Datasheet

MCP6S21/2/6/8
DS21117B-page 22  2003-2012 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
Register 5-1). In this shutdown mode, most pins are
high impedance (Section 4.4, “Shutdown Mode”, and
Section 5.1, “SPI Timing”, cover the exceptions at pins
V
REF,
V
OUT
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 C
S
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
SO
CS
SCK
SI
CS
SCK
SO
Device 1
Device 1
00100000 00000000
SO
CS
SCK
SI
Device 2
Device 2
00000000 00000000
1. Set CS low.
2. Clock out the instruction and data
for Device 2 (16 clocks) to Device 1.
3. Device 1 automatically clocks out all
zeros (first 16 clocks) to Device 2.
4. Clock out the instruction and data
for Device 1 (16 clocks) to Device 1.
5. Device 1 automatically shifts data
from Device 1 to Device 2 (16
clocks).
6. Raise CS
.
Device 1
01000001 00000111
Device 2
00100000 00000000
PIC
®