7. Programmable I/O Features Description
|Programmable Output Slew Rate Control||
Each I/O pin contains a slew rate control, allowing you to specify the slew rate on a pin-by-pin basis. The slew rate control affects both the rising and falling edges of the signal.
A faster slew rate provides high-speed transitions for high-performance systems while a slower slew rate reduces system noise and crosstalk but adds a nominal delay to the rising and falling edges.
|Programmable IOE Delay||
You can activate the programmable IOE delays to ensure zero hold time, minimize setup time, or increase the clock-to-output time. This feature helps read and write timing margins because it minimizes the uncertainties between signals on the bus.
Each pin can have a different input delay from the pin-to-input register or a delay from output register-to-output pin values. This is to ensure that the signals within a bus have the same delay going into or out of the device.
|Programmable Open-Drain Output||
The programmable open-drain output provides a high-impedance state on output when logic to the output buffer is high. If logic to the output buffer is low, the output is low.
You can attach several open-drain outputs to a wire. This connection type is like a logical OR function and is commonly called an active-low wired-OR circuit. If at least one of the outputs is in logic 0 state (active), the circuit sinks the current and brings the line to low voltage.
You can use open-drain output if you are connecting multiple devices to a bus. For example, you can use the open-drain output for system-level control signals that can be asserted by any device or as an interrupt.
Each I/O pin supports an optional bus-hold feature that is active only after configuration. When the device enters user mode, the bus-hold circuit captures the value that is present on the pin by the end of the configuration.
The bus-hold circuitry uses a resistor to weakly pull the signal level to the last-driven state of the pin. The bus-hold circuitry holds this pin state until the next input signal is present. Therefore, you do not require an external pull-up or pull-down resistor to hold a signal level when the bus is tri-stated.
For each I/O pin, you can individually specify that the bus-hold circuitry pulls non-driven pins away from the input threshold voltage—where noise can cause unintended high-frequency switching. To prevent over-driving signals, the bus-hold circuitry drives the voltage level of the I/O pin lower than the I/O bank power supply level.
|Programmable Pull-Up Resistor||
Each I/O pin on supported banks provides an optional programmable pull-up resistor during user mode. The pull-up resistor weakly holds the I/O to the I/O bank power supply level.
|Programmable Pull-Down Resistor||
Each I/O pin on supported banks provides an optional programmable pull-down resistor during user mode. The pull-down resistor weakly holds the I/O to the ground level.
Pre-emphasis momentarily boosts the high-frequency component of the output signal during switching to increase the output slew rate. The amount of pre-emphasis required depends on the attenuation of the high-frequency component along the transmission line.
For more information, refer to Programmable Pre-Emphasis.
De-emphasis attenuates the I/O signal height when the symbol is longer than the specified duration. You can use de-emphasis to alter the signal amplitude to compensate for signal degradation over long transmission path.
For more information, refer to Programmable De-Emphasis.
|Programmable Differential Output Voltage||
The programmable VOD settings allow you to adjust the output eye-opening to optimize the trace length and power consumption. A higher VOD swing improves voltage margins at the receiver end, and a smaller VOD swing reduces power consumption.
For more information, refer to Programmable Differential Output Voltage.
The Schmitt Trigger allows input buffers to respond to slow input edge rates with a fast output edge rate. Most importantly, Schmitt Triggers provide hysteresis on the input buffer, preventing slow-rising noisy input signals from ringing or oscillating on the input signal driven into the logic array.
This feature provides system noise tolerance on the device inputs but adds a small, nominal input delay.
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