exercised when performing two

successive read-modify-write instructions (SETB or

CLRB operations) on an 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 recommended 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

analog comparator function.

Port configuration is performed by writing to a set of

control 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

control registers for each port. To specify which one you

want to access, you use another register called the MODE

3.2.1. MODE Register

The MODE register controls access to the port

configuration registers. Because the MODE register is not

memory-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 2-1. MODE

future expansion 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.

Table 3-2: 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

The following code example shows how to program the

pullup control registers.

mov M,#$0E

;MODE=0Eh to access port pullup

;registers

mov W,#$03

;W = 0000 0011

mov !RA,W

;disable pullups for RA0 and RA1

mov W,#$FF

;W = 1111 1111

mov !RB,W

;disable all pullups for RB0-RB7

mov W,#$00

;W = 0000 0000

mov !RC,W

;enable all pullups for RC0-RC7

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

connects 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-

impedance input. Clear the bit to 0 to make the pin

operate as an output.

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