This paper deals with a relatively new SPICE model of a P-i-N photodiode. The model includes a change of velocities of electrons and holes, due to the voltage drop on the edges of the photodiode, which depends on the time form of input excitation. We have derived the model of the P-i-N photodiode for digital input excitation i.e. for Heaviside's square wave time excitation. The model is incorporated in SPICE program and simulated with it. The model and the limitations of the model itself are observed. The output results are compared with the similar ones. It is suggested when it is practical to use the model, and determined the photodiode working domain regime when the model gives accurate results.