Controller Area Network (CAN) - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 651

7 6 5 4 3 2 1 0

CANE CLKSRC LOOPB LISTEN 0 WUPM SLPAK INITAK

Reset: 0 0 0 1 0 0 0 1

CANE: MSCAN enable
0 = the MSCAN module is disabled.
1 = the MSCAN module is enabled.
CLKSRC: MSCAN clock source
0 = the MSCAN clock source is the oscillator clock.
1 = the MSCAN clock source is the bus clock.
LOOPB: loopback self-test mode
0 = loopback self-test disabled.
1 = loopback self-test enabled.
LISTEN: listen-only mode
0 = normal operation.
1 = listen-only mode activated.
WUPM: wake-up mode
0 = MSCAN wakes up the CPU after any recessive to dominant edge on the
CAN bus and WUPE bit of the CANCTL0 register is set to 1.
1 = MSCAN wakes up the CPU only in case of a dominant pulse on the CAN
bus that has a length of T WUP and the WUPE bit is set to 1.
SLPAK: sleep mode acknowledge
0 = running (MSCAN functions normally).
1 = sleep mode active (MSCAN has entered sleep mode).
INITAK: initialization mode acknowledge
0 = normal operation (MSCAN operates normally).
1 = initialization mode active (MSCAN is in initialization mode).

Figure 13.18 ■ MSCAN control register 1 (CAN x CTL, x 5 0, 1, 2, 3, or 4)

is entered by setting the LISTEN bit. When operating in a noisy environment, one can choose
to enable the filter to filter out the noise on the CAN bus so that the CAN module won't be
woken up by the noise.
There is a time delay from requesting to enter the sleep mode or initialization mode until
the desired mode is entered. To make sure the requested mode is entered, one should check the
SLPAK and INITAK bits.

MSCAN B US T IMING R EGISTER 0 (CAN X BTR0)
The CAN x BTR0 register allows the user to select the synchronization jump width and
the divide factor to the selected clock source to derive the time quantum. The contents of
the CAN x BTR0 register are shown in Figure 13.19. The MSCAN uses the time quantum ( t Q ) as
the minimum unit for timing control. The synchronization jump width defines the maximum
number of time quantum ( t Q ) clock cycles a bit time can be shortened or lengthened to achieve
resynchronization to data transitions on the bus. The time quantum ( t Q ) is derived by dividing
the prescale factor into the selected clock source. The value of the prescale factor is defined by
bits BRP5,BRP0.