16

LTC1091/LTC1092

LTC1093/LTC1094

S

APPLICATI

I FOR ATIO

3. Accommodating Microprocessors with

Different Word Lengths

The LTC1091/LTC1093/LTC1094 will fill zeros indefinitely

after the transmitted data until CS is brought high. At that

to MPU serial ports with different transfer increments

including 4 bits (e.g., COP400) and 8 bits (e.g., SPI and

MICROWIRE/PLUS

dated by the correct positioning of the start bit in the

LTC1091 input word.

Figure 1 shows examples of LTC1091 input and output

words for 4-bit and 8-bit processors. A complete data

exchange can be implemented with two 4-bit MPU outputs

and three inputs in 4-bit systems and one 8-bit output and

two inputs in 8-bit systems. The resulting data winds up

left justified in the MPU with zeros automatically filled in

the unused low order bits by the LTC1091. In section 5

another example is given using the MC68HC05C4 which

MICROWIRE/PLUS is a trademark of National Semiconductor Corp.

eliminates one 8-bit transfer and positions data right

justified inside the MPU.

The LTC1091/LTC1093/LTC1094 can be operated with

required to communicate to the MPU. Data is transmitted

in both directions on a single wire. The processor pin

connected to this data line should be configurable as either

an input or an output. The LTC1091, for example, will take

control of the data line and drive it low on the 4th falling

CLK edge after the start bit is received (see Figure 2).

Therefore, the processor port line must be switched to an

input before this happens, to avoid a conflict.

In the next section, an example is made of interfacing

8051 MPU.

FILL ZEROS

X = DON’T CARE

1091/2/3/4 F01

CLK

CS

MPU SENDS

4-BIT

TRANSFERS

MPU READS BACK

START

BIT

Hi-Z

START

MSBF

MSBF

X

0

0

0

1

• • •

B9

B8

B7

B6

B5

B4

B3

B2

B1

B0

SGL/

DIFF

ODD/

SIGN

SGL/

DIFF

ODD/

SIGN

MSBF

X

SGL/

DIFF

ODD/

SIGN

B9

B8

B7

B6

B5

B4

B3

B2

B1

B0

0

0

B9

B8

B7

B6

B5

B4

B3

B2

B1

B0

0

0

0

0

0

0

START

BIT

0

0

0

1

MPU SENDS

8-BIT

TRANSFERS

MPU READS BACK

Figure 1. LTC1091 Input and Output Word Arrangements for 4-Bit and 8-Bit Serial Port Microprocessors