I'm trying out basic CAN communication on a Hercules microcontroller. In this third blog I'll design driver PCBs. The first part checks the design rules from the datasheet.
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Real CAN driver
In the previous post I was using a makeshift CAN bus. But CAN buses that comply with the standard require a differential bus driver.
I bought 2 TCAN332DTCAN332D driver ICs. I've selected those because they are simple and require only a few additional components (in fact, if you don't care about protection: none. But that's not OK in automotive settings).
The PCB I design will have place for all protection components listed in the datasheet. I can choose to populate them or not. As long as I don't build them into a car, I can leave them off.
The board will also have optional components for the case when the device is an end point of the bus and needs to properly close the line with the correct impedance.
Here's the layout guideline from the tech ref.
(message to Texas Instruments: This is the ugliest Layout Example drawing I've ever seen in a datasheet. Where's your aesthetic feeling?)
My device doesn't have Fault, Sleep and Shutdown pins. I chose the simplest one. The grey components don't apply.
From the data sheet: I removed instructions for signals not available on the TCAN332D |
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13.1 Layout Guidelines TCAN33x family of devices incorporates integrated IEC 61000-4-2 ESD protection. Should the system requires additional protection against ESD, EFT or surge, additional external protection and filtering circuitry may be needed.
In order for the PCB design to be successful, start with design of the protection and filtering circuitry. Because ESD and EFT transients have a wide frequency bandwidth from approximately 3 MHz to 3 GHz, high frequency layout techniques must be applied during PCB design. Design the bus protection components in the direction of the signal path. Do not force the transient current to divert from the signal path to reach the protection device. Below is a list of layout recommendations when designing a CAN transceiver into an application.
• Transient Protection on CANH and CANL: Transient Voltage Suppression (TVS) and capacitors (D1, C5 and C7 shown in Figure 40) can be used for additional system level protection. These devices must be placed as close to the connector as possible. This prevents the transient energy and noise from penetrating into other nets on the board.
• Bus Termination on CANH and CANL: Figure 40 shows split termination where the termination is split into two resistors, R5 and R6, with the center or split tap of the termination connected to ground through capacitor C6. Split termination provides common mode filtering for the bus. When termination is placed on the board instead of directly on the bus, care must be taken to ensure the terminating node is not removed from the bus, as this causes signal integrity issues if the bus is not properly terminated on both ends.
• Decoupling Capacitors on VCC: Bypass and bulk capacitors must be placed as close as possible to the supply pins of transceiver (examples are C2 and C3).
• Ground and power connections: Use at least two vias for VCC and ground connections of bypass capacitors and protection devices to minimize trace and via inductance.
• Digital inputs and outputs: To limit current of digital lines, serial resistors may be used. Examples are R1, R2, R3 and R4.
• Filtering noise on digital inputs and outputs: To filter noise on the digital I/O lines, a capacitor may be used close to the input side of the I/O as shown by C1, C8 and C4.
• TXD input pin: If an open-drain host processor is used to drive the TXD pin of the device, an external pullup resistor between 1 kΩ and 10 kΩ must be used to help drive the recessive input state of the device (weak internal pullup resistor) |
For the termination, I can use symmetric or simple:
I'll provide footprints for both options on the PCB. All in all, this will be an easy design.
I selected SOIC ICs, but smaller footprints are available.
I've designed some PCBs with smaller ones in the past but they turned out hard to reproduce.
In the next blog I'll start creating the PCB. And I'll use my favourite KiCAD tool Kipart to generate the IC schematic symbol.
For the disclaimer inclined reader: I purchased the ICs. The shipping was free. I have a free shipping agreement with the supplier.