ST10F296 datasheet(193/346 Pages) STMICROELECTRONICS

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CAN modules

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17.2

Configuration support

Both CAN controllers can work on the same CAN bus and support up to 64 message

objects. In this configuration, both receive and transmit signals are linked together when

using the same CAN transceiver. This configuration is particularly supported by providing

open-drain outputs for the CAN1_Txd and CAN2_TxD signals. The open-drain function is

controlled with the ODP4 register for Port P4. In this way it is possible to connect pins P4.4

with P4.5 (receive lines) and pins P4.6 with P4.7 (transmit lines configured to be open-

drain).

The user is also allowed to map both CAN modules internally on the same pins, P4.5 and

P4.6. In this way, pins P4.4 and P4.7 may be used either as general purpose I/O lines or for

I2C interface. This is done by setting the CANPAR bit of the XMISC register. To access this

respectively must be set.

Note:

CAN parallel mode is effective only if both CAN1 and CAN2 are enabled by setting bits

CAN1EN and CAN2EN in the XPERCON register. If CAN1 is disabled, CAN2 remains on

P4.4/P4.7 even if bit CANPAR is set.

17.3

Clock prescaling

The XMISC register also provides a bit (CANCK2) which modifies the clock frequency

driving both the CAN modules. Due to architectural limitations of the CAN module, when the

CPU frequency is higher than 40 MHz, it is recommended to provide each CAN module with

the CPU clock divided by 2. It is sufficient to supply 20 MHz for the CAN module to produce

the maximum baud rate defined by the protocol standard. The CPU frequency can be

reduced down to 8 MHz. It is still possible to obtain the maximum CAN speed (1Mbaud) by

feeding the CAN module directly with the CPU clock disabling the prescaler factor.

After reset, the prescaler is enabled, the CPU clock is divided by two, and provides the CAN

modules. According to the system clock frequency, the application can disable the prescaler

to obtain the required baud rate.

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ST10F296 датащи(PDF) 193 Page - STMicroelectronics