MCP6S21 datasheet(22/43 Pages) MICROCHIP

22 / 43 page

MCP6S21/2/6/8

DS21117A-page 22

 2003 Microchip Technology Inc.

5.2.4

SHUTDOWN COMMAND

The software Shutdown command allows the user to

put the amplifier into a low power mode (see

high impedance (Section 4.4, “Shutdown Mode”, and

Section 5.1, “SPI Timing”, cover the exceptions at pins

VREF, VOUT and SO).

Once the PGA has entered shutdown mode, it will

remain in this mode until either a valid command is sent

to the device (other than NOP or Shutdown), or the

device is powered down and back up again. The

internal registers maintain their values while in

shutdown.

Once brought out of shutdown mode, the part comes

back to its previous state (see Section 5.4 for excep-

tions to this rule). This makes it possible to bring the

device out of shutdown mode using one command;

send a command to select the current channel (or gain)

and the device will exit shutdown with the same state

that existed before shutdown.

5.3

Daisy Chain Configuration

Multiple devices can be connected in a daisy chain

configuration by connecting the SO pin from one device

to the SI pin on the next device and using common SCK

and CS lines (Figure 5-3). This approach reduces PCB

layout complexity.

The example in Figure 5-3 shows a daisy chain config-

uration with two devices, although any number of

devices can be configured this way. The MCP6S21 and

MCP6S22 can only be used at the far end of the daisy

chain because they do not have a serial data out (SO)

pin. As shown in Figure 5-4 and Figure 5-5, both SI

and SO data are sent in 16-bit (2 byte) words. These

devices abort any command that is not a multiple of 16

bits.

When using the daisy chain configuration, the maxi-

mum clock speed possible is reduced to

≈ 5.8 MHz

because of the SO pin’s propagation delay (see

Electrical Specifications).

The internal SPI shift register is automatically loaded

with zeros whenever CS goes high (a command is exe-

cuted). Thus, the first 16-bits out of the SO pin once CS

line goes low are always zeros. This means that the

first command loaded into the next device in the daisy

chain is a NOP. This feature makes it possible to send

shorter command and data byte strings when the far-

thest devices do not need to change. For example, if

there were three devices on the chain and only the mid-

dle device needed changing, only 32 bytes of data

need to be transmitted (for the first and middle

devices), and the last device on the chain would

receive a NOP when the CS pin is raised to execute the

command.

FIGURE 5-3:

Daisy Chain Configuration.

Microcontroller

SCK

Device 1

00100000 00000000

SCK

Device 2

00000000 00000000

Set CS low.

Clock out the instruction and data

for Device 2 (16 clocks) to Device 1.

Device 1 automatically clocks out all

zeros (first 16 clocks) to Device 2.

Clock out the instruction and data

for Device 1 (16 clocks) to Device 1.

Device 1 automatically shifts data

from Device 1 to Device 2 (16

clocks).

Raise CS.

Device 1

01000001 00000111

Device 2

00100000 00000000

PICmicro®

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