COP685BC datasheet(23/57 Pages) NSC

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Functional Block Description of

the CAN Interface (Continued)

Transceive Logic (TCL)

The TCL is a state machine which incorporates the bit stuff

logic and controls the output drivers, CRC logic and the

Rx/Tx shift registers. It also controls the synchronization to

the bus with the CAN clock signal generated by the BTL.

Error Management Logic (EML)

The EML is responsible for the fault confinement of the CAN

protocol. It is also responsible for changing the error

counters, setting the appropriate error flag bits and interrupts

and changing the error status (passive, active and bus off).

Cyclic Redundancy Check (CRC) Generator and

The CRC Generator consists of a 15-bit shift register and the

logic required to generate the checksum of the destuffed bit-

stream. It informs the EML about the result of a receiver

checksum.

The checksum is generated by the polynomial:

χ15 + χ14 + χ10 + χ8 + χ7 + χ4 + χ3 +1

Receive/Transmit (Rx/Tx) Registers

The Rx/Tx registers are 8-bit shift registers controlled by the

TCL and the BSP. They are loaded or read by the Interface

Management Logic, which holds the data to be transmitted

or the data that was received.

Bit Time Logic (BTL)

The bit time logic divider divides the CKI input clock by the

value defined in the CAN prescaler (CSCAL) and bus timing

by the formula:

Where

divider is the value of the clock prescaler, PS is the

programmable value of phase segment 1 and 2 (1..8) and

PPS the programmed value of the propagation segment

(1..8) (located in CTIM).

Bus Timing Considerations

The internal architecture of the CAN interface has been op-

timized to allow fast software response times within mes-

sages of more than two data bytes. The TBE (Transmit

Buffer Empty) bit is set on the last bit of odd data bytes when

CAN internal sample points are high.

It is the user’s responsibility to ensure that the time between

setting TBE and a reload of TxD2 is longer than the length of

phase segment 2 as indicated in the following equation:

Table 2 shows examples of the minimum required t

LOAD for

different CSCAL settings based on a clock frequency of

10 MHz. Lower clock speeds require recalculation of the

CAN bit rate and the mimimum t

LOAD.

TABLE 2. CAN Timing (CKI = 10 MHz t

c = 1 µs)

CSCAL

CAN Bit Rate (kbit/s)

Minimum

LOAD (µs)

250

2.0

100

5.0

8.0

12.5

439

499

199

100

Figure 19 illustrates the minimum time required for t

LOAD.

DS012067-50

FIGURE 18. Bit Rate Generation

DS012067-51

FIGURE 19. TBE Timing

COP685BC Datasheet(PDF) 23 Page - National Semiconductor (TI)

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