ACIS columns can be read out continuously in Continuous Clocking (CC) mode. This provides much higher time resolution (3 msec) than the standard frame readout modes. However CC-mode prevents any information about the row number of an event reaching the ground. (A source appears as a stripe along a column.) Event grades are reconstructed on-board.

For more details:

• Continuous Clocking (CC) Mode section of the Proposers' Observatory Guide (POG)
• Why topic: Continuous Clocking Mode