Datasheet

CAN: Controller Area Network Lab using ST STM32 Cortex-M processors. www.keil.com

A Node Schematic:

This is the schematic diagram from the Keil MCBSTM32E™ evaluation board. IC1 is a Texas Instruments CAN transceiver

which performs the conversion between the single-ended CAN controller CAN Tx and CAN Rx signals to the bi-directional

differential pair of the CAN bus called CAN Hi and CAN Lo (High and Low). This schematic is complete. The STM32

CAN I/O is TTL, CMOS and 5 volt tolerant, all at the same time making it exceptionally easy to design the interface.

This transceiver IC1 connects to the STM32 microprocessor IC2 which contains an integral CAN controller via two pins: D

(Driver input) and R (Receiver output). The corresponding nomenclature on the STM32 is CAN Rx and CAN Tx. CAN Tx

connects to D. CAN Rx connects to R. It is that simple. Some processors have multiple CAN controllers. These are usually

used in routers, gateways or to create more receiver FIFO memory for intentionally slowed down CPUs (for EMI reasons).

For general use a node normally needs only one controller. If it had at least two, it could talk to itself.

RS on IC1 (slope control) is used to adjust the rise and fall times of the output edges to limit EMI from the twisted pair.

Note R4, the 120 ohm termination resistor. This evaluation board is meant to be used with one other board as a small test

network. If this board is used as a node, and is not at one of the ends, this resistor should be removed and external resistors

used. P17 corresponds to a generally accepted standard for CAN on DB9 connectors. P17 Pin 7 is the CAN Hi bus line and

pin 6 is CAN Lo. TIP: If the CAN Hi and CAN Lo wires are reversed, the network will not operate.

The MCBSTM32E board has one CAN controller. Since there must always be two CAN nodes for a network, you need

another board with a CAN node. Most CAN analyzers can act as the 2

node. MCBSTM32C and STM32F4 Discovery

boards have two CAN nodes. Many other boards, including those from STMicroelectronics, contain two CAN nodes.

A Tiny Network without Transceiver ICs:

Sometimes you have a CAN equipped processor on a low cost board but it has no CAN transceiver chips. Here is a method

that can be used to create a small experimental network with such a board. There will be no noise immunity and you might

have to lower the speed….but many experimenters have made this work satisfactorily. Use a signal diode similar to 1N914

or 1N4148. Power supply diodes usually do not have a fast enough recovery time for CAN to function.

The processor on the STM32F4 Discovery board has two CAN controllers but

it has has no transceiver chips. How to install this on the Discovery board is

detailed in the section on “Running a CAN network…” on page 21.

A three node CAN network. Uses two CAN nodes

on STM32F107 (Keil MCBSTM32C

™) and

another on a STM32F407 Cortex-M4

(MCBSTM32F400™).

Termination is on the boards. This can be

disconnected on the MCBSTM32F400 with a

jumper. CAN Transceiver chips are used. These

boards can be connected to any Hi-Sped network.

This network works perfectly even though the

wires are a bit sloppy and are hardly a twisted pair.

This is because of the network’s small size and

robustness of CAN in general: even at 500 Kbps.