Data Sheet

ADGS1408/ADGS1409

Rev. 0 | Page 25 of 34

The ADGS1408/ADGS1409 can only enter daisy-chain mode

when in address mode by sending the 16-bit SPI command,

0x2500 (see Figure 49). When the ADGS1408/ADGS1409

receive this command, the SDO of the device sends out the

same command because the alignment bits at SDO are 0x25,

which allows multiple daisy-connected devices to enter daisy-

chain mode in a single SPI frame. A hardware reset is required to

exit daisy-chain mode.

For the timing diagram of a typical daisy-chain SPI frame, see

Figure 50. When CS goes high, Device 1 writes Command 0,

Bits[7:0] to its switch data register, Device 2 writes Command 1,

Bits[7:0] to its switches, and so on. The SPI block uses the last

eight bits it received through SDI to update the switches. After

entering daisy-chain mode, the first eight bits sent out by SDO

on each device in the chain are 0x00. When CS goes high, the

internal shift register value does not reset back to zero.

An SCLK rising edge reads in data on SDI while data is

propagated out of SDO on an SCLK falling edge. The expected

number of SCLK cycles must be a multiple of eight before CS

goes high. When the expected number of SCLK cycles is not a

multiple of eight, the SPI interface sends the last eight bits

received to the switch data register.

POWER-ON RESET

The digital section of the ADGS1408/ADGS1409 goes through an

a reset, ensure a minimum of 120 μs from the time of power-up or

not drop out during the 120 μs initialization phase because it

may result in incorrect operation of the ADGS1408/ADGS1409.

0

0

1

0

0

1

0

1

0

0

0

0

0

0

0

0

SDO

0

0

1

0

0

1

0

1

0

0

0

0

0

0

0

0

12

3

4

56

7

89

10

11

12

13

14

15

16

SDI

SCLK

CS

Figure 49. SPI Command to Enter Daisy-Chain Mode

SDO

COMMAND3[7:0]

8’h00

COMMAND2[7:0]

COMMAND3[7:0]

COMMAND1[7:0]

COMMAND2[7:0]

COMMAND0[7:0]

COMMAND1[7:0]

SDI

SDO3

8’h00

8’h00

8’h00

8’h00

COMMAND3[7:0]

8’h00

COMMAND2[7:0]

COMMAND3[7:0]

SDO2

DEVICE 2

DEVICE 1

DEVICE 4

DEVICE 3

CS

NOTES

1. SDO2 AND SDO3 ARE THE OUTPUT COMMANDS FROM DEVICE 2 AND DEVICE 3, RESPECTIVELY.

Figure 50. Example of an SPI Frame Where Four ADGS1408/ADGS1409 Devices Connect in Daisy-Chain Mode