![]() ![]() The PCI local bus provides pull-up resistors, but they would require several clock cycles to pull a signal high given the bus's large distributed capacitance. Circuit designers will often use pull-up or pull-down resistors (usually within the range of 1–100 kΩ) to influence the circuit when the output is tri-stated. When outputs are tri-stated (in the Hi-Z state) their influence on the rest of the circuit is removed, and the circuit node will be "floating" if no other circuit element determines its state. A ROM or static RAM chip with an output enable line will typically list two access times: one from chip select asserted and address valid, and a second, shorter time beginning when output enable is asserted. This can be done while the bus is in use for other purposes, and when output enable is finally asserted, the data will appear with minimal delay. When chip select is asserted, the chip internally performs the access, and only the final output drivers are disabled by deasserting output enable. (An advantage of course, is that the chip consumes minimal power in this case.) When chip select is deasserted, the chip does not operate internally, and there will be a significant delay between providing an address and receiving the data. The difference lies in the time needed to output the signal. If CS is not asserted, the outputs are high impedance. Many memory devices designed to connect to a bus (such as RAM and ROM chips) have both CS ( chip select) and OE (output enable) pins, which superficially appear to do the same thing. Three-state logic can reduce the number of wires needed to drive a set of LEDs (tri-state multiplexing or Charlieplexing). Three-state buffers are essential to the operation of a shared electronic bus. Three-state buffers can also be used to implement efficient multiplexers, especially those with large numbers of inputs. If more than one device is electrically connected to another device, putting an output into the Hi-Z state is often used to prevent short circuits, or one device driving high (logical 1) against another device driving low (logical 0). The basic concept of the third state, high impedance (Hi-Z), is to effectively remove the device's influence from the rest of the circuit. The term tri-state should not be confused with ternary logic ( 3-value logic). Many devices are controlled by an active-low input called OE (Output Enable) which dictates whether the outputs should be held in a high-impedance state or drive their respective loads (to either 0- or 1-level). Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals. Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 74 series as well as in other types, but also internally in many integrated circuits. ![]() This allows multiple circuits to share the same output line or lines (such as a bus which cannot listen to more than one device at a time). In digital electronics three-state, tri-state, or 3-state logic allows an output or input pin/pad to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels. For the diskette controller format, see 3mode. ![]()
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