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Chapter 34 CAN Controller
3.Functional Description
The IF1 Registers are used as Transmit Buffer. The transmission of the contents of the IF1 Registers is
requested by writing the Busy bit of the IF1 Command Request Register to '1'. The IF1 Registers are locked
while the Busy bit is set. The Busy bit indicates that the transmission is pending.
As soon as the CAN bus is idle, the IF1 Registers are loaded into the shift register of the CAN Core and the
transmission is started. When the transmission has completed, the Busy bit is reset and the locked IF1
Registers are released.
A pending transmission can be aborted at any time by resetting the Busy bit in the IF1 Command Request
Register while the IF1 Registers are locked. If the CPU has reset the Busy bit, a possible retransmission in
case of lost arbitration or in case of an error is disabled.
The IF2 Registers are used as Receive Buffer. After the reception of a message the content of the shift
register is stored into the IF2 Registers, without any acceptance filtering.
Additionally, the actual contents of the shift register can be monitored during the message transfer. Each time
a read Message Object is initiated by writing the Busy bit of the IF2 Command Request Register to '1', the
content of the shift register is stored in the IF2 Registers.
In Basic Mode the evaluation of all Message Object related control and status bits and of the control bits of the
IFx Command Mask Registers is turned off. The message number of the Command request registers is not
evaluated. The NewDat and MsgLst bits of the IF2 Message Control Register retain their function, DLC3-0
will show the received DLC, the other control bits will be read as '0'.
3.9 Software control of Pin CAN_TX
Four output functions are available for the CAN transmit pin CAN_TX. Additionally to its default function – the
serial data output – it can drive the CAN Sample Point signal to monitor CAN_Core's bit timing and it can drive
constant dominant or recessive values. The last two functions, combined with the readable CAN receive pin
CAN_RX, can be used to check the CAN bus' physical layer.
The output mode of pin CAN_TX is selected by programming the Test Register bits Tx1 and Tx0 as described
in section
2.3 "CAN Protocol Related Registers" on page 696.
The three test functions for pin CAN_TX interfere with all CAN protocol functions. CAN_TX must be left in its
default function when CAN message transfer or any of the test modes Loop Back Mode, Silent Mode, or Basic
Mode are selected.
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