PC912XDP512F0MPVR datasheet(437/1348 Pages) FREESCALE

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

MC9S12XDP512 Data Sheet, Rev. 2.21

Freescale Semiconductor

437

NOTE

The CANTBSEL register is held in the reset state when the initialization

mode is active (INITRQ = 1 and INITAK=1). This register is writable when

not in initialization mode (INITRQ = 0 and INITAK = 0).

Read: Find the lowest ordered bit set to 1, all other bits will be read as 0

Write: Anytime when not in initialization mode

The following gives a short programming example of the usage of the CANTBSEL register:

To get the next available transmit buffer, application software must read the CANTFLG register and write

this value back into the CANTBSEL register. In this example Tx buffers TX1 and TX2 are available. The

value read from CANTFLG is therefore 0b0000_0110. When writing this value back to CANTBSEL, the

Tx buffer TX1 is selected in the CANTXFG because the lowest numbered bit set to 1 is at bit position 1.

Reading back this value out of CANTBSEL results in 0b0000_0010, because only the lowest numbered

bit position set to 1 is presented. This mechanism eases the application software the selection of the next

available Tx buffer.

•

LDD CANTFLG; value read is 0b0000_0110

•

STD CANTBSEL; value written is 0b0000_0110

•

LDD CANTBSEL; value read is 0b0000_0010

If all transmit message buffers are deselected, no accesses are allowed to the CANTXFG registers.

10.3.2.12 MSCAN Identiﬁer Acceptance Control Register (CANIDAC)

The CANIDAC register is used for identiﬁer acceptance control as described below.

Table 10-15. CANTBSEL Register Field Descriptions

Field

Description

2:0

TX[2:0]

Transmit Buffer Select — The lowest numbered bit places the respective transmit buffer in the CANTXFG

buffer TX1). Read and write accesses to the selected transmit buffer will be blocked, if the corresponding TXEx

bit is cleared and the buffer is scheduled for transmission (see Section 10.3.2.7, “MSCAN Transmitter Flag

0 The associated message buffer is deselected

1 The associated message buffer is selected, if lowest numbered bit

76543210

IDAM1

IDAM0

IDHIT2

IDHIT1

IDHIT0

Reset:

00000000

= Unimplemented

Figure 10-15. MSCAN Identiﬁer Acceptance Control Register (CANIDAC)

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