Actually digital I/O cells using if your working frequency is low. not exceed ~200 to 400 MHz. because digital level shifter can't operating normally in high frequency. the solution is use much more difficult construction of digital level shifter or replace the digital level shifter with analog level shifter, for example diff-amp. with diff-amp I/O cells can work up to ~600MHz and more (special diff-amp). But replacing digital component with analog we increasing the power consumption. Now most of DDR2 or DDR3 memories using mixed type of I/O cells (logic parts are digital, the other part is analog ) and can reach up to 1666MHz frequency.
New high-speed drivers using Current mode logic (CML), voltage mode logic (VML), and Low-voltage differential signaling (LVDS).