ADBMS1818ASWZ-R7 datasheet(56/92 Pages) AD

56 / 92 page

Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. B | 56 of 92

Figure 86. isoSPI Timing Diagram

Waking Up the Serial Interface

The serial ports (SPI or isoSPI) enter the low power idle state

if there is no activity on Port A or Port B for a time of tIDLE.

The wake-up circuit monitors activity on Pin 61 through Pin 64. If

ISOMD = V–, Port A is in SPI mode. Activity on the CSB pin or

SCK pin wakes up the SPI. If ISOMD = VREG, Port A is in isoSPI

mode. Differential activity on IPA to IMA (or IPB to IMB) wakes up

the isoSPI. The ADBMS1818 is ready to communicate when the

isoSPI state changes to ready within tWAKE or tREADY, depending on

the core state (see Figure 53 and the Sleep State, Standby State,

REFUP State, and Measure State sections for details).

Figure 87 shows the timing and the functionally equivalent circuit

(only Port A shown). Common-mode signals do 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 VWAKE = 200 mV for a minimum duration of tDWELL = 240 ns to

qualify as a wake-up signal that powers up the serial interface.

Waking a Daisy Chain—Method 1

The ADBMS1818 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, in turn, wakes up the next

device. If there are N devices in the stack, all the devices are

powered up within the time N × tWAKE or N × tREADY, depending on

the core state. For large stacks, the time N × tWAKE may be equal to

or larger than tIDLE. In this case, after waiting longer than the time of

N × tWAKE, the host can send another dummy byte and wait for the

time N × tREADY 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, which guarantees that the devices propagate the

wake-up signal up the daisy chain. However, this method fails to

wake up all devices when a device in the middle of the chain is in

the ready state instead of the idle state. When this happens, the

device in the ready state does not propagate the wake-up pulse,

so the devices above it remain in the idle state. This situation can

occur when attempting to wake up the daisy chain after only tIDLE of

idle time (some devices may be idle, some may not).

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