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Application Brief

Meeting the Timing Requirements of CAN FD in Isolated
CAN Systems for HEV/EVs

Max Robertson, Transceivers Applications Manager; Abhi Aarey, ISO Applications Manager; Neel Seshan, ISO
Marketing Manager
For automotive applications, Controller Area Network
Flexible Data Rate (CAN FD) has become the defacto standard for most automakers. Unquestionably,
the higher speed of CAN FD over Classical CAN
enables faster communication of data allowing for
more bandwidth, lower latency for the messages, orincreased nodes on the CAN bus. Additionally, if theCAN communication is used between different voltage
domains as in hybrid electric vehicles (HEVs) andelectric vehicles (EVs), digital isolators are often used
to help with protection of the low-voltage side fromhigh-voltage side and improve noise immunity.
The CAN protocol is preferred in automotive
applications due to its prioritization and arbitration
features, making it popular in time-critical applications,such as belt starter generators, traction inverters,
battery management systems. Hence the timing
specifications of the CAN device become critical whenselecting the right device for your system. Table
1 shows the main specifications to consider when
designing a system that meets the physical layer
requirements of the ISO 11898-2:2016 high-speed
CAN specification.

Table 1. ISO 11898-2:2016 high-speed CAN timing
specifications (continued)
Parameter

Figure 1 and Figure 2 show the test setup forthe measurement of these parameters for the CAN
transceiver by itself. Figure 1 shows the measurementof the total loop delay from the driver input (TXD)
to receiver output (RXD). This measurement is donein the recessive to dominant state as shown bytPROP(LOOP1) and the dominant to recessive stateas shown by tPROP(LOOP2). Figure 2 measures thevariation in the bit width as it propagates throughthe transmitter to the output and back through thereceiver to the receiver output.
CANH

tPROP(LOOP1) Total loop delay, driver input(TXD) to receiver output (RXD),
recessive to dominant
tPROP(LOOP2) Total loop delay, driver input(TXD) to receiver output (RXD),
dominant to recessivetBIT(BUS)

RL

CL
50%

TXD

0V

CANL

STB

0V

tPROP(LOOP2)tPROP(LOOP1)

RXD

VOH

CL_RXD
50%RXDVOLMAX
255 nsCopyright © 2016, Texas Instruments Incorporated

Figure 1. tPROP(LOOP)Test Circuit and Measurement
VI
70%
TXD

255 nsCANH

30%30%0V

TXD
VI

RL

5 x tBIT

CLtBIT(TXD)
CANL

Bit time on CAN bus output pins
435 ns 530 ns
with tBIT(TXD) = 500 ns (for 2 Mbps
CAN FD)
Bit time on CAN bus output pins
155 ns 210 ns
with tBIT(TXD) = 200 ns (for 5 Mbps
CAN FD)tBIT(RXD)

VCC

TXD
VIVO

MIN

MAX

Bit time on RXD output pins with120 ns 220 nstBIT(TXD) = 200 ns (for 5 Mbps CANFD)

Table 1. ISO 11898-2:2016 high-speed CAN timing
specifications
ParameterMIN

Bit time on RXD output pins with
400 ns 550 ns
tBIT(TXD) = 500 ns (for 2 Mbps CANFD)

0V

tBIT(BUS)

STB

900mV
VDIFFRXD

VO

500mV

CL_RXD
VOH
70%
RXD
30%
tBIT(RXD)

VOL

Figure 2. CAN FD Bit Timing Parameter
Measurement
CAN devices such as the Grade 0 CAN
FD transceiver TCAN1044EV-Q1 can meet theSLLA479 – MARCH 2020
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Meeting the Timing Requirements of CAN FD in Isolated CAN Systems forHEV/EVs
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requirements over the temperature range of -40 °C
to 150 °C as shows in Table 2 .
Table 2. CAN transceiver timing specifications
Parameter

ISO 11898-2:2016

TCAN1044EV-Q1

MINMINMAX

MAX

tPROP(LOOP1)

255 ns210 nstPROP(LOOP2)

255 ns210 nstBIT(BUS)
2 Mbps CAN FD

435 ns

530 ns

460 ns

510 nstBIT(BUS)
5 Mbps CAN FD

155 ns210 ns160 ns210 nstBIT(RXD)
2 Mbps CAN FD

400 ns

550 ns445 ns

515 nstBIT(RXD)
5 Mbps CAN FD

120 ns

220 ns145 ns215 nsFor applications that require isolation between theCAN transceiver and the microcontroller, simply
looking at the CAN transceiver timing performance
is not enough. It is important to add the timing
performance of the digital isolator that is in series
with the CAN transceiver to ensure the two-chipsolution still meets the timing specification. Isolator
datasheets provide the propagation delay and pulse
width distortion specifications.

Figure 3. Total loop delay for isolated CAN system
Minimum bit time on the bus output pins = Minimum
CAN tBIT(BUS) time -Maximum Isolator pulse width
distortion. Maximum bit time on the bus output pins= Maximum CAN tBIT(BUS) time + Maximum Isolator
pulse width distortion. Minimum bit time on thereceiver output pins = Minimum CAN tBIT(RXD) time 2 x Maximum Isolator pulse width distortion. Maximumbit time on the receiver output pins = Maximum CANtBIT(RXD) time + 2 x Maximum Isolator pulse widthdistortion.

For example, these timing specifications for the Grade
0 ISO7741E-Q1 are shown in Table 3:
Table 3. Digital isolator timing specifications
ISO7741E-Q1 (-40 °C to +150 °C)
ParametertPLH, tPHL

Propagation delay time

PWD

Pulse width distortion|tPHL – tPLH|MINMAX

6 ns

16.5 ns
4.9 nsBased on the individual timing specifications for thedigital isolator and the CAN transceiver, the combined
data can be calculated as follows (as shown in Figure
3 and Figure 4):
Total loop delay from the driver input to the receiver
output: CAN loop delay + 2 x Isolator propagation
delay time

Figure 4. Total bit time for isolated CAN
Based on the formulae discussed above, thecombined timing specifications of the two-chip
isolated CAN solution is shown in Table 4.
Table 4. Discrete isolated CAN timing
specifications

Parameter2

ISO 11898-2:2016

ISO7741E-Q1 +TCAN1044EV-Q1(-40 °C to +150 °C)

MIN

MIN

MAX

MAX

tPROP(LOOP1)

255 ns

243 nstPROP(LOOP2)

255 ns

243 nstBIT(BUS)
2 Mbps CAN FD

435 ns

530 ns455.1 ns

514.9 nstBIT(BUS)
5 Mbps CAN FD

155 ns210 ns155.1 ns214.9 nstBIT(RXD)
2 Mbps CAN FD

400 ns

550 ns435.2 ns524.8 ns

Meeting the Timing Requirements of CAN FD in Isolated CAN Systems forHEV/EVs
Copyright © 2021 Texas Instruments Incorporated

SLLA479 – MARCH 2020
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Table 4. Discrete isolated CAN timing
specifications (continued)
ISO 11898-2:2016

ISO7741E-Q1 +TCAN1044EV-Q1(-40 °C to +150 °C)

ParameterMIN

MAXMINMAX

tBIT(RXD)
5 Mbps CAN FD

120 ns

220 ns135.2 ns224.8 ns

From the table it is clear that the Grade 0 isolator
ISO7741E-Q1 when used along with the Grade 0
CAN FD transceiver TCAN1044EV-Q1, can meet theISO 11898-2:2016 specifications up to CAN FD speed
for 2 Mbps.
This article discussed the key timing specs andcalculation examples to determine if the chosen
discrete isolator and CAN transceiver can meet
the CAN standards. Selecting the right components
upfront will ensure the proper CAN system operationand speed up release to production.

SLLA479 – MARCH 2020
Submit Document Feedback

Meeting the Timing Requirements of CAN FD in Isolated CAN Systems for
HEV/EVs
Copyright © 2021 Texas Instruments Incorporated

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