Motorola MCF5281, MCF5282 user manual Interrupts, Auto-Power Save Mode

Models: MCF5282 MCF5281

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Functional Overview

If the FlexCAN is in debug mode when the STOP bit is set, the FlexCAN will assume that debug mode should be exited. As a result, it will try to synchronize with the CAN bus, and only then will it await the conditions required for entry into low-power stop mode.

Unlike other modules, the FlexCAN does not come out of reset in low-power stop mode. The basic FlexCAN initialization procedure (see Section 25.4.10, “FlexCAN Initialization Sequence”) should be executed before placing the module in low-power stop mode.

If the FlexCAN is in low-power stop mode with the self-wake mechanism engaged and is operating with a single system clock per time quantum, there can be extreme cases in which FlexCAN wake-up on recessive to dominant edge may not conform to the CAN protocol. FlexCAN synchronization will be shifted one time quantum from the wake-up event. This shift lasts until the next recessive to dominant edge, which resynchronizes the FlexCAN to be in conformance with the CAN protocol. The same holds true when the FlexCAN is in auto-power save mode and awakens on a recessive to dominant edge.

25.4.11.3 Auto-Power Save Mode

Auto-power save mode enables normal operation with optimized power savings. Once the auto-power save (APS) bit in CANMCR is set, the FlexCAN looks for a set of conditions in which there is no need for its clocks to be running. If these conditions are met, the FlexCAN stops its clocks, thus saving power. The following conditions will activate auto-power save mode.

No Rx/Tx frame in progress.

No transfer of Rx/Tx frames to and from an SMB, and no Tx frame awaiting transmission in any message buffer.

No CPU access to the FlexCAN module.

The FlexCAN is not in debug mode, low-power stop mode, or the bus off state.

While its clocks are stopped, if the FlexCAN senses that any one of the aforementioned conditions is no longer true, it restarts its clocks. The FlexCAN then continues to monitor these conditions and stops/restarts its clocks accordingly.

25.4.12 Interrupts

The module can generate up to 19 interrupt sources (16 interrupts due to message buffers and 3 interrupts due to Bus-off, Error and Wake-up). Each one of the message buffers can be an interrupt source, if its corresponding IMASK bit is set.

There is no distinction between Tx and Rx interrupts for a particular buffer, under the assumption that the buffer is initialized for either transmission or reception, and thus its

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Chapter 25. FlexCAN

25-19

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Motorola MCF5281, MCF5282 user manual Interrupts, Auto-Power Save Mode