33784 Datasheet, PDF (16/30 Pages) Freescale Semiconductor, Inc – DSI 2.02 Sensor Interface

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.
Bus Error
CRC
Error
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.
CRC
Error
Master
Command N
Command N+1 _{Command N+2}
Command N+3
Command N+4
Slave
Response N-1
Response N
No Response
Response N+2
Response N+3
CRC
Error
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.
33784
16
Analog Integrated Circuit Device Data
Freescale Semiconductor