ADBMS1818ASWZ-R7 datasheet(53/89 Pages) AD

53 / 89 page

Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. 0 | 53 of 89

Figure 87. Wake-Up Detection and Idle Timer

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 a minimum, a pair of long isoSPI

pulses (–1 and +1) is needed for each device, separated by more

than tREADY or tWAKE (if the core state is standby mode or sleep

mode, respectively), but less than tIDLE. 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 ADBMS1818 with ISOMD = 0) to

generate the long isoSPI pulses. Alternatively, dummy commands

(such as RDCFGA) can be executed to generate the long isoSPI

pulses.

DATA LINK LAYER

All data transfers on ADBMS1818 occur in byte groups. Every byte

consists of 8 bits. Bytes are transferred with the MSB first. CSB

must remain low for the entire duration of a command sequence,

including between a command byte and subsequent data. On a

write command, data is latched in on the rising edge of CSB.

NETWORK LAYER

Packet Error Code

The PEC is a 15-bit cyclic redundancy check (CRC) value calculat-

ed for all of the bits in a register group in the order they are passed,

using the initial PEC value of 000000000010000 and the following

characteristic polynomial: x15 + x14 + x10 + x8 + x7 + x4 + x3 + 1.

To calculate the 15-bit PEC value, a simple procedure can be

established:

1. Initialize the PEC to 000000000010000 (PEC is a 15‑bit register

group).

2. For each bit DIN coming into the PEC register group, set:

►

IN0 = DIN XOR PEC, Bit 14

►

IN3 = IN0 XOR PEC, Bit 2

►

IN4 = IN0 XOR PEC, Bit 3

►

IN7 = IN0 XOR PEC, Bit 6

►

IN8 = IN0 XOR PEC, Bit 7

►

IN10 = IN0 XOR PEC, Bit 9

►

IN14 = IN0 XOR PEC, Bit 13

3. Update the 15-bit PEC as follows:

►

PEC, Bit 14 = IN14

►

PEC, Bit 13 = PEC, Bit 12

►

PEC, Bit 12 = PEC, Bit 11

►

PEC, Bit 11 = PEC, Bit 10

►

PEC, Bit 10 = IN10

►

PEC, Bit 9 = PEC, Bit 8

►

PEC, Bit 8 = IN8

►

PEC, Bit 7 = IN7

►

PEC, Bit 6 = PEC, Bit 5

►

PEC, Bit 5 = PEC, Bit 4

►

PEC, Bit 4 = IN4

►

PEC, Bit 3 = IN3

►

PEC, Bit 2 = PEC, Bit 1

►

PEC, Bit 1 = PEC, Bit 0

►

PEC, Bit 0 = IN0

4. Go back to Step 2 until all the data is shifted. The final PEC

(16 bits) is the 15-bit value in the PEC register with a 0 bit

appended to its LSB.

Figure 88 shows the 15-bit PEC algorithm. An example to calculate

the PEC for a 16-bit word (0x0001) is listed in Table 43. The PEC

for 0x0001 is computed as 0x3D6E after stuffing a 0 bit at the LSB.

For longer data streams, the PEC is valid at the end of the last bit of

data sent to the PEC register.

The ADBMS1818 calculates the PEC for any command or data

received and compares it with the PEC following the command or

data. The command or data is regarded as valid only if the PEC

matches. ADBMS1818 also attaches the calculated PEC at the end

of the data it shifts out. Table 44 shows the format of PEC while

writing to or reading from ADBMS1818.

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