Linear Technology LTC6804-1 - Page 46

LTC6804-1/LTC6804-2
46
680412fc
For more information www.linear.com/LTC6804-1
Table 25. Write/Read PEC Format
NAME RD/WR BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
PEC0 RD/WR PEC[14] PEC[13] PEC[12] PEC[11] PEC[10] PEC[9] PEC[8] PEC[7]
PEC1 RD/WR PEC[6] PEC[5] PEC[4] PEC[3] PEC[2] PEC[1] PEC[0] 0
operaTion
Daisy-chained (LTC6804-1) configurations support broad-
cast commands only, because they have no addressing.
All devices in the chain receive the command bytes simul-
taneously. For example, to initiate ADC conversions in a
stack of devices, a single ADCV command is sent, and all
devices
will start conversions at the same time. For read
and write commands, a single command is sent, and then
the stacked devices effectively turn into a cascaded shift
register, in which data is shifted through each device to
the next device in the stack. See the Serial Programming
Examples section.
Polling Methods
The simplest method to determine ADC completion is
for the controller to start an ADC conversion and wait for
the specified conversion time to pass before reading the
results. Polling is not supported with daisy-chain com
-
munication (LTC6804-1).
In parallel configurations that communicate in SPI mode
(ISOMD pin tied low), there are two methods of poll
-
ing. The first method is to hold CSB low after an ADC
conversion command is sent. After entering a conversion
command, the SDO line is driven low when the device is
busy per
forming conversions (Figure 23). SDO is pulled
high when the device completes conversions. However
, the
SDO will also go back high when CSB goes high even if the
device has not completed the conversion. An addressed
device drives the SDO line based on its status alone. A
problem with this method is that the controller is not free
to do other serial communication while waiting for ADC
conversions to complete.
The next method overcomes this limitation. The controller
can send an ADC start command, perform other tasks, and
then send a poll ADC converter status (PLADC) command
to determine the status of the ADC conversions (Figure24).
After entering the PLADC command, SDO will go low if
the device is busy performing conversions. SDO is pulled
high at the end of conversions. However, the SDO will also
Broadcast vs Address Commands
CONFIGURATION TYPE OF COMMAND
DEVICE INTERFACE READ WRITE POLL
LTC6804-2
(Address/Parallel)
SPI Address-
Only
Address
or
Broadcast
Address
or
Broadcast
isoSPI Address-
Only
†
LTC6804-1
(Daisy-Chain)
SPI or
isoSPI
Broadcast-Only N/A
†
The LTC6804-2 will not return data pulses when using broadcast
commands in isoSPI mode. Therefore, ADC commands will execute, but
polling will not work.
Address Commands (LTC6804-2 Only)
An address command is one in which only the addressed
device on the bus responds. Address commands are used
only with LTC6804-2 parts. All commands are compatible
with addressing. See Bus Protocols for Address command
format.
Broadcast Commands (LTC6804-1 or LTC6804-2)
A broadcast command is one to which all devices on the
bus will respond, regardless of device address. This com
-
mand format can be used with LTC6804-1 and LTC6804-2
parts. See Bus Protocols for Broadcast command format.
With broadcast commands all devices can be sent com
-
mands simultaneously.
In parallel (LTC6804-2) configurations, broadcast com-
mands are useful for initiating ADC conversions or for
sending
write commands when
all parts are being written
with the same data. The polling function (automatic at the
end of ADC commands, or manual using the PLADC com
-
mand) can also be used with broadcast commands, but
only with parallel SPI interfaces. Polling is not compatible
with parallel isoSPI. Likewise, broadcast read commands
should not be used in a parallel configuration (either SPI
or isoSPI).