COBHAM GR740 User Manual

GR740-UM-DS, Nov 2017, Version 1.7 266 www.cobham.com/gaisler
GR740
CAN controller will be performed. The CAN controller can thus hold two CAN messages for trans-
mission: one in the fetch buffer, which is fed to the CAN core, and one in the prefetch buffer.
After a message has been successfully transmitted, the prefetch-buffer contents are moved to the fetch
buffer (provided that there is message ready). The read pointer (CanTxRD.READ) is automatically
incremented after a successful transmission, i.e. after the fetch-buffer contents have been transmitted,
taking wrap around effects of the circular buffer into account. If there is at least an additional CAN
message available in the circular buffer, a new prefetch will be performed.
If the write and read pointers are equal, no more prefetches and fetches will be performed, and trans-
mission will stop.
If the single shot mode is enabled for the transmit channel (CanTxCTRL.SINGLE=1), any message
for which the arbitration is lost, or failed for some other reason, will lead to the disabling of the chan-
nel (CanTxCTRL.ENABLE=0), and the message will not be put up for re-arbitration.
Interrupts are provided to aid the user during transmission, as described in detail later in this section.
The main interrupts are the Tx, TxEmpty and TxIrq which are issued on the successful transmission
of a message, when all messages have been transmitted successfully and when a predefined number of
messages have been transmitted successfully. The TxLoss interrupt is issued whenever transmission
arbitration has been lost, could also be caused by a communications error. The TxSync interrupt is
issued when a message matching the SYNC Code Filter Register.SYNC and SYNC Mask Filter Reg-
ister.MASK registers is successfully transmitted. Additional interrupts are provided to signal error
conditions on the CAN bus and AMBA bus.
17.5.5 Straight buffer
It is possible to use the circular buffer as a straight buffer, with a higher granularity than the 1kbyte
address boundary limited by the base address (CanTxADDR.ADDR) field.
While the channel is disabled, the read pointer (CanTxRD.READ) can be changed to an arbitrary
value pointing to the first message to be transmitted, and the write pointer (CanTxWR.WRITE) can
be changed to an arbitrary value.
When the channel is enabled, the transmission will start from the read pointer and continue to the
write pointer.
17.5.6 AMBA AHB error
Definition:
• a message fetch occurs when no other messages is being transmitted
• a message prefetch occurs when a previously fetched message is being transmitted
• the local fetch buffer holds the message being fetched
• the local prefetch buffer holds the message being prefetched
• the local fetch buffer holds the message being transmitted by the CAN core
• a successfully prefetched message is copied from the local prefetch buffer to the local fetch buf-
fer when that buffer is freed after a successful transmission.
An AHB error response occurring on the AMBA AHB bus while a CAN message is being fetched
will result in a TxAHBErr interrupt.
If the CanCONF.ABORT bit is set to 0b, the channel causing the AHB error will skip the message
being fetched from memory and will increment the read pointer. No message will be transmitted.
If the CanCONF.ABORT bit is set to 1b, the channel causing the AHB error will be disabled
(CanTxCTRL.ENABLE is cleared automatically to 0 b). The read pointer can be used to determine
which message caused the AHB error. Note that it could be any of the four word accesses required to
read a message that caused the AHB error.

COBHAM GR740 Specifications

General

Brand	COBHAM
Model	GR740
Category	Computer Hardware
Language	English