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SN65HVD257 EVM Setup and Operation for Redundant (Parallel Networks)

3.4 Using Customer Installable IO Options for Current Limiting, Pull up or down, Noise

Filtering

The EVM has footprints on the PCB for the installation of various filtering and protection options to adapt

the EVM to match CAN network topology requirements if the EVM is being used as a CAN node.

Each digital input or output pin has footprints to allow for series current limiting resistors (default populated

with 0Ω), pull up or down resistors depending on pin use and a capacitor to GND which, configured with

the serial resistor, implements RC filters (for noisy environments). The table below lists these features for

each of the digital input and output pins of the EVM. Replace or populate the RC components as

necessary for the application. The RC output filter pads for may be reused as a resistor divider network to

level shift the outputs down to 3.3V levels. The SN65HVD257 already has 3.3V compatible inputs on TXD

and S pins.

Table 5. EVM Digital IO Configuration

Signal Jumper

Pull Up or

Series R C to GND Description

Pull

Down

Description Type Pull Up

Down

TXD U1 Input NA NA R8 (R4/R10) NA NA TXD input from JMP1 to TXD U1

R32

TXD U2 Input NA NA NA NA TXD input from JMP1 to TXD U2

(R28/R34)

R44 PD RXD U1 output to AND Gate for

RXD U1 Output NA NA R17 C10

(10k) combined RXD redundant output

R43 PD RXD U1 output to AND Gate for

RXD U2 Output NA NA R17 C16

(10k) combined RXD redundant output

RXDprime is the combined RXD output

from the parallel CAN buses via AND

RXDprime Output NA NA R20 NA C15

gate U2 which is routed to JMP1 as

RXD

R3 S (Mode) pin input from JMP1 or PU or

S U1 Input R1 (JMP2) R2 NA C1

(JMP2) PD to S U1

R27 S (Mode) pin input from JMP1 or PU or

S U2 Input R25 (JMP2) R26 NA C21

(JMP2) PD to S U2

FAULT3 is the combined RXD output

from the parallel CAN buses via XOR

FLT3 Output NA NA R47 (3.3k) NA C28 (1nF)

gate U6 with the RC filter populated

which is routed to JMP1 as FLT3.

3.5 Using customer installable IO options for 3.3V IO

The EVM may be configured to have a 3.3V level output through the repurposing of the RC output filter

pads. These RC pads may be reused as a resistor divider network to level shift the outputs down to 3.3V

levels. The SN65HVD257 already has 3.3V compatible inputs on the TXD and S pins. Table 6 shows

some examples. For use in applications, calculations must be made to ensure the resistor divider network

chosen adheres to the application requirement. Considerations should include: current biasing in the

resistor network (loading, power), ensuring that the V

and V

of the divider will meet the V

and V

input threshold levels of the host processor, and that the output of the resistor divider will be below the

absolute maximum rating of the host processor at the absolute maximum rating of the transceiver (or the

worst case corner the application will provide).

Table 6. EVM Digital IO Configuration

Output R1 Pad and Value R2 Pad and Value Description

RXDprime R20 = 3.9 kΩ C15 = 6.8 kΩ C15 pad is repurposed as R2.

FLT1 R15 = 0 Ω C6 = 8.2 kΩ R1 is the pull up R16. C6 pad is repurposed as R2.

R16 = 4.7k Ω

FLT2 "R39 = 0 Ω C26 = 8.2 kΩ R1 is the pull up R 40. C26 pad is repurposed as R2.

R40 = 4.7kΩ

FLT3 R47 = 3.9 kΩ C28 = 1nF and 6.8 kΩ C28 pad is repurposed as R2 and filter C (stacked

components).

SLLU172–August 2012 SN65HVD257 CAN EVM: Functional Safety and Redundant CAN Network

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