Meaning
Slew rate limits specifying the maximum rate of voltage change on an output pin of a digital driver define the switching transition speed. The output driver slew rate determines both the signal integrity of the transmission line and the generation of electromagnetic interference. Balancing this transition speed is necessary to meet the timing requirements while minimizing high-frequency noise.
Noise Generation
Fast transitions generate high-frequency harmonic energy that can easily radiate from board traces. This radiation causes electromagnetic interference issues during product compliance testing. Rapid transitions also induce transient voltages across package and trace inductances, leading to ground bounce and power rail noise.
Slowing down the transition reduces these unwanted noise components.
Timing Budget
High-speed serial links require fast transitions to maintain a wide eye opening and meet the setup time requirements. If the transition is too slow, the signal will not reach the logic threshold before the next clock cycle. This delay can lead to setup time violations and data errors.
System designers must configure the transition speed to match the channel length.
Slew Control
Modern integrated circuits feature programmable driver stages that allow engineers to select the transition speed via register settings. Adjusting the drive strength or active pull-up current changes the transition rate on the physical pin. This programmability allows designers to tune the signal behavior after the board has been manufactured.
The selection of the transition speed allows the system to be optimized for either low noise or high speed.
