Low voltage differential driver with multiple drive strengths

摘要

The first pulse problem for a low-voltage differential SCSI bus driver is remedied by supplying greater power for a first pulse of a bus line after a steady state condition. Activity detection circuitry detects when a signal has remained in a steady state for a number of bus cycles and enables an additional power boosting differential driver to deliver an appropriate amount of power for a limited amount of time in order to produce a quality first pulse while minimizing power output. The extra power needed to remedy the quality of the first pulse is only supplied for the duration of the first pulse so that the output driver strength is minimized and the total power over time that an integrated circuit must dissipate is reduced. In another embodiment, instead of greater than normal power being delivered for a first pulse, the output driver is decreased in its output drive strength while an output remains in a particular state. When the output does finally switch states, it switches at normal strength; the net effect is an increased drive strength from the steady state to the new state. Step down control circuitry is used to determine after a specified number of clock pulses how much power should be stepped down and in how many increments.