MC33780 Datasheet, PDF (21/37 Pages) Freescale Semiconductor, Inc – Dual DBUS Master with Differential Drive and Frequency Spreading

FUNCTIONAL DEVICE OPERATION
LOGIC COMMANDS AND REGISTERS
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-- Calculates the 4-bit CRC (x^4 + 1) serially for 8 to 16 bits of data.
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constant CRCPoly: std_logic_vector: = “0001”; -- x^4 +1
constant InitCrc: std_logic_vector: = “1010”;
procedure SerialCalculateCRC4(CRC: input std_logic_vector;Data: in std_logic) is variable
Xor1: std_logic;
begin
Xor1: = CRC(3) xor Data;
CRC: = CRC(2 downto 0) & ‘0’; -- Shift left 1 bit
if Xor1 = ‘1’ then
CRC: = CRC xor CRCPoly
end if;
end SerialCalculateCRC4;
Figure 16. CRC Algorithm
C3
C2
C1
C0
T
T
T
T
Input Data
1X4 + 0X3 + 0X2 + 0X1 + +X0 = X4+1
Figure 17. Default CRC Block Diagram
MESSAGE SIZE SPECIAL CASES
The response to any 8- to 15-bit message is expected to
be another 8- to 15-bit message and the response to any
16-bit message is expected to be another 16-bit message.
This gives rise to some special cases when there is a
transition from one message size to a different message size.
Some messages must be long words (16 bits of data), others
can be short words (8 to 15 bits of data).
The following are examples where the word is a standard
DSI formatted short word (8 bits of data and 4 bits of CRC).
Example 1: If the previous message was a short word and
the current message is a long word, the response message
(which is also a short word) finishes before the current
message frame and the CRC bits look like data bits in the
long word format. Since the CRC validation of this short word
message response is not reliable, this short word response
should not be used.
Example 2: If the previous message was a long word and
the current message is a short word, the response message
(which is also a long word) cannot finish before the current
message frame. Bits three to zero of the data and the CRC
bits are lost. Data bits seven to four of the 16-bit response
message look like the CRC bits of an 8-bit response and
almost certainly would not be correct. Because the response
is incomplete and the CRC check is probably not valid, this
response is not useful.
The long word to short word message size transition
normally only occurs after setting up the DBUS peripherals.
During address setup, a message with address 0000 is sent
to attempt to set the address of the next peripheral on the
daisy-chained bus. Before any peripherals have been
assigned an address, their bus switches are opened so the
addressing message only goes to the first peripheral in line.
As each peripheral gets an address, it closes its bus switch
so the next address assignment command can reach the next
peripheral in line on the bus. Each peripheral responds to an
address assignment only once (during the next message
after the command that set its address). After the last
peripheral has been assigned an address, any subsequent
address assignments will receive no response. When the
master MCU fails to receive a response, it knows it has
passed the last peripheral. At this point, short word messages
may be sent. The first such message will have no meaningful
response associated with it.
The first message after reset is also a special case
because there was no previous message, therefore there will
be no meaningful response during the first message transfer.
Analog Integrated Circuit Device Data
Freescale Semiconductor
33780
21