MC9S08DZ96F2CLL datasheet(298/458 Pages) FREESCALE

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Chapter 12 Freescale’s Controller Area Network (S08MSCANV1)

MC9S08DZ128 Series Data Sheet, Rev. 1

298

Freescale Semiconductor

12.5.5.4

MSCAN Sleep Mode

The CPU can request the MSCAN to enter this low power mode by asserting the SLPRQ bit in the

CANCTL0 register. The time when the MSCAN enters sleep mode depends on a ﬁxed synchronization

delay and its current activity:

•

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 12-44. 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 12-44). 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.

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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