ST72521 datasheet(128/211 Pages) STMICROELECTRONICS

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ST72521

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CONTROLLER AREA NETWORK (Cont’d)

10.8.2 Main Features

– Support of CAN specification 2.0A and 2.0B pas-

sive

– Three prioritized 10-byte Transmit/Receive mes-

sage buffers

– Two programmable global 12-bit message ac-

ceptance filters

– Programmable baud rates up to 1 MBit/s

– Buffer flip-flopping capability in transmission

– Maskable interrupts for transmit, receive (one

per buffer), error and wake-up

– Automatic low-power mode after 20 recessive

bits or on demand (standby mode)

– Interrupt-driven wake-up from standby mode

upon reception of dominant pulse

– Optional dominant pulse transmission on leaving

standby mode

– Automatic message queuing for transmission

upon writing of data byte 7

– Programmable loop-back mode for self-test op-

eration

– Advanced error detection and diagnosis func-

tions

– Software-efficient buffer mapping at a unique ad-

dress space

– Scalable architecture.

10.8.3 Functional Description

10.8.3.1 Frame Formats

A summary of all the CAN frame formats is given

in Figure 68 for reference. It covers only the stand-

ard frame format since the extended one is only

acknowledged.

A message begins with a start bit called Start Of

Frame (SOF). This bit is followed by the arbitration

field which contains the 11-bit identifier (ID) and

the Remote Transmission Request bit (RTR). The

RTR bit indicates whether it is a data frame or a re-

mote request frame. A remote request frame does

not have any data byte.

The control field contains the Identifier Extension

bit (IDE), which indicates standard or extended

format, a reserved bit (ro) and, in the last four bits,

a count of the data bytes (DLC). The data field

ranges from zero to eight bytes and is followed by

the Cyclic Redundancy Check (CRC) used as a

frame integrity check for detecting bit errors.

The acknowledgement (ACK) field comprises the

ACK slot and the ACK delimiter. The bit in the ACK

slot is placed on the bus by the transmitter as a re-

cessive bit (logical 1). It is overwritten as a domi-

nant bit (logical 0) by those receivers which have

at this time received the data correctly. In this way,

the transmitting node can be assured that at least

one receiver has correctly received its message.

Note that messages are acknowledged by the re-

ceivers regardless of the outcome of the accept-

ance test.

The end of the message is indicated by the End Of

Frame (EOF). The intermission field defines the

minimum number of bit periods separating con-

secutive messages. If there is no subsequent bus

access by any station, the bus remains idle.

10.8.3.2 Hardware Blocks

The CAN controller contains the following func-

tional blocks (refer to Figure 67):

– ST7 Interface: buffering of the ST7 internal bus

and address decoding of the CAN registers.

– TX/RX Buffers: three 10-byte buffers for trans-

mission and reception of maximum length mes-

sages.

– ID Filters: two 12-bit compare and don’t care

masks for message acceptance filtering.

– PSR: page selection register (see memory map).

– BRPR: clock divider for different data rates.

– BTR: bit timing register.

– ICR: interrupt control register.

– ISR: interrupt status register.

– CSR: general purpose control/status register.

– TECR: transmit error counter register.

– RECR: receive error counter register.

– BTL: bit timing logic providing programmable bit

sampling and bit clock generation for synchroni-

zation of the controller.

– BCDL: bit coding logic generating a NRZ-coded

datastream with stuff bits.

– SHREG: 8-bit shift register for serialization of

data to be transmitted and parallelisation of re-

ceived data.

– CRC: 15-bit CRC calculator and checker.

– EML: error detection and management logic.

– CAN Core: CAN 2.0B passive protocol control-

ler.

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