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 synchronous 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 further 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.
PIN NAME FUNCTION
1 SCLK Serial Clock Input
2 V+ Positive Supply Voltage
3 DIN Serial Data Input
4 GND Ground
5 NOØ Switch 0
6 COMØ Switch 0
7 NO1 Switch 1
8 COM1 Switch 1
9 NO2 Switch 2
10 COM2 Switch 2
11 NO3 Switch 3
12 COM3 Switch 3
13 COM4 Switch 4
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 VLLogic Supply/Reset
24 CS Chip Select
Pin Description
MAX335 Serial Controlled, 8-Channel SPST Switch
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