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EP1C12F100C6ES Datasheet(PDF) 57 Page - Altera Corporation |
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EP1C12F100C6ES Datasheet(HTML) 57 Page - Altera Corporation |
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57 / 104 page  Altera Corporation 2–51 January 2007 Preliminary I/O Structure Slew-Rate Control The output buffer for each Cyclone device I/O pin has a programmable output slew-rate control that can be configured for low noise or high- speed performance. A faster slew rate provides high-speed transitions for high-performance systems. However, these fast transitions may introduce noise transients into the system. A slow slew rate reduces system noise, but adds a nominal delay to rising and falling edges. Each I/O pin has an individual slew-rate control, allowing the designer to specify the slew rate on a pin-by-pin basis. The slew-rate control affects both the rising and falling edges. Bus Hold Each Cyclone device I/O pin provides an optional bus-hold feature. The bus-hold circuitry can hold the signal on an I/O pin at its last-driven state. Since the bus-hold feature holds the last-driven state of the pin until the next input signal is present, an external pull-up or pull-down resistor is not necessary to hold a signal level when the bus is tri-stated. The bus-hold circuitry also pulls undriven pins away from the input threshold voltage where noise can cause unintended high-frequency switching. The designer can select this feature individually for each I/O pin. The bus-hold output will drive no higher than VCCIO to prevent overdriving signals. If the bus-hold feature is enabled, the device cannot use the programmable pull-up option. Disable the bus-hold feature when the I/O pin is configured for differential signals. The bus-hold circuitry uses a resistor with a nominal resistance (RBH) of approximately 7 k Ωto pull the signal level to the last-driven state. Table 4–15 on page 4–6 gives the specific sustaining current for each VCCIO voltage level driven through this resistor and overdrive current used to identify the next-driven input level. The bus-hold circuitry is only active after configuration. When going into user mode, the bus-hold circuit captures the value on the pin present at the end of configuration. Programmable Pull-Up Resistor Each Cyclone device I/O pin provides an optional programmable pull- up resistor during user mode. If the designer enables this feature for an I/O pin, the pull-up resistor (typically 25 k Ω) holds the output to the VCCIO level of the output pin's bank. Dedicated clock pins do not have the optional programmable pull-up resistor. |
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