The basic sdc to be introduced today is the system interface sdc. Mainly including set_drive, set_driving_cell, set_input_transition, set_load these four commands, are related to the driver load on the port.
Set_drive: set the resistance value of the input port or bidirectional port, mainly to estimate the delay of the input circuit more accurately. We need to know the transition time of the signal to the input port. The set drive uses a certain value to estimate the input resistance of the input. Get the delay of the input port, the smaller the value, the stronger the drive capability.
Example: set_drive 2.0 [get_ports in1]
Note: You cannot set the set_drive value to 0, so the input has an infinite drive capability, meaning the transition time is 0.
Set_driving_cell: This constraint is actually the same function as set_drive. It refers to using a unit in the design library to drive the input port, and then calculating the tranition time according to the input resistance of the unit, thereby obtaining the delay of the input port. Usually used more than set_drive, generally used at the block level, because it will consider OCV and other factors, it will be more accurate.
Example: set_driving_cell-lib_cell AND -from_pin B -pin Y IN1
Set_input_transition: Set the transition time of the input port, which is generally used for the chip io pad constraint. Because the voltage on the Pad is relatively high, the corresponding capacitance and transition are also large, and the driving cell cannot be used.
Example: set_input_transition 0.8 [get_ports DATA_IN*]
Set_load: Sets the load of the output circuit. Since the load on the external circuit will affect the delay of the unit connected to the port, it is necessary to limit the load capacity of the port. In order to calculate the delay of the output circuit more accurately, we need to know all the loads of the output circuit. The larger the circuit load capacitance, the smaller the delay time.
Example: set_load 1.5 -min -pin_load [get_ports in1]
Note: The load capacitance cannot be made zero, so the output port has infinite drive capability.
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