PC912XDP512F0MPVR datasheet(469/1348 Pages) FREESCALE

469 / 1348 page

Chapter 10 Freescale’s Scalable Controller Area Network (S12MSCANV3)

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

469

•

If there are one or more message buffers scheduled for transmission (TXEx = 0), the MSCAN will

continue to transmit until all transmit message buffers are empty (TXEx = 1, transmitted

successfully or aborted) and then goes into sleep mode.

•

If the MSCAN is receiving, it continues to receive and goes into sleep mode as soon as the CAN

bus next becomes idle.

•

If the MSCAN is neither transmitting nor receiving, it immediately goes into sleep mode.

Figure 10-45. Sleep Request / Acknowledge Cycle

NOTE

The application software must avoid setting up a transmission (by clearing

one or more TXEx ﬂag(s)) and immediately request sleep mode (by setting

SLPRQ). Whether the MSCAN starts transmitting or goes into sleep mode

directly depends on the exact sequence of operations.

If sleep mode is active, the SLPRQ and SLPAK bits are set (Figure 10-45). The application software must

use SLPAK as a handshake indication for the request (SLPRQ) to go into sleep mode.

When in sleep mode (SLPRQ = 1 and SLPAK = 1), the MSCAN stops its internal clocks. However, clocks

that allow register accesses from the CPU side continue to run.

If the MSCAN is in bus-off state, it stops counting the 128 occurrences of 11 consecutive recessive bits

due to the stopped clocks. The TXCAN pin remains in a recessive state. If RXF = 1, the message can be

read and RXF can be cleared. Shifting a new message into the foreground buffer of the receiver FIFO

(RxFG) does not take place while in sleep mode.

It is possible to access the transmit buffers and to clear the associated TXE ﬂags. No message abort takes

place while in sleep mode.

If the WUPE bit in CANCTL0 is not asserted, the MSCAN will mask any activity it detects on CAN. The

RXCAN pin is therefore held internally in a recessive state. This locks the MSCAN in sleep mode

(Figure 10-46). WUPE must be set before entering sleep mode to take effect.

The MSCAN is able to leave sleep mode (wake up) only when:

•

CAN bus activity occurs and WUPE = 1

SYNC

Bus Clock Domain

CAN Clock Domain

MSCAN

in Sleep Mode

CPU

Sleep Request

SLPRQ

Flag

SLPAK

Flag

SLPRQ

sync.

SLPAK

sync.

SLPRQ

SLPAK

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