2.5.4. I/O Power Guidelines
Nonterminated I/O Standards
Nonterminated I/O standards such as LVTTL and LVCMOS have a rail-to-rail output swing. The voltage difference between logic-high and logic-low signals at the output pin is equal to the VCCIO supply voltage. If the capacitive loading at the output pin is known, the following expression determines the dynamic power consumed in the I/O buffer:
- F is the output transition frequency
- C is the total load capacitance being switched
- V is equal to VCCIO supply voltage
Transistor-to-transistor logic (TTL) I/O buffers consume very little static power. As a result, the total power that a LVTTL or LVCMOS output consumes is highly dependent on load and switching frequency.
Resistively Terminated I/O Standards
In resistively terminated I/O standards like SSTL and HSTL, the output load voltage swings by a small amount around a bias point. The dynamic power equation above is valid as well, but V is the actual load voltage swing. This voltage is much smaller than VCCIO, resulting in lower dynamic power when comparing to nonterminated I/O under similar conditions.
Resistively terminated I/O standards dissipate significant static (frequency-independent) power, because the I/O buffer is constantly driving current into the resistive termination network. However, the lower dynamic power of these I/O standards means they often have lower total power than LVCMOS or LVTTL for high-frequency applications. As a best practice, when using resistively terminated standards choose the lowest drive strength I/O setting that meets the speed and waveform requirements to minimize I/O power.
You can save a small amount of static power by connecting unused I/O banks to the lowest possible VCCIO voltage.