AN1105 datasheet(55/100 Pages) STMICROELECTRONICS

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ST7 pCAN PERIPHERAL DRIVER

3.3 A FEW WORDS ABOUT DRIVER PERFORMANCE

3.3.1 CPU Load

Let’s imagine the following situation:

– Bus rate: 100 kilobauds

– Bus load: 80%

– FCPU: 8 MHz

– 14 identifiers (typical value) have to be received.

Let’s measure the time spent inside the interrupt function when a message is received. The

application processing inside the notification functions must be minimal: they only return a

buffer to be filled.

We retrieve the following results:

– 8 bytes data message, filter match: 105 µs

– 0 bytes data message, filter match: 90 µs

– Any message, filter no match: 50 µs

Note: Any message, if only the eight most significant bits match, takes 100 µs. That is to say almost as

much time as with matching identifier. So it is better to chose the identifiers wisely, in order to prevent

this from occurring as much as possible.

The longest possible message (2.0A standard) is 130 bits long (including stuffing bits). Im-

agine that all messages on the bus are that long. Consequently, there will be 615 messages

per second on the bus, that’s to say one message every 1.625 µs. The saving of data lasts

105 µs, so the CPU load is in this case 6.5%.

The same calculation for 0-byte messages (shortest possible) leads us to 15.3% CPU load.

Note that these are worst case scenarios! Not all the messages on the bus have to be saved.

And the bus load chosen here is pretty heavy (typical values are rather around 40%).

Do not forget to multiply or divide these values if you want an estimation of CPU load under

other bus speed/bus loading conditions and other FCPU speeds.

AN1105 Datasheet(PDF) 55 Page - STMicroelectronics