Hardware Reference
In-Depth Information
After the transmission/reception of the last bit of the data field, CRC_RG(14:0) contains the
CRC sequence. The
CRC delimiter
is a single recessive bit.
ACK F
IELD
As shown in Figure 13.6, the ACK
field is 2 bits long and contains the ACK slot and the
ACK delimiter. A transmitting node sends 2 recessive bits in the ACK field. A receiver that has
received a valid message reports this to the transmitter by sending a dominant bit in the ACK
slot (i.e., it sends ACK). A node that has received the matching CRC sequence overwrites the
recessive bit in the ACK slot with a dominant bit. This bit will be received by the data frame
transmitter and learn that the previously transmitted data frame has been correctly received.
The
ACK delimiter
has to be a recessive bit. As a consequence, the ACK slot is surrounded by 2
recessive bits (the CRC delimiter and the ACK delimiter).
ACK field
CRC field
End-of-frame
ACK slot
ACK delimiter
Figure 13.6
■
ACK field
E
ND
-
OF
-F
RAME
F
IELD
Each data frame and remote frame is delimited by a flag sequence consisting of 7 recessive
bits. This 7-bit sequence is the
end-of-frame
sequence.
A node that needs certain data can request the relevant source node to transmit the data by
sending a remote frame. The format of a remote frame is shown in Figure 13.7. A remote frame
Interframe
space or
overload
frame
Interframe
space
Remote frame
Start-of-
frame
Arbitration
field
Control
field
CRC
field
ACK
field
End-of-
frame
Figure 13.7
■
Remote frame
consists of six fields: start of frame, arbitration, control, CRC, ACK, and end of frame. The
polarity of the RTR bit in the arbitration field indicates whether a transmitted frame is a
data
frame
(RTR bit dominant) or a
remote frame
(RTR bit recessive).
The error frame consists of two distinct fields. The first field is given by the superposition
of error flags contributed from different nodes. The second field is the error delimiter. The for-
mat of the error frame is shown in Figure 13.8. In order to terminate an error frame correctly, an
error-passive
node
may need the bus to be idle for at least 3 bit times (if there is a local error at
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