[SpiROSE] Drivers tradeoff

A lot of effort has been done this week to reduce the cost of our project. Luckily this process also help to refine our needs and design. One example is the LEDs’ driver: with thousands of LED to drive, we first compute that we needed almost a thousand driver, which was a nightmare to control and a money sink. Thus we decide that multiplexing will be the way to go.

This imply to use a FPGA and have several over consequences. If we use 32-multiplexing on a 48 channel driver, we can drive 512 LEDs with one driver. However when 16 LEDs are switched on, the multiplexing causes the 494 other to be turned off, thus this divides the LEDs’ intensity by 32 !  Moreover it makes the refresh slower, and because we rotate at high speed (18.85 m/s tip speed at 30 fps) we may observe an offset due to the driver being too slow.

Thus we have made some computation to know if this offset will be visible:

The driver we intent to use ( works at 25 MHz, so if we refresh the outermost LEDs at 30 fps they will have moved 0.4 mm at the end of the refresh, which is a third of the voxel size. Thus we should not see an offset between the inner LEDs and the outer ones.

So we have a tradeoff (as in any engineer project) between the number of driver (which are expensive) and the LEDs intensity.

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