CAN FD Loopback

Example Path: examples/PlatformSpecific/MODRT1171/CAN/CanFdLoopback

Overview

CAN FD (Flexible Data-rate) extends classic CAN in two ways: payloads grow from 8 bytes to up to 64 bytes per frame, and with bit-rate switching (BRS) the data phase of each frame runs at a higher bit rate than the arbitration phase. All three of the i.MX RT1171's FlexCAN controllers support CAN FD.

This example demonstrates CAN FD on a single bare MODRT1171 using the FlexCAN loopback self-test mode (the transmitter connected directly to the receiver inside the chip) – no transceiver, no wiring, no second node:

1. Functional test – one 64-byte frame with BRS (500 kbit/s arbitration, 2 Mbit/s data phase) is sent through the internal loopback and the received payload is verified against what was sent.
2. Throughput comparison – the same 4096-byte payload is moved once as 512 classic 8-byte frames and once as 64 FD 64-byte BRS frames, timed with the NetBurner StopWatch class, and the effective data rates are printed side by side.

The FlexCAN driver (fsl_flexcan, NXP SDK version 2.11.6) is built into the NetBurner MODRT1171 platform library; the example includes <fsl_flexcan.h> directly and bundles no driver source.

Hardware requirements

• MODRT1171 module
• Personal Computer
• Serial terminal program connected to the MODRT1171 UART0 serial port

Board settings

No special settings are required. Loopback mode uses no package pins.

Note

This example is written for the MODRT1171 rev 1.5 module, and uses FlexCAN1 to match the other MODRT1171 CAN examples. At rev 1.6 (which makes the module a MOD5441X drop-in replacement), the FlexCAN1 signals move from the P1 header to the P2 header: RX moves P1.26 -> P2.41 and TX moves P1.27 -> P2.44. The FlexCAN1 peripheral instance and this example's code are unchanged; only the connector pins move – and this example uses no pins at all. Designs that need more than one CAN bus have options beyond FlexCAN1: FlexCAN3 is available on the I2C pins for designs that do not use I2C (or that bit-bang I2C on spare GPIO), and on rev 1.5 FlexCAN2 also reaches module pins – all three controllers support CAN FD. For pin details see the MODRT1171 Software Developer Guide and the MODRT1171 Pinout Update document: https://www.netburner.com/download/MODRT1171_Rev1p5_to_Rev1p6_Pinout_Update/

Running the demo

Build and load the application, then open the serial terminal:

==FlexCAN FD loopback example -- Start.==
 
Send 64-byte FD frame (BRS on) from MB9 to MB10
Received frame: edl=1 brs=1, 64 bytes
  word0  = 0x00000000  word1  = 0x01010101  word2  = 0x02020202  word3  = 0x03030303
  ...
Payload verified: received data matches transmitted data.
 
Throughput comparison, 4096 bytes of payload each way:
  Classic CAN:  512 frames x  8 bytes: 141309 us  (231 kbit/s effective)
  CAN FD BRS:    64 frames x 64 bytes:  23228 us  (1410 kbit/s effective)
  CAN FD moved the same payload 6.0x faster.
 
==FlexCAN FD loopback example -- Finish.==

The throughput difference comes from both FD improvements at once: 64-byte frames amortize the frame overhead (ID, CRC, acknowledgement) over 8x more payload, and bit-rate switching runs the data bits at 4x the arbitration rate.

For CAN FD on a real bus, see the CanFdSimple example. For the classic CAN version of this self-test, see FlexcanLoopback.