33784 datasheet(17/30 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33784

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

When an enhanced long word or short word is sent on the

bus, the 33784 will calculate a CRC as each bit is received.

The CRC is calculated using the polynomial and seed that

have been programmed into the IC via the bus. At the

conclusion of the transmission, the 33784 will compare the

calculated CRC with the CRC included within the message. If

the two match, the message is considered valid and the

33784 will act on the message accordingly. If the calculated

CRC does not match the CRC included within the message,

the 33784 will ignore the transmission and the message will

be discarded.

STANDARD DBUS RESPONSE STRUCTURE

There are two standard response lengths to correspond

with the two standard command word lengths. A standard

long response always consists of 16 data bits and 4 CRC bits.

A standard short response always consists of 8 data bits and

4 CRC bits. Refer to Table 8.

In both cases, the data bits are sent first, starting with the

MSB, and are followed by the CRC bits. The CRC bits are

calculated from the data bits using the standard polynomial

X4+1 and seed 1010. The polynomial and seed cannot be

changed when responding in standard mode.

Normally, standard long responses will be sent for

standard long commands, and standard short responses will

be sent for standard short commands. However, if a long

command is followed by a short command, then the response

to the long command will occur during the short command

and will be truncated. In this case, the response to the long

command is considered invalid.

Similarly, if a short command is followed by a long

command, then the response to the short command will occur

during the long command and will contain extra bits. In this

case the response to the short command is considered

invalid.

ENHANCED DBUS RESPONSE STRUCTURE

There are two enhanced response lengths to correspond

with the two enhanced command word lengths. Like the

standard long word, an enhanced long response always

consists of 16 data bits and 4 CRC bits. The data bits are sent

first, starting with the MSB, and are followed by the CRC bits.

The CRC bits are calculated from the data bits using the

polynomial and seed that was programmed into the IC via the

bus.

An enhanced short response consists of either 8 or 10

data bits and 4 CRC bits. The enhanced short response will

have 8 data bits if the enhanced short command did not use

the optional 2 bits, and it will have 10 data bits if the enhanced

short command did use the optional 2 bits.

In certain cases, the optional 2 bits might be used in the

command, but due to the nature of the command, the

response only contains 8 bits of data. In this circumstance,

the response will be right-padded with zeros so that 10 data

bits are sent, followed by the CRC.

In other cases, the optional 2 bits might not be used in the

command, but due to the nature of the command, the

response contains 10 bits of data. In this circumstance, the 2

least significant bits of the response data will be dropped and

only the 8 most significant data bits are sent, followed by the

CRC. This is illustrated in logic Commands and Registers,

page 18.

Table 7. Standard and Enhanced DBUS Command Structure

Word Type

Symbol First

Data

Address

Command

CRC

Last

Standard Long Word

Enhanced Long Word

ELW

Standard Short Word

8-Bit Enhanced Short Word

8-Bit ESW

10-Bit Enhanced Short Word 10-Bit ESW

Table 8. Standard and Enhanced DBUS Response Structure

Word Type

Symbol

First

Response

CRC

Last

Standard Long Word

D15

D14

D13

D12

D11

D10

Enhanced Long Word

ELW

D15

D14

D13

D12

D11

D10

Standard Short Word

8-Bit Enhanced Short Word

8-Bit ESW

10-Bit Enhanced Short Word 10-Bit ESW

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