An FPGA based backup version of the TileCal digitizer.

The ATLAS Tile Calorimeter front end digitization and readout system comprises about 1800 digitizer boards with two TileDMU ASICs on each board. The TileDMUs are responsible for storing, derandomising and reading out digitized data from twelve ADCs. An ample number of board spares are available. However, a backup solution is desirable in the event of unexpected failure modes. The original version contains both outdated and custom made circuits that are difficult or impossible to find in sufficient numbers. We have developed a new version using inexpensive off the shelf FPGAs (Spartan 6). The FPGAs have all the necessary functionality to emulate the TileDMU and will be readily available for a considerable time. The new board is functionally compatible with the current version and to a large extent uses the same code. The design goal was to leave the digitizer design as intact as possible since it is well tested and performs well. As radiation tolerance is an issue we have implemented triple mode redundancy in the FPGA. To further improve the system we added in system programmability via TTCrx for both the FPGA and the configuration memory using one way JTAG. This provides a way to recover from radiation damage to the configuration PROM or to remotely upgrade system firmware.