LTC6804-1/LTC6804-2
43
680412fc
For more information www.linear.com/LTC6804-1
operaTion
Port A is in isoSPI mode. Differential activity on IPA-IMB
wakes up the isoSPI interface. The LTC6804 will be ready
to communicate when the isoSPI state changes to READY
within t
WAKE
or t
READY
, depending on the Core state (see
Figure 1 and state descriptions for details.)
Figure 21 illustrates the timing and the functionally
equivalent circuit. Common mode signals will not wake
up the serial interface. The interface is designed to wake
up after receiving a large signal single-ended pulse, or a
low-amplitude symmetric pulse. The differential signal
|SCK(IPA) – CSB(IMA)|, must be at least V
WAKE
= 200mV
for a minimum duration of t
DWELL
= 240ns to qualify as a
wake up signal that powers up the serial interface.
Waking a Daisy Chain — Method 1
The LTC6804-1 sends a Long +1 pulse on Port B after it is
ready to communicate. In a daisy-chained configuration,
this pulse wakes up the next device in the stack which will,
in turn, wake up the next device. If there are ‘N’ devices in
the stack, all the devices are powered up within the time
N • t
WAKE
or N • t
READY
, depending on the Core State. For
large stacks, the time N • t
WAKE
may be equal to or larger
than t
IDLE
. In this case, after waiting longer than the time
of N • t
WAKE
, the host may send another dummy byte and
wait for the time N • t
READY
, in order to ensure that all
devices are in the READY state.
Method 1 can be used when all devices on the daisy chain
are in the IDLE state. This guarantees that they propagate
the wake-up signal up the daisy chain. However, this
method will fail to wake up all devices when a device in
the middle of the chain is in the READY state instead of
IDLE. When this happens, the device in READY state will
not propagate the wake-up pulse, so the devices above it
will remain IDLE. This situation can occur when attempt
-
ing to wake up the daisy chain after only t
IDLE
of idle time
(some devices may be IDLE, some may not).
Waking a Daisy Chain — Method 2
A more robust wake-up method does not rely on the built-in
wake-up pulse, but manually sends isoSPI traffic for enough
time to wake the entire daisy chain. At minimum, a pair of
long isoSPI pulses (–1 and +1) is needed for each device,
separated by more than t
READY
or t
WAKE
(if the core state is
STANDBY or SLEEP, respectively), but less than t
IDLE
. This
allows each device to wake up and propagate the next pulse
to the following device. This method works even if some
devices in the chain are not in the IDLE state. In practice,
implementing method 2 requires toggling the CSB pin
(of the LTC6820, or bottom LTC6804-1 with ISOMD=0)
to generate the long isoSPI pulses. Alternatively, dummy
commands (such as RDCFG) can be executed to generate
the long isoSPI pulses.
Figure 21. Wake-Up Detection and IDLE Timer
680412 F21
CSB OR IMA
SCK OR IPA
WAKE-UP
STATE
MODE NOISE
WAKE-UP
CSB OR IMA
SCK OR IPA
LOW POWER MODE
t
IDLE
> 4.5ms
t
READY
< 10µs
t
DWELL
= 240ns
V
WAKE
= 200mV
LOW POWER MODE OK TO COMMUNICATE
t
DWELL
= 240ns
DELAY
RETRIGGERABLE
t
IDLE
=
5.5ms
ONE-SHOT
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