7. System Architecture

Thanks for reading what of the tuturial has been completed so far. It probably won’t ever be done. Anyway, I’d like to discuss the system architecture quickly.

First, the OpenMV Cam is based off of the STM32 Microcontroller architecture because the MicroPython pyboard is based off of the STM32 Microcontroller architecture. If the project was started using some other system things might have been totally different.

Moving on, we choose not to use DRAM with the original OpenMV Cam because it made the system too expensive to produce at low volumes. SDR DRAM (which is what the STM32 supports) isn’t the cheapest at low manufacturing volumes and greatly increases board design complexity (e.g. you need to do 8-layer boards to route all the signals). As we’ve revved the OpenMV Cam with a faster and faster main processor the SDR DRAM speed also has not kept up with the internal RAM speed. On the STM32H7 for example the internal RAM bandwidth is 3.2GB/s versus a maximum SDR RAM bandwidth of 666MB/s even if we built the system with an 8-layer board using a 32-bit DRAM bus requring 50+ I/O pins for the DRAM.

So, since we’re built on the STM32 architecture and limited to using expensive and slow SDR DRAM for now we haven’t added it as our internal SRAM is way faster. As production volumes go up and technology improves hopefully we’ll be able to have more memory while still keeping the OpenMV Cam simple to use.

7.1. Memory Architecture

Given the above memory architecture limitations we built all of our code to run inside of the STM32 microcontroller memory. However, the STM32 doesn’t have one large contigous memory map. It features different segments of RAM for different situations.

First, there’s a segment of RAM which contains global variables, the heap, and the stack. The heap and global variables are fixed in size so only the stack grows and shrinks. For performance reasons heap/stack collision is not checked constantly so don’t use recursive functions on the OpenMV Cam.

As for the heap, it’s fixed in size versus growing towards the stack and managed by MicroPython’s garbage collector. MicroPython automagically free’s up unused blocks inside of the heap. However, the design of the MicroPython heap does not allow it to be arbitrarily large (e.g. in the megabyte range) like heaps on PCs. So, even if we have DRAM it would be hard to leverage using MicroPython’s heap.

Next, there’s a larger memory segment for the frame buffer to store images in. On the bottom of the frame buffer new images are stored when functions like sensor.snapshot() are called. Any unused space in the frame buffer is then available to be used as a “frame buffer stack” that builds from the top of the frame buffer down. This memory architecture design is what allows a lot of our computer vision methods to execute without having to allocate large data structures inside of the MicroPython heap.

That said, the frame buffer stack is still a stack and doesn’t support random allocations and deallocations. Luckily, most computer vision algorithms have very predictable memory allocations. For ones that don’t (like AprilTags) we allocate a temporary heap inside of the frame buffer stack when we need it (again to avoid fragmenting the MicroPython heap).

Finally, vision algorithms return their results (which are small usually) by allocating objects in the MicroPython heap. The results can then be garbage collected easily by MicroPython while the frame buffer stack is fully cleared after any computer vision algorithms finish executing.

Now while this works great it means you can only have one big image in the frame buffer in RAM. As the MicroPython heap is optimized for small objects storing large 100KB images in it doesn’t make sense. To enable more images to fit in RAM we allow the frame buffer stack to be used for secondary image storage using sensor.alloc_extra_fb(). By allocating a secondary frame buffer on the frame buffer stack you can now have two or more images in RAM at the cost of reducing memory space for more complex algorithms (like AprilTags).

So, that’s the memory architecture. And... we allow images to be stored in the frame buffer, heap, and the frame buffer stack. Yes, our code is rather complex to handle all of this and it would have been great to just throw everything in a large DRAM. But, now you know why this isn’t the case.