33784 datasheet(16/30 Pages) FREESCALE

16 / 30 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

33784

FUNCTIONAL DEVICE OPERATION

OPERATIONAL MODES

In case there is a bus error (due to induced noise or a bus

fault), both the master and slave devices will likely read bad

data. The slave reacts to bad data by not sending a response

during the next frame, and clears the any pending response.

The master will detect a CRC error (if enabled) once it

receives the corrupted data sent by the slave, and once again

when the slave fails to respond. This is illustrated in Figure 8.

When this error occurs, the system software needs to

acknowledge this condition and resend a command of the

same size so that it can receive the proper response.

Failure to take corrective action will result in unintended

errors as shown in Figure 8. In this case, the master will miss

Responses N and N+1.

Figure 8. Bus Traffic With Receive Error and Recovery

STANDARD DBUS COMMAND STRUCTURE

Two word sizes are available for standard DBUS

commands. These are termed “long word” and “short word”.

A standard long word always consists of 8 data bits,

4 address bits, 4 command bits, and 4 cyclic redundancy

check (CRC) bits. The data bits are always sent first, starting

with the MSB, and are followed by the address bits, then the

command bits, and ending with the CRC bits. Refer to

Table 7, page 17.

A standard short word consists of 4 address bits,

4 command bits, and 4 CRC bits. The address bits are

always sent first, starting with the MSB, followed by the

command bits, and ending with the CRC bits. This is also

shown in Table 7.

Some commands can be sent in either standard long word

or standard short word format as desired. If these commands

are sent in long word format, the data bits are “don’t-care” for

the 33784, but should all be set to 0 to maintain future

compatibility.

When a standard 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 X4+1 and seed

1010. The polynomial and seed cannot be changed when

communicating in standard mode. 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.

ENHANCED DBUS COMMAND STRUCTURE

In addition to standard DBUS commands, the 33784 can

accept enhanced DBUS commands. Like standard

commands, there are two word sizes available for enhanced

commands. These, like the standard long word, are termed

“enhanced long word” and “enhanced short word”. An

enhanced long word always consists of 8 data bits, 4 address

bits, 4 command bits, and 4 CRC bits. The data bits are

always sent first, starting with the MSB, and are followed by

the address bits, then the command bits, and ending with the

CRC bits. Refer to Table 7.

However, an enhanced long word differs from a standard

long word in that the CRC polynomial and seed are not fixed

and can be programmed into the IC via the bus. The method

of programming the polynomial and seed is discussed in

Format Control Command and Response, page 23.

Likewise, enhanced short words will also use the

polynomial and seed that have been programmed into the IC.

Enhanced short words consist of 0 or 2 data bits, 4 address

bits, 4 command bits, and 4 CRC bits. The data bits (if any)

are sent first, followed by the address bits, followed by the

command bits, and ending with the CRC bits. This is shown

in Table 7. The optional data bits are only place holders and

are used so that longer responses can be transmitted. If the

optional data bits are used, they are “don’t-care” for the

33784, but should both be set to 0 to maintain future

compatibility.

Some commands can be sent in either enhanced long

word or enhanced short word format as desired. If these

commands are sent in enhanced long word format, the data

bits are “don’t-care” for the 33784, but should all be set to 0

to maintain future compatibility.

Command N

Bus Error

Command N+1

Command N+2

Response N-1

Response N

No Response

Response N+2

Command N+3

Command N+4

Response N+3

CRC

Error

CRC

Error

CRC

Master

Slave

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