Serially Controlled, Low-Voltage,

8-Channel SPST Switch

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11

_______________Detailed Description

Basic Operation

The MAX395’s interface can be thought of as an 8-bit

shift register controlled by CS (Figure 2). While CS is

low, input data appearing at DIN is clocked into the

shift register synchronously with SCLK’s rising edge.

The data is an 8-bit word, each bit controlling one of

eight switches in the MAX395 (Table 1). DOUT is the

shift register’s output, with data appearing synchro-

nously with SCLK’s falling edge. Data at DOUT is sim-

ply 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 previous configuration. When the eight

bits of data have been shifted in, CS is driven 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 first input bit (D7) at its output.

More or less than eight clock cycles can be entered

during the CS low period. When this happens, the shift

register will contain only the last eight serial data bits,

regardless of when they were entered. On the rising

edge of CS, all the switches will be set to the corre-

sponding states.

The MAX395’s three-wire serial interface is compatible

with SPI™, QSPI™, and Microwire™ standards. If inter-

facing with a Motorola processor serial interface, set

CPOL = 0. The MAX395 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 updated on

the rising edge of CS, regardless of SCLK’s state. This

meets all the SPI and QSPI requirements.

Daisy Chaining

For a simple interface using several MAX395s, “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 MAX395s in series.

When CS is brought high, all switches are updated

simultaneously. Additional shift registers may be includ-

ed anywhere in series with the MAX395 data chain.

• • •

• • •

• • •

• • •

• • •

SCLK

DIN

DOUT

COM OUT

CS

Figure 1. Timing Diagram