NEC V850E/PH2, User Manual

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Chapter 18 AFCAN Controller
User’s Manual U16580EE3V1UD00
a data frame or remote frame first can be checked. The LOPT pointer is utilized as a write pointer that
indicates to what part of the THL a message buffer number is recorded. Any time a data frame or
remote frame is transmitted, the corresponding message buffer number is recorded to the THL element
indicated by the LOPT pointer. Each time recording to the THL has been completed, the LOPT pointer
is automatically incremented. In this way, the number of the message buffer that has received and
stored a frame will be recorded chronologically.
The TGPT pointer is utilized as a read pointer that reads a recorded message buffer number from the
THL. This pointer indicates the first THL element that the CPU has not yet read. By reading the CnT-
GPT register by software, the number of a message buffer that has completed transmission can be
read. Each time a message buffer number is read from the CnTGPT register, the TGPT pointer is auto-
matically incremented.
If the value of the TGPT pointer matches the value of the LOPT pointer, the THPM bit (transmit history
list pointer match) of the CnTGPT register is set to 1. This indicates that no message buffer numbers
that have not been read remain in the THL. If a new message buffer number is recorded, the LOPT
pointer is incremented and because its value no longer matches the value of the TGPT pointer, the
THPM bit is cleared. In other words, the numbers of the unread message buffers exist in the THL.
If the LOPT pointer is incremented and matches the value of the TGPT pointer minus 1, the TOVF bit
(transmit history list overflow) of the CnTGPT register is set to 1. This indicates that the THL is full of
message buffer numbers that have not been read. If a new message is received and stored, the mes-
sage buffer number recorded last is overwritten by the message buffer number that transmitted its mes-
sage afterwards. In this case, after the TOVF bit has been set (1), therefore, the recorded message
buffer numbers in the THL do not completely reflect the chronological order. However the other trans-
mitted messages can be found by a CPU search applied to all transmit message buffers unless the
CPU has not overwritten a transmit object in one of these buffers beforehand. In total up to six transmis-
sion completions can occur without overflowing the THL.
Caution: If the history list is in the overflow condition (TOVF is set), reading the history list
contents is still possible, until the history list is empty (indicated by THPM flag set).
Nevertheless, the history list remains in the overflow condition, until TOVF is cleared
by software. If TOVF is not cleared, the THPM flag will also not be updated (cleared)
upon successful transmission of a new message. This may lead to the situation, that
THPM indicates an empty history list, although a successful transmission has taken
place, while the history list is in the overflow state (TOVF and THPM are set).