Data Input/Output

The DIO line is an input/output pin used to issue com-

mands to the MAX1459 (input mode) or read the

EEPROM contents (output mode).

In input mode (the default mode), data on DIO is

latched on each rising edge of SCLK. Therefore, data

on DIO must be stable at the rising edge of SCLK and

should transition on the falling edge of SCLK.

DIO will switch to output mode after receiving either the

READ EEPROM command or the READ EEPROM

MATRIX command. See the Read EEPROM section for

detailed information.

Communication Protocol

To initiate communication, the first 8 bits on DIO after

CS transitions from low to high must be 101010U0 (AA

hex or A8 hex, defined as the INIT sequence). The

MAX1459 will then begin accepting 16-bit control

words (Figure 4).

If the INIT SEQUENCE is not detected, all subsequent

data on DIO is ignored until CS again transitions from

low to high and the correct INIT SEQUENCE is received.

The U bit of the INIT SEQUENCE controls the updating

of the DACs and configuration register from the internal

EEPROM. If this bit is low (U = 0, INIT SEQUENCE = A8

hex), all four internal DACs and the configuration regis-

ter will be updated from the EEPROM on the next rising

edge of CS (this is also the default on power-up). If the

U bit is high (INIT SEQUENCE = AA hex), the DACs

and configuration register will not be updated from the

internal EEPROM; they will retain their current value on

any subsequent CS rising edge. The MAX1459 contin-

ues to accept control words until CS is brought low.

Control Words

After receiving the INIT SEQUENCE on DIO, the

MAX1459 begins latching in 16-bit control words, MSB

first (Figure 5).

The first 4 bits of the control word (the MSBs,

CM3–CM0) are the command field. The last 12 bits

(D11–D0) represent the data field. The MAX1459 sup-

ports the commands listed in Table 5.

No-OP Command (0 hex)

The no-operation (No-OP) command must be issued

before and after the commands ERASE EEPROM and

WRITE EEPROM BIT. In the case of the ERASE EEP-

ROM command, the control word must be 0000 hex. In

the case of the WRITE EEPROM BIT command, the

command field must be 0h, and the data field must

have, in its lower bits, the EEPROM address to be writ-

ten (Figure 6). For example, to write location 1C hex of

2-Wire, 4–20mA

Smart Signal Conditioner

______________________________________________________________________________________

11

Figure 5. Control-Word Timing Diagram

SCLK

DIO

COMMAND

MSB

LSB

LSB

MSB

MSB

LSB

DATA

16-BIT CONTROL WORD

D9

D6

D8

D5

D7

D4

D3

D2

D1

D0

CM3

CM0

CM2

D11

CM1

D10

Figure 4. Communication Sequence

CS

SCLK

DIO

16 CLK

CYCLES

INIT

SEQUENCE

CONTROL

WORD

CONTROL

WORD

CONTROL

WORDS

16 CLK

CYCLES

n x 16 CLK

CYCLES

X

1

0

1

CM3

CM3

CM2

CM2

DO

DO

0U

0

1

0

8 CLK

CYCLES