LTC6804-1/LTC6804-2
42
680412fc
For more information www.linear.com/LTC6804-1
operaTion
Figure 20. isoSPI Timing Diagram
The lower isoSPI port (Port A) never transmits long
(CSB) pulses. Furthermore, a slave isoSPI port will only
transmit short –1 pulses, never a +1 pulse. The master
port recognizes a null response as a logic 1. This allows
for multiple slave devices on a single cable without risk
of collisions (Multidrop).
Figure 20 shows the isoSPI timing diagram for a READ
command to daisy-chained LTC6804-1 parts. The ISOMD
pin is tied to V
–
on the bottom part so its Port A is config-
ured as a SPI port (CSB, SCK, SDI and SDO). The isoSPI
signals of three stacked devices are shown, labeled with
the port (A or B) and part number. Note that ISO
B1 and
ISO A2 is actually the same signal, but shown on each
end of the transmission cable that connects parts 1 and 2.
Likewise, ISO B2 and ISO A3 is the same signal, but with
the cable delay shown between parts 2 and 3.
Bits W
n
-W
0
refers to the 16-bit command code and the
16-bit PEC of a READ command. At the end of bit W
0
the
3 parts decode the READ command and begin shifting out
data which is valid on the next rising edge of clock SCK.
Bits X
n
-X
0
refer to the data shifted out by Part 1. Bits Y
n
-Y
0
refer to the data shifted out by Part 2 and bits Z
n
-Z
0
refer
to the data shifted out by Part 3. All this data is read back
from the SDO port on Part 1 in a daisy-chained fashion.
Waking Up the Serial Interface
The serial ports (SPI or isoSPI) will enter the low power
IDLE state if there is no activity on Port A for a time of t
IDLE
.
The WAKEUP circuit monitors activity on pins 41 and 42.
If ISOMD = V
–
, Port A is in SPI mode. Activity on the CSB
or SCK pin will wake up the SPI interface. If ISOMD = V
REG
,
SDI
SCK
SDO
CSB
ISO A2
ISO B2
ISO A3
ISO B1
READ DATACOMMAND
6000500040003000200010000
t
7
t
6
t
5
t
RTN
t
11
t
10
t
2
t
1
t
CLK
t
4
t
3
t
RISE
t
DSY(CS)
t
8
t
9
t
DSY(CS)
Z
n-1
Z
n-1
Z
n
Z
n
W
0
W
0
W
n
W
n
Y
n-1
Y
n-1
Y
n
Y
n
W
0
W
0
X
n-1
X
n
Z
0
W
n
W
n
t
DSY(D)
t
10
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