CP3BT26

e

Bus

Signal

CAN

Clock

PREVIOUS

 

A

TSEG1

TSEG2

BIT

 

 

 

 

 

 

 

 

 

 

"NORMAL" BIT TIME

 

 

 

 

 

 

PREVIOUS

 

 

 

 

 

 

A

TSEG1

SJW

 

BIT

 

 

 

 

 

 

BIT TIME LENGTHENED BY SJW

NEXT BIT

TSEG2

NEXT BIT

DS029

Figure 45. Resynchronization (e > SJW)

e

Bus

Signal

CAN

Clock

PREVIOUS

A

TSEG1

TSEG2

 

 

 

BIT

 

 

 

 

 

 

 

 

 

 

 

 

"NORMAL" BIT TIME

 

 

 

 

 

 

 

 

 

PREVIOUS

 

 

 

 

 

 

 

 

 

 

 

 

A

TSEG1

TSEG2

NEXT BIT

BIT

 

 

 

 

 

 

BIT TIME SHORTENED BY SJW

DS030

Figure 46. Resynchronization (e < -SJW)

19.2.7Clock Generator

The CAN prescaler (PSC) is shown is Figure 47. It divides the CKI input clock by the value defined in the CTIM register. The resulting clock is called time quanta clock and defines the length of one time quantum (tq).

Please refer to CAN Timing Register (CTIM) on page 135 for a detailed description of the CTIM register.

Note: PSC is the value of the clock prescaler. TSEG1 and TSEG2 are the length of time segment 1 and 2 in time quan- ta.

The resulting bus clock can be calculated by the equation:

busclock =

CKI

(------------------------------------------------------------------------------------PSC)x(1 + TSEG1 + TSEG2)

The values of PSC, TSEG1, and TSEG2 are specified by the contents of the registers PSC, TSEG1, and TSEG2 as follows:

PSC = PSC[5:0] + 2

TSEG1 = TSEG1[3:0] + 1

TSEG2 = TSEG2[2:0] + 1

CKI

 

 

÷ PSC

 

 

 

÷ (1+TSEG1+TSEG2)

 

Bit Rate

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Internal Time

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Quanta Clock (1/tq)

 

DS031

Figure 47. CAN Prescaler

19.3MESSAGE TRANSFER

The CAN module has access to 15 independent message buffers, which are memory mapped in RAM. Each message buffer consists of 8 different 16-bit RAM locations and can be individually configured as a receive message buffer or as a transmit message buffer.

A dedicated acceptance filtering procedure enables soft- ware to configure each buffer to receive only a single mes- sage ID or a group of messages. One buffer uses an

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National CP3BT26 manual Message Transfer, Cki