The diversity of hardware modchips operate on and varying methods they use mean that while modchips are often used for the same goal, they may work in vastly different ways, even if they are intended for use on the same console. Some of the first modchips for the Nintendo Wii, known as drive chips, modify the behaviour and communication of the optical drive to bypass security. On the Xbox 360, a common modchip took advantage of the fact short periods of instability in the CPU could be used to fairly reliably lead it to incorrectly compare security signatures. The precision required in this attack meant that the modchip had to make use of a CPLD. Other modchips, such as the XenoGC and clones for the Nintendo GameCube, invoke a debug mode where security measures are reduced or absent (in which case a stock Atmel AVR microcontroller was used). A more recent innovation are optical disk drive emulators or ODDE, which replace the optical disk drive and allow data to come from another source bypassing the need to circumvent any security. These often make use of FPGAs to enable them to accurately emulate timing and performance characteristics of the optical drives.