AN1105 datasheet(28/100 Pages) STMICROELECTRONICS

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ST7 pCAN PERIPHERAL DRIVER

– Identifiers may exist on the bus that do not concern our cell.

2.2.2 Cell Configuration

Open then the can_custom.h file. You can see the two first lines of code:

//#define WAKE_UP_PULSE

//#define RUN_ON_START_UP

According to our requirements (See Section 2.2.1 "Application"), we only want to define the

RUN_ON_STARTUP option.

So modify the code the following way:

//#define WAKE_UP_PULSE

#define RUN_ON_START_UP

2.2.2.1 Bit Timing

First of all, we have to decide the speed of our network. Let’s say, for example, 250 kilobaud.

Look in your can_custom.h file. You see the following code:

#define INIT_BRPR 0x00

#define INIT_BTR 0x00

The following two registers define the bit timing in the cell.

The BRPR register contains the size of time quantum, in number of clock ticks, in a bit field

named BRP (See Section 1.3.2 "Bit timing").This field ranges from 0 to 63. The true value for

a time quanta is then BRPR+1.

The BTR register contains the size of the two synchronization segments in time quanta, in two

different bit fields: BS1 and BS2(“). BS1 ranges from 0 to 7and BS2 from 0 to 3. The actual

length of bit segments in time quanta is then BS1+1 and BS2+1.

You must always chose BS1>BS2, so that the sampling point takes place in the second half of

the bit.

Use the following formula:

For example, in our case, fcpu = 8 MHz.

So, fbus = fcpu/32.

We have then the following possibilities:

BTR+1

BS1+BS2+3

216

fbus Bauds

()

fcpu Hz

()

BRP

()

BS1

BS2

()

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