L9803 datasheet(75/126 Pages) STMICROELECTRONICS

75 / 126 page

L9803

On-Chip Peripherals

75/126

5.6.2

Main Features

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Support of CAN specification 2.0A and 2.0B passive

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Three prioritized 10-byte Transmit/Receive message buffers

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Two programmable global 12-bit message acceptance filters

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Programmable baud rates up to 1 MBit/s

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Buffer flip-flopping capability in transmission

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Maskable interrupts for transmit, receive (one per buffer), error and wake-up

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Automatic low-power mode after 20 recessive bits or on demand (standby mode)

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Interrupt-driven wake-up from standby mode upon reception of dominant pulse

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Optional dominant pulse transmission on leaving standby mode

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Automatic message queuing for transmission upon writing of data byte 7

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Programmable loop-back mode for self-test operation

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Advanced error detection and diagnosis functions

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Software-efficient buffer mapping at a unique address space

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Scalable architecture.

5.6.3

Functional Description

Frame Formats

A summary of all the CAN frame formats is given in Figure 38 for reference. It covers only

the standard 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 remote 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 recessive bit (logical 1). It is

overwritten as a dominant 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

receivers regardless of the outcome of the acceptance 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 consecutive messages. If there is no

subsequent bus access by any station, the bus remains idle.

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