The N-FEE operates the 4 CCDs of a normal camera, digitizes the image data and transfers them to the DPU. Each normal CCD has an integration time of 22.5 s and a readout time of ~2.5 s. The readouts are staggered at every 6.25 s, in equal intervals of the 25 s frame time. The readout and data transfer to the DPU are arranged such that the readout of one CCD is finished before the next begins, in order to minimise crosstalk and interference effects.
An FPGA is the core of the N-FEE, and receives command packets from the DPU and timing and synchronisation data from the AEU. It generates all the clocks necessary for driving the 4 CCDs and drives the DACs responsible for providing the bias voltages.
The interface between N-FEE and N-DPU is made by two SpaceWire links. The protocol used is RMAP in all cases, but the command interface is actually simulated RMAP with e.g. control registers, HK data, memory mapped for simple access.
The F-FEEs operate the CCDs of the two fast cameras. It has many aspects in common with the N-FEE: commanding, CCD bias supplies, clock waveforms, and housekeeping. Other aspects are significantly different due to the use of frame-transfer devices and shorter integration times: FPGA and programming, number of SpaceWire interfaces and data rate.
For the fast cameras, the 4 CCDs are read out simultaneously every 2.5 s. Due to less critical noise requirements, the F-FEE uses an integrated analogue front-end (AFE) electronics, instead of the non- integrated 16-bit AFE used by N-FEE. As for the N-FEE, synchronisation of the two cameras is ensured by receiving from the associated F-AEU a high frequency signal (50 MHz) and a signal giving the information of the 2.5 s period beginning, also synchronised with the 25.0 s period of the normal cameras.
The N-FEE is responsability of UK, while the F-FEE is responsibility of Germany, with the involvement of France and Belgium.