Circbuf

Overview

Elphel cameras use FPGA to compress images and video, the compressed frames are transferred from the FPGA to the system memory through pseudo-DMA channel implemented in the CPU (Axis Etrax FS). It is "pseudo" as physically the data still goes (over the system bus) from the FPGA to the CPU chip, buffered there in small chunks and then sent to the system memory. Overall on average it takes 5 bus cycles (100MHz clock) to transfer one 32-bit word (4 bytes) of data from the FPGA to the system memory - 80MB/sec. There are hardware provisions in the model 353 to support new feature of ETRAX FS (not available in earlier ETRAX 100LX used in Elphel earlier models 303,313,323 and 333 cameras) - allowing external device (FPGA in our case) to become the bus master. That can allow to increase the data rate to virtually full 400MB/sec (in large bursts), but no FPGA code is yet developed to support this mode.

Circular buffer in Elphel 353 camera

The compressed frames are stored in a large circular buffer, it is done similar in the previous cameras and with other compressor (Ogg Theora) but the following description applies to the current state of the firmware (7.1.0.1) of the model 353 camera with JPEG compressor (Theora branch is not ported yet as of September 2007). Currently buffer size - CCAM_DMA_SIZE is ~19MB as defined in include/asm-cris/elphel/c313a.h).