______________Detailed Description

Serial Digital Interface

Basic Operation

Refer to Figure 2. The MAX335 interface can be

thought of as an 8-bit shift register controlled by

CS.

While

CS is low, input data appearing at DIN is clocked

into the shift register synchronous with SCLK’s rising

edge. The data is an 8-bit word, each bit controlling

one of eight switches in the MAX335 (Table 1). DOUT is

the output of the shift register, with data appearing syn-

chronous with SCLK’s falling edge. Data at DOUT is

simply the input data delayed by eight clock cycles.

When shifting the input data, D7 is the first bit in and

out of the shift register. While shifting data, the switches

remain in their original configuration. When the 8 bits of

data have been shifted in,

CS is brought high. This

updates the new switch configuration and inhibits fur-

ther data from entering the shift register. Transitions at

DIN and SCLK have no effect when

CS is high, and

DOUT holds the last bit in the shift register.

The MAX335 three-wire serial interface is compatible

with the SPI™ and Microwire™ standards. If interfacing

with a Motorola processor serial interface, set CPOL = 0.

The MAX335 is considered a slave device (Figures 2

and 3). Upon power-up, the shift register contains all

zeros, and all switches are off.

The latch that drives the analog switch is only

updated on the rising edge of

CS when SCLK is low.

If SCLK is high when

CS rises, the latch will not be

updated until SCLK goes low. The CPOL = 1, CPHA = 1

SPI configuration does not update the latch correctly.

Daisy Chaining

For a simple interface using several MAX335s, “daisy

chain” the shift registers as shown in Figure 5. The

CS

pins of all devices are connected together, and a

stream of data is shifted through the MAX335s in

series. When

CS is brought high, all switches are

updated simultaneously. Additional shift registers may

be included anywhere in series with the MAX335 data

chain.

Addressable Serial Interface

When several serial devices are configured as slaves,

addressable by the processor, DIN pins of each

MAX335 are connected together (Figure 6). Address

decode logic individually controls

CS of each slave

device. When a slave is selected, its

CS is brought low,

data is shifted in, and

CS is brought high to latch the

data. Typically, only one slave is addressed at a time.

DOUT is not used.

Digital Feedthrough

Digital feedthrough energy measures 100nV-sec, which

means that with no filtering at the signal channel,

feedthrough from a sharply rising clock edge into an

unfiltered switch channel can be measured at 1Vp-p for

100ns. However, even 100pF capacitance in the switch

channel, when combined with the switch resistance,

yields a filter that reduces this transient to 10mVp-p

typical. To reduce digital feedthrough, hysteresis

(150mV typ) was added to the SCLK input so triangle

or sine waves may be used.

Serial Controlled, 8-Channel SPST Switch

6

_______________________________________________________________________________________

NAME

FUNCTION

1

SCLK

Serial Clock Input

2

V+

Positive Supply Voltage

PIN

_____________________Pin Description

3

DIN

Serial Data Input

4

GND

Ground

8

COM1

Switch 1

7

NO1

Switch 1

6

COMØ

Switch 0

5

NOØ

Switch 0

13

COM4

Switch 4

12

COM3

Switch 3

11

NO3

Switch 3

10

COM2

Switch 2

9

NO2

Switch 2

14

NO4

Switch 4

15

COM5

Switch 5

16

NO5

Switch 5

17

COM6

Switch 6

18

NO6

Switch 6

19

COM7

Switch 7

20

NO7

Switch 7

21

V-

Negative Supply Voltage

22

DOUT

Serial Data Output

23

Logic Supply/Reset

24

C

—

S

–

Chip Select