The Fast AEQ mode is used to distinguish two channels (short channel and a long channel) and choose the appropriate receiver equalization setting for that channel. Because Fast AEQ only distinguishes between two choices, the AEQ time is a lot shorter than Full AEQ mode which minimizes impact to USB link training.

When Fast AEQ is enabled and channel is determined to be short, the TUSB1142 will use the value programmed into the CEQx_SEL, where x = 1 or 2. If the TUSB1142 determines channel is not short, the TUSB1142 will switch to EQ value programmed into LONG_EQx register, where x = 1 or 2. During initial system evaluation, it is recommended to perform both short and long channel USB 3.1 RX JTOL Gen2 testing and program CEQx_SEL and LONG_EQx to the value which produced the best results for each channel configuration.

The TUSB1142 will determine short and long based on the estimate eye height. The value programmed into FASTAEQ_LIMITS register will determine the eye height limits. Software can change the defaults of this register to lower or raise the limits.