DS80C390_00 Datasheet, PDF (146/155 Pages) Dallas Semiconductor – High-Speed Microcontroller User’s Guide Supplement

DS80C390 High-Speed Microcontroller User’s Guide Supplement
overwrite (WTOE=0) function prevents the module from potentially losing data during a high-speed data
transfer. In transmit mode, the WTOE bit prevents a message center ID from being overwritten by an
incoming remote frame.
The following examples demonstrate the use of the WTOE and other bits when using multiple message
centers with the same arbitration value. Case 2 illustrates the approach described above for configuring
multiple message centers to capture a large amount of data at a relatively high rate.
Case 1: WTOE=1 (Overwrites allowed)
1. Software configures message center 1 & 2 with the same arbitration value (abbreviated AV).
2. Software configures message center 1 & 2 to receive (T/ R = 0) and to allow message overwrite
WTOE=1.
3. The first message received that matches AV will be stored into message center 1, DTUP = 1.
4. The second message that matches AV will be stored into message center 1, DTUP = ROW = 1.
5. The third message that matches AV will be stored into message center 1.
6. etc.
Note that in this example message center 2 will never receive a message, and that if software does not
read message center 1 before the second message is received, the first message will be lost.
Case 2: WTOE=0 (Overwrites disabled)
1. Software configures message center 1 & 2 with the same arbitration value (abbreviated AV).
2. Software configures message center 1 & 2 to receive (T/ R = 0) and to disable message overwrite
WTOE=0.
3. The first message received that matches AV will be stored in message center 1, DTUP = 1.
4. The second message received that matches AV will be 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 will be 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 will be stored into message center 2, DTUP = 1
9. etc.
Note that in this example message center 1 or 2 will never be overwritten. The user must insure 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 may 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 Writeover Enable
(WTOE) bit. When overwrite is enabled (WTOE = 1) and there is unread data (DTUP =1) or a pending
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