Size matters — Using emCompress to accelerate firmware updates via CAN bus

To address these speed limitations, SEGGER’s development team was approached by a customer from the automotive industry. This customer regularly manages firmware updates for ECUs from their suppliers. The only available connection with these ECUs is the CAN bus. Figure 1 shows an example of the basic structure of a setup in which the firmware update is imported via CAN. In this case, a firmware update of 1.5 MB took almost 40 minutes, an unacceptably long time.

Since the speed limitations of the CAN bus cannot be changed, the only option was to reduce the amount of data to be transferred. To this end, SEGGER’s compression software emCompress was integrated into the customer’s bootloader.

emCompress offers solutions to compress data for all kinds of applications where size matters. Designed for embedded systems, emCompress can compress and decompress data even on the smallest microcontrollers.

emCompress is available in three editions – emCompress-ToGo, emCompress-LZMA, and emCompress-Embed – that cover different use cases for compression. Each edition is optimized for its specific use to provide the most efficient and convenient way to compress and decompress data.

There is sufficient RAM and non-volatile memory on the automotive ECUs, so the goal was not so much to achieve the smallest possible footprint as to achieve maximum data compression in order to minimize the transmission time via the CAN bus.

This led to emCompress-LZMA, with its LZMA algorithm, as the go-to solution. The Lempel–Ziv–Markov chain algorithm (LZMA) is an algorithm used to perform lossless data compression. LZMA uses a dictionary-based compression algorithm, whose output is then encoded with a range encoder, using a complex model to make a probability prediction of each bit. Independent studies show that – depending on the data set – compressions with a factor of more than 6 to 1 are possible with this algorithm [1]. Typical compression rates for firmware images range from 2 to 4 reducing the update time between 50 and 75 percent.

emCompress-Flex supports LZMA as well as other algorithms (e.g. LZMA2). In this use case, the decompression algorithm also supports files that were created by PC-based tools such as “7-ZIP” or “Lzip” which use LZMA as well.

The compressed firmware image is then sent via the CAN bus to the system. emCompress-Flex directly receives and decompresses it in chunks. The decompressed chunks are passed to the firmware update function.