CP3BT26

DRIVE

The Drive bit shows the output value on the

 

CANTX pin at the time of the error. Note that

 

a receiver will not drive the bus except during

 

ACK and during an active error flag.

19.10.17 CAN Timer Register (CTMR)

The CTMR register reports the current value of the Time Stamp Counter as described in Section 19.8.

15

0

CTMR15:0

0

R

The CTMR register is a free running 16-bit counter. It con- tains the number of CAN bits recognized by the CAN mod- ule since the register has been cleared. The counter starts to increment from the value 0000b after a hardware reset. If the Timer Stamp Enable bit (TSTPEN) in the CAN global configuration register (CGCR) is set, the counter will also be cleared on a message transfer of the message buffer 0.

19.11.1 External Connection

The CAN module uses the CANTX and CANRX pins to con- nect to the physical layer of the CAN interface. They provide the functionality described in Table 64.

Table 64 External CAN Pins

Signal Name

Type

Description

 

 

 

CANTX

Output

Transmit data to the CAN bus

 

 

 

CANRX

Input

Receive data from the CAN bus

 

 

 

The logic levels are configurable by the CTX and CRX bits of the Global Configuration Register CGCR (see “CAN Glo- bal Configuration Register (CGCR)” on page 133).

19.11.2 Transceiver Connection

An external transceiver chip must be connected between the CAN block and the bus. It establishes a bus connection in differential mode and provides the driver and protection requirements. Figure 64 shows a possible ISO-High-Speed configuration.

120 Termination

The contents of CTMR are captured into the Time Stamp register of the message buffer after successfully sending or receiving a frame, as described in “Time Stamp Counter” on page 126.

19.11SYSTEM START-UP AND MULTI-INPUT WAKE-UP

After system start-up, all CAN-related registers are in their reset state. The CAN module can be enabled after all con- figuration registers are set to their desired value. The follow- ing initial settings must be made:

„ Configure the CAN Timing register (CTIM). See “Bit

Time Logic” on page 117.

„ Configure every buffer to its function as receive/transmit.

See “Buffer Status/Control Register (CNSTAT)” on

page 128.

„ Set the acceptance filtering masks. See “Acceptance Fil-

tering” on page 119.

CPU Bus

CR16CAN

 

Transceiver Chip

 

5

 

VCC

CANRX

REF

BUS_H

4

RX

BUS_L

 

1

TX

CANTX

 

 

RS

GND

 

 

 

 

8

2

CAN bus signals

To other modules

VCC

3

7

6

120

„ Enable the CAN interface. See “CAN Global Configura-

tion Register (CGCR)” on page 133.

Before disabling the CAN module, software must make sure that no transmission is still pending.

Note: Activity on the CAN bus can wake up the device from a reduced-power mode by selecting the CANRX pin as an input to the Multi-Input Wake-Up module. In this case, the CAN module must not be disabled before entering the re- duced-power mode. Disabling the CAN module also dis- ables the CANRX pin. As an alternative, the CANRX pin can be connected to any other input pin of the Multi-Input Wake- Up module. This input channel must then be configured to trigger a wake-up event on a falling edge (if a dominant bit is represented by a low level). In this case, the CAN module can be disabled before entering the reduced-power mode. After waking up, software must enable the CAN module again. All configuration and buffer registers still contain the same data they held before the reduced-power mode was entered.

DS048

Figure 64. External Transceiver

19.11.3 Timing Requirements

Processing messages and updating message buffers re- quire a certain number of clock cycles, as shown in Table 65. These requirements may lead to some restrictions regarding the Bit Time Logic settings and the overall CAN performance which are described below in more detail. Wait cycles need to be added to the cycle count for CPU access to the object memory as described in CPU Access to CAN Registers/Memory on page 127. The number of occurrenc- es per frame is dependent on the number of matching iden- tifiers.

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National CP3BT26 manual System START-UP and MULTI-INPUT WAKE-UP, Can Timer Register Ctmr, External Connection