[Hardware Engineer] about signal level driving capability

Drive capability refers to the current flowing into the output port / Outflow size , But in most cases, it refers to the output port Fan out Ability .

Fan out ： One output port is connected to multiple input ports .

In terms of DC characteristics

The output ports of digital devices are Maximum current limit , Current limit exceeded , It is possible to make the output level amplitude lower / Burn device . For example , A single chip computer GPIO The maximum output current at high level of the port is 20mA, So this one 20mA The specification represents the I/O The level driving ability of the port . And if the load is too large （ For example, small resistance ）, Then the load current may exceed its maximum output current , At this time, we say that the driving capacity is insufficient . The direct consequence of insufficient driving capacity is the drop of output voltage , For logic circuits , Failure to maintain its high level may even cause logic confusion , This phenomenon is generally not allowed .

therefore , From the perspective of DC characteristics , Driving capacity refers to the maximum current flow into the output port / Outflow capacity .

In terms of communication characteristics

Due to the input capacitance at the input port , So the output port Level reversal process , It can be considered as the charging and discharging process of the capacitor at the input port . The larger the fan out of the output port , Then charge and discharge speed / The slower the level reversal , There is a maximum fan out value at a certain switching frequency , Therefore, when the actual fan out value is greater than the maximum fan out value , The level reversal speed is too slow to meet the requirements .

therefore , From the perspective of AC characteristics , Drive capability can also refer to the fan out capability of the device output port , However, the fan out ability is still related to the inflow and outflow ability of the current at the upper port .