An I/O standard with a voltage-referenced signal, input (V_REF) of 1.5, an output (V_CCIO) voltage of 3.3 V, a GTL+/SSTL input buffer and a 3.3-V/2.5-V TTL output buffer. The CTT I/O standard is supported by the APEX^™ 20KC, APEX 20KE, APEX^™ II, ARM^®-based Excalibur^™, Stratix^™, and Stratix^™ GX device families.