Chapter 23 Controller Area Network (CAN).pdf
Chapter 23 Controller Area Network (CAN).pdf
Chapter 23 Controller Area Network (CAN).pdf
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Functional Description www.ti.com<br />
<strong>23</strong>.3.8.2.1 Automatic Retransmission<br />
According to the <strong>CAN</strong> Specification (ISO11898), the D<strong>CAN</strong> provides a mechanism to automatically<br />
retransmit frames which have lost arbitration or have been disturbed by errors during transmission. The<br />
frame transmission service will not be confirmed to the user before the transmission is successfully<br />
completed.<br />
By default, this automatic retransmission is enabled. It can be disabled by setting bit disable automatic<br />
retransmission ( DAR) in the <strong>CAN</strong> Control register. Further details to this mode are provided in<br />
Section <strong>23</strong>.3.15.3.<br />
<strong>23</strong>.3.8.2.2 Auto-Bus-On<br />
By default, after the D<strong>CAN</strong> has entered Bus-Off state, the CPU can start a Bus-Off-Recovery sequence by<br />
resetting the Init bit. If this is not done, the module will stay in Bus-Off state.<br />
The D<strong>CAN</strong> provides an automatic Auto-Bus-On feature which is enabled by bit ABO in the <strong>CAN</strong> control<br />
register. If set, the D<strong>CAN</strong> will automatically start the Bus-Off-Recovery sequence. The sequence can be<br />
delayed by a user-defined number of L3_SLOW_GCLK cycles which can be defined in the Auto-Bus-On<br />
Time register (D<strong>CAN</strong> ABOTR).<br />
NOTE: If the D<strong>CAN</strong> goes to Bus-Off state due to a massive occurrence of <strong>CAN</strong> bus errors, it stops<br />
all bus activities and automatically sets the Init bit. Once the Init bit has been reset by the<br />
CPU or due to the Auto-Bus-On feature, the device will wait for 129 occurrences of bus Idle<br />
(equal to 129 * 11 consecutive recessive bits) before resuming normal operation. At the end<br />
of the Bus-Off recovery sequence, the error counters will be reset.<br />
<strong>23</strong>.3.8.3 Test Modes<br />
The D<strong>CAN</strong> module provides several test modes which are mainly intended for production tests or self test.<br />
For all test modes, the Test bit in the <strong>CAN</strong> control register needs to be set to one. This enables write<br />
access to the test register (D<strong>CAN</strong> TEST).<br />
<strong>23</strong>.3.8.3.1 Silent Mode<br />
The silent mode may be used to analyze the traffic on the <strong>CAN</strong> bus without affecting it by sending<br />
dominant bits (e.g., acknowledge bit, overload flag, active error flag). The D<strong>CAN</strong> is still able to receive<br />
valid data frames and valid remote frames, but it will not send any dominant bits. However, these are<br />
internally routed to the <strong>CAN</strong> core.<br />
Figure <strong>23</strong>-5 shows the connection of signals <strong>CAN</strong>_TX and <strong>CAN</strong>_RX to the <strong>CAN</strong> core in silent mode. Silent<br />
mode can be activated by setting the Silent bit in the Test register to one. In ISO 11898-1, the silent mode<br />
is called the bus monitoring mode.<br />
Figure <strong>23</strong>-5. <strong>CAN</strong> Core in Silent Mode<br />
D<strong>CAN</strong><br />
<strong>CAN</strong>_TX <strong>CAN</strong>_RX<br />
=1<br />
•<br />
Tx Rx<br />
<strong>CAN</strong> Core<br />
4046 <strong>Controller</strong> <strong>Area</strong> <strong>Network</strong> (<strong>CAN</strong>) SPRUH73E–October 2011–Revised May 2012<br />
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