data MSB first; therefore, IO7, MASK7, and data 7 bit cor-

respond to the MSB (first bit of the data byte).

Other Registers

RAP uses the send-byte protocol to resample the

address pins. Do not use read- and write-byte protocols

to RAP because data is redirected to NDR1 although the

ADD_ pins will be sampled.

SPOR uses the send-byte protocol to resample the

address pins and reset the registers to the POR state. Do

not use read- and write-byte protocols to SPOR because

data is redirected to NDR1 although the function will be

performed.

MFID uses the read-byte protocol to access the ID regis-

ter. Do not use write-byte protocol to MFID because data

is redirected to NDR1.

SMBSUS (Suspend-Mode) Input

The state of the SMBSUS input selects which register

contents (NDR1 or SDR1) are applied to the IO_ pins and

which set of registers are used to mask the interrupts

(NDR2, NDR3 or NDR2, SDR3). Driving SMBSUS low

selects the suspend-mode registers, while driving SMB-

SUS high selects the normal registers. This feature allows

the system to select between two different I/O configura-

tions asynchronously, eliminating latencies introduced by

the serial bus.

Octal SMBus-to-Parallel I/O Expanders

_______________________________________________________________________________________

9

SMBCLK

AB

C

D

E

FG

H

I

J

K

SMBDATA

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

E = SLAVE PULLS SMBDATA LINE LOW

L

M

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO SLAVE

H = LSB OF DATA CLOCKED INTO SLAVE

I = SLAVE PULLS SMBDATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO MASTER

K = ACKNOWLEDGE CLOCK PULSE

L = STOP CONDITION, DATA EXECUTED BY SLAVE

M = NEW START CONDITION

Figure 3. SMBus Write Timing

SMBCLK

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

AB

C

D

E

FG

H

I

J

SMBDATA

K

E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO MASTER

H = LSB OF DATA CLOCKED INTO MASTER

I = ACKNOWLEDGE CLOCK PULSE

J = STOP CONDITION

K = NEW START CONDITION

Figure 4. SMBus Read Timing