6

feature and function (mask or data) of the byte immediately

following the control byte, the data byte (See Addressing the

Single Port I/O). Each bit of the data register may be

individually programmed as an input or output. A logic low in

a data direction bit programs that pin as an input, a logic high

makes it an output. A read operation of data register pins

programmed as inputs reflects the current logic level present

at the buffered port pins. A read operation of those data

register pins programmed as outputs indicates the last value

written to that location. At power-up, all port pins are

configured as unterminated inputs. Two chip identify pins are

used to allow up to 4 I/O ports to share the same chip enable

signal. The first two bits shifted in are compared with the

hardwired levels at the chip identify pins to enable the

selected I/O for serial data transfer. Note that when chip

enable becomes true, the compare flag is latched for all

devices sharing the same chip enable.

Compare Function

The value of a port pin (D0 - D7), configured as an input, is

compared with the corresponding bit value (DR0 - DR7)

stored in the Data Register. Pins configured as outputs are

assumed to have the same value as the corresponding bit

stored in the Data Register. The compare function is

programmed via C01 and C00 (CM1, CM0) of the Address

Byte. As shown in Table 1, the values for CM1 and CM0 will

sense one of four separate conditions.

The compare flag is set to one when the programmed

condition is satisfied. Otherwise, the flag is cleared to zero.

The compare flag is latched when the device is enabled (a

transition of CE from “High” to “Low”).

Data Format

During write operations, the data byte that follows the control

byte is normally the data word that is transferred to the data

or data direction register. Control bits 2 and 3 (DF0 and DF1)

change the interpretation of this data as shown in Table 2.

Note that one or more bits can be set or cleared in either

register without having to write to bits not requiring change.

TABLE 1.

CM1

CM0

CONDITION

0

0

At least one non-match

01

All match

1

0

All are non-match

1

1

At least one match

TABLE 2.

C03 DF1 C02 DF0

OPERATION

0

X

Data following the control word will be written to the selected register.

1

0

Data following the control word is a mask. Those bits which are a 1 will cause that register flip-flop to be cleared to 0. Those

which are a 0 will cause that register flip-flop to be unchanged.

1

1

Data following the control word is a mask. Those bits which are a 1 will cause that register flip-flop to be set to 1; those which

are a 0 will cause that register flip-flop to be unchanged.

TABLE 3. EXAMPLE

CONTROL

DATA

PREVIOUS

REGISTER VALUE

NEW

REGISTER VALUE

C07 C06 C05 1 0 X C01 C00

11110000

10101010

11110000

C07 C06 C05 1 1 1 C01 C00

11110000

10101010

11111010

C07 C06 C05 1 1 0 C01 C00

11110000

10101010

00001010

C07 C06 C05 1 1 X C01 C00

00000000

10101010

10101010

X = Don’t Care

FIGURE 6. CONTROL BYTE

CE

SCK

MISO

MOSI

OR

SCK

X

C07

C06

C05

C04

C03

C02

C01

C01

INPUT

Z

Z

Z

C07

C06

C05

C04

C03

*

OUTPUT

X = DON’T CARE

Z = HIGH IMPEDANCE

* = COMPARE FLAG

CDP68HC68P1