ADBMS1818CSWAZ-R7 datasheet(59/92 Pages) AD

59 / 92 page

Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. B | 59 of 92

While writing any command to ADBMS1818, the command bytes

CMD0 and CMD1 (see Table 49 and Table 50) and the PEC bytes

PEC0 and PEC1 are sent on Port A in the following order:

CMD0, CMD1, PEC0, PEC1

After a write command to daisy-chained ADBMS1818 devices,

data is sent to each device followed by the PEC. For example,

when writing Configuration Register Group A to two daisy-chained

devices (primary device P, stacked device S), the data is sent to the

primary device on Port A in the following order:

CFGAR0(S), … , CFGAR5(S), PEC0(S), PEC1(S), CFGAR0(P),

… , CFGAR5(P), PEC0(P), PEC1(P)

After a read command for daisy-chained devices, each device shifts

out its data and the PEC that it computed for its data on Port

A followed by the data received on Port B. For example, when

reading Status Register Group B from two daisy -chained devices

(primary device P, stacked device S), the primary device sends out

data on Port A in the following order:

STBR0(P), … , STBR5(P), PEC0(P), PEC1(P), STBR0(S), … ,

STBR5(S), PEC0(S), PEC1(S)

See the Bus Protocols section for the command format.

All devices in a daisy-chained configuration receive the command

bytes simultaneously. For example, to initiate ADC conversions in a

stack of devices, a single ADCV command is sent, and all devices

start conversions at the same time. For read and write commands,

a single command is sent, and the stacked devices effectively turn

into a cascaded shift register, in which data is shifted through each

device to the next higher (on a write) or the next lower (on a read)

device in the stack. See the Serial Interface Overview 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.

If using a single ADBMS1818 that communicates in SPI mode

(ISOMD pin tied low), there are two methods of polling. 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 performing conversions. SDO

is pulled high when the device completes conversions. However,

SDO also goes high when CSB goes high even if the device has

not completed the conversion (see Figure 89). A problem with this

method is that the controller is not free to perform other serial

communications 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 (see Figure 90). After entering the PLADC

command, SDO goes low if the device is busy performing conver-

sions. SDO is pulled high at the end of conversions. However, SDO

also goes high when CSB goes high even if the device has not

completed the conversion.

If using a single ADBMS1818 that communicates in isoSPI mode,

the low-side port transmits a data pulse only in response to a

master isoSPI pulse received by it. Therefore, after entering the

command in either method of polling described previously, isoSPI

data pulses are sent to the part to update the conversion status.

These pulses can be sent using the LTC6820 by simply clocking its

SCK pin. In response to this pulse, the ADBMS1818 sends back a

low isoSPI pulse if it is still busy performing conversions or a high

data pulse if it has completed the conversions. If a CSB high isoSPI

pulse is sent to the device, the device exits the polling command.

In a daisy-chained configuration of N stacked devices, the same

two polling methods can be used. If the bottom device communi-

cates in SPI mode, the SDO of the bottom device indicates the

conversion status of the entire stack. That is, SDO remains low until

all the devices in the stack have completed the conversions. In the

first method of polling, after an ADC conversion command is sent,

clock pulses are sent on SCK while keeping CSB low. The SDO

status becomes valid only at the end of N clock pulses on SCK.

During the first N clock pulses, the bottom ADBMS1818 in the daisy

chain outputs a 0 or a low data pulse. After N clock pulses, the

output data from the bottom ADBMS1818 gets updated for every

clock pulse that follows (see Figure 91). In the second method, the

PLADC command is sent followed by clock pulses on SCK while

keeping CSB low. Similar to the first method, the SDO status is

valid only after N clock cycles on SCK and gets updated after every

clock cycle that follows (see Figure 92).

If the bottom device communicates in isoSPI mode, isoSPI data

pulses are sent to the device to update the conversion status.

Using the LTC6820, this action can be achieved by just clocking

the SCK pin. The conversion status is valid only after the bottom

ADBMS1818 device receives N isoSPI data pulses and the status

gets updated for every isoSPI data pulse that follows. The device

returns a low data pulse if any of the devices in the stack is busy

performing conversions and returns a high data pulse if all the

devices are free.

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