Receive History List Function - NEC V850ES/F 3-L Series User Manual

Chapter 18

18.9.3 Receive history list function

Caution
604
The receive history list (RHL) function records in the receive history list the
number of the receive message buffer in which each data frame or remote
frame was received and stored. The RHL consists of storage elements
equivalent to up to 23 messages, the last in-message pointer (LIPT) with the
corresponding CnLIPT register and the receive history list get pointer (RGPT)
with the corresponding CnRGPT register.
The RHL is undefined immediately after the transition of the CAN module from
the initialization mode to one of the operation modes.
The CnLIPT register holds the contents of the RHL element indicated by the
value of the LIPT pointer minus 1. By reading the CnLIPT register, therefore,
the number of the message buffer that received and stored a data frame or
remote frame first can be checked. The LIPT pointer is utilized as a write
pointer that indicates to what part of the RHL a message buffer number is
recorded. Any time a data frame or remote frame is received and stored, the
corresponding message buffer number is recorded to the RHL element
indicated by the LIPT pointer. Each time recording to the RHL has been
completed, the LIPT 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 RGPT pointer is utilized as a read pointer that reads a recorded message
buffer number from the RHL. This pointer indicates the first RHL element that
the CPU has not read yet. By reading the CnRGPT register by software, the
number of a message buffer that has received and stored a data frame or
remote frame can be read. Each time a message buffer number is read from
the CnRGPT register, the RGPT pointer is automatically incremented.
If the value of the RGPT pointer matches the value of the LIPT pointer, the
RHPM bit (receive history list pointer match) of the CnRGPT register is set to
1. This indicates that no message buffer number that has not been read
remains in the RHL. If a new message buffer number is recorded, the LIPT
pointer is incremented and because its value no longer matches the value of
the RGPT pointer, the RHPM bit is cleared. In other words, the numbers of the
unread message buffers exist in the RHL.
If the LIPT pointer is incremented and matches the value of the RGPT pointer
minus 1, the ROVF bit (receive history list overflow) of the CnRGPT register is
set to 1. This indicates that the RHL is full of numbers of message buffers that
have not been read. When further message reception and storing occur, the
last recorded message buffer number is overwritten by the number of the
message buffer that received and stored the newly received message. In this
case, after the ROVF bit has been set (1), the recorded message buffer
numbers in the RHL do not completely reflect the chronological order. However
messages itself are not lost and can be located by CPU search in message
buffer memory with the help of the DN-bit.
If the history list is in the overflow condition (ROVF is set), reading the history
list contents is still possible, until the history list is empty (indicated by RHPM
flag set). Nevertheless, the history list remains in the overflow condition, until
ROVF is cleared by software. If ROVF is not cleared, the RHPM flag will also
not be updated (cleared) upon a message storage of newly received frame.
This may lead to the situation, that RHPM indicates an empty history list,
although a reception has taken place, while the history list is in the overflow
state (ROVF and RHPM are set).
User's Manual U18743EE1V2UM00
CAN Controller (CAN)