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 0
Ω
), 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.
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
OH
and V
OL
of the divider will meet the V
IH
and V
IL
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 k
Ω
C15 = 6.8 k
Ω
C15 pad is repurposed as R2.
FLT1
R15 = 0
Ω
C6 = 8.2 k
Ω
R1 is the pull up R16. C6 pad is repurposed as R2.
R16 = 4.7k
Ω
FLT2
"R39 = 0
Ω
C26 = 8.2 k
Ω
R1 is the pull up R 40. C26 pad is repurposed as R2.
R40 = 4.7k
Ω
FLT3
R47 = 3.9 k
Ω
C28 = 1nF and 6.8 k
Ω
C28 pad is repurposed as R2 and filter C (stacked
components).
11
SLLU172 – August 2012
SN65HVD257 CAN EVM: Functional Safety and Redundant CAN Network
Copyright © 2012, Texas Instruments Incorporated
