Cyclic vs non-cyclic electron flow:
Under certain conditions, the photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I (P700) but not photosystem II (P680). This process produces no NADPH and no O2, but it does make ATP. This is called cyclic photophosphorylation.
The chloroplast shifts to this process when the ATP supply drops and the level of NADPH rises. Often the amount of ATP needed to drive the Calvin cycle exceeds what is produced in non-cyclic photophosphorylation.
Without sufficient ATP, the Calvin cycle will slow or even stop. The chloroplast will continue cyclic photophosphorylation until the ATP supply has been replenished. ATP is produced through chemiosmosis in both cyclic and non-cyclic photophosphorylation.
Therefore, the difference between cyclic and non-cyclic electron flow is that in cyclic photophosphorylation happens only in the photosystem I but non-cyclic photophosphorylation occurs in both the photosystems I and II. In the cyclic photophosphorylation, only ATP is produced, whereas, in the non-cyclic photophosphorylation both NADPH and ATP are produced.