(Firstly If you edit your post, please tick the little box that says Minor Edit, otherwise everyone gets sent loads of emails)
I took a quick look at the OV5640 spec and it looks very similar to the OV7670.
I think anyone would struggle to create a "simple" interface to this, mainly because it streams video data on its output.
My guess would be that the PPI in the nRF52 could handle being triggered by the pixel clock and then read the GPIO parallel data into memory in the nRF52.
I don't know anything about eMMC. I presume its not a parallel interface, and if its like SD memory, it will have latency issues.
Your best bet is probably to read the frame into the nRF52 then save it to eMMC, but its not going to be a simple job to get it all working.
Even the camera config via I2C is a pain, there are hundreds of registers you have to set correctly to get a decent image.
The ArduCAM uses a FPGA and a memory chip for frame buffering and to convert the parallell interface to SPI. You can see the power consumption in the specs on the site I linked to:
Normal :5V/70mA
Low power mode: 5V/20mA
The fps may be less than you want as the site says:
3~10fps video output at low resolution
If you want to interface a camera you need a framebuffer. I would also guess you will get problems if you try to use parallell interface on the nRF, especially if you try to do other things as well in your code (for example using the SoftDevice). The gpio can be read at maximum 8 MHz (gpio peripheral runs at 16MHz) minus some CPU cycles (CPU runs at 64MHz) for storing the value and accessing the gpio again. But this needs to be synced with other signals so the actual read speed is probably be lower.
SPI in comparison uses DMA so this can work in the background while you do other things in your code. Maximum clock for the SPI is 8MHz, so in theory 1MB/s.
i don't know before tomorrow or friday if I can share the code with you.
Perhaps this code should be in your github repo, to make it easier for people to find.
BTW. i am sure the nRF52 would be able to directly read data from the OV7670 ( because people managed to do it with a 16MHz AVR), but I doubt anyone has the time or enthusiasm to try it on the nRF52.
