Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement - Manual (Page 159)

High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
159
Case 1: WTOE = 1 (Overwrites allowed)
1. Software configures message centers 1 and 2 with the same arbitration value (abbreviated AV).
2. Software configures message centers 1 and 2 to receive (T/R= 0) and allow message overwrite (WTOE = 1).
3. The first message received that matches AV is stored in message center 1, DTUP = 1.
4. The second message that matches AV is stored in message center 1, DTUP = ROW = 1.
5. The third message that matches AV is stored in message center 1.
6. Etc.
Note: In this example, message center 2 never receives a message and that, if software does not read message center 1 before the
second message is received, the first message is lost.
Case 2: WTOE = 0 (Overwrites disabled)
1. Software configures message centers 1 and 2 with the same arbitration value (abbreviated AV).
2. Software configures message centers 1 and 2 to receive (T/R = 0) and to disable message overwrite (WTOE = 0).
3. The first message received that matches AV is stored in message center 1, DTUP = 1.
4. The second message received that matches AV is stored in message center 2, DTUP = 1
5. Software reads message center 1 and then programs message center 1, DTUP = 0.
6. The third message received that matches AV is stored into message center 1, DTUP = 1.
7. Software reads message center 2 and then programs message center 2, DTUP = 0.
8. The fourth message received that matches AV is stored into message center 2, DTUP = 1
9. Etc.
Note: In this example, message center 1 or 2 is never overwritten. The user must ensure that the proper number of message centers
be allocated to the same arbitration value when using this arrangement. If software fails to read the allocated message group, an
incoming message can be lost without software realizing it (ROW is never set when WTOE = 0). To put a message center back into
operation, software must force DTUP = 0 and EXTRQ = 0. This indicates that software has read the message center.
Special Considerations for Message Center 15
Message center 15 incorporates a shadow message center used to buffer incoming messages, in addition to the standard message
center registers. When the message center is empty (DTUP = EXTRQ = 0), incoming messages are loaded directly into the message
center registers. When the message center has unread data (DTUP = 1) or a pending remote frame request (EXTRQ = 1), incoming
messages are loaded into the shadow message center. Unread contents of the shadow message center are automatically loaded into
the message center when it becomes empty (DTUP = 0). An overwrite condition is possible when both the message center 15 and
shadow message centers are full.
The response of message center 15 to the overwrite condition is dependent on the write-over enable (WTOE) bit. When overwrite is
enabled (WTOE = 1) and there is unread data (DTUP = 1) or a pending remote frame request (EXTRQ = 1), successfully received mes-
sages are stored in the shadow message center, overwriting existing data. If the shadow message center contained previously unread
data at the time of the overwrite, the message center 15 ROW bit is set. If the shadow message center was empty at the time, then the
incoming message is simply loaded into the shadow buffer and ROW is not set to a 1. Note that the message center 15 ROW bit reflects
only an overwrite of the shadow message center, not the message center registers (as with message centers 1–14).
When WTOE = 0, there is unread data (DTUP = 1) or a pending remote frame request (EXTRQ = 1) in message center 15, and there
is already a message stored in the shadow buffer. Incoming messages are not stored in either the message center or shadow buffer.
Using the Autobaud Feature
It is sometimes necessary to connect a CAN node to a network with an unknown baud rate. The autobaud feature of the DS80C400
provides a simple way for the CAN module to determine the baud rate of the network and reconfigure the DS80C400 to operate at that
baud rate. Special hardware inside the CAN module allows it to interface to a fully functional CAN bus and perform the autobaud fea-
ture without disturbing other CAN nodes.
Maxim Integrated