Quad/Octal, 2-Wire Serial 8-Bit DACs

with Rail-to-Rail Outputs

______________________________________________________________________________________

13

Setting the RST bit high clears all DAC input latches.

The DAC outputs remain unchanged until a STOP con-

dition is detected (Figure 10a). If a reset is issued, the

following output byte is ignored. Subsequent pairs of

command/output bytes overwrite the input latches

(Figure 10b).

All changes made during a transmission affect the

MAX520/MAX521’s outputs only when the transmission

ends and a STOP has been recognized. The R0, R1,

and R2 bits are reserved bits that must be set to zero.

The MAX520/MAX521 are fully compatible with existing

SDA has an open drain which pulls the data line low

during the 9th clock pulse. Figure 11 shows a typical

Additional START Conditions

It is possible to interrupt a transmission to a MAX520/

MAX521 with a new START (repeated start) condition

(perhaps addressing another device), which leaves the

input latches with data that has not been transferred to

the output latches (Figure 12). Only the currently

addressed device will recognize a STOP condition and

transfer data to its output latches. If the device is left

with data in its input latches, the data can be trans-

ferred to the output latches the next time the device is

addressed, as long as it receives at least one com-

mand byte and a STOP condition.

(

)

(

)

(

)

SDA

0

START

CONDITION

ADDRESS BYTE

ACK

1

0

1

AD1 AD0 0

0

0 0010

0

(RST)

(RST)

STOP

CONDITION

COMMAND BYTE

ACK

ALL OUTPUTS

SET TO 0

(

)

ALL INPUT LATCHES

SET TO 0

ALL INPUT LATCHES

SET TO 0

SDA

0

START

CONDITION

ADDRESS BYTE

ACK

10

1

AD1 AD0 0

0 00010

0

0

STOP

CONDITION

COMMAND BYTE

ACK

"DUMMY"

OUTPUT BYTE

ACK

(a)

(b)

ALL DAC OUTPUTS SET TO 0 UNLESS

CHANGED BY ADDITIONAL COMMAND

BYTE/OUTPUT BYTE PAIRS

NOTE: X = DON'T CARE

ADDITIONAL

COMMAND BYTE/

OUTPUT BYTE PAIRS

X

X

X

X

X

X

X

X

X

X

X

X

X

X

0 OR AD2

0 OR AD2

Figure 10. Resetting DAC Outputs

MAX520

SDA SCL

µC

SDA

SCL

E

2

PROM

XICOR

X24C04

SDA

SCL

AD1

AD2

AD0

QUAD

DAC

SDA

SCL

AD1

AD0

OCTAL

DAC

+5V

MAX521