Figure 26. Continuous Cleans {Clean Until Trigger} Flowchart - Princeton PIXIS-XB System User Manual

52 PIXIS-XB System Manual Version 2

Figure 26. Continuous Cleans {Clean Until Trigger} Flowchart

Once the External Sync pulse trigger edge is received, cleaning of the array stops as soon
as the current row is shifted, and frame collection begins: a delay time of up to one row
shift can be expected. With Normal Shutter {Normal} operation the shutter is opened for
the set exposure time. With PreOpen Shutter {Open Before Trigger} operation, the
shutter is open during the Continuous Cleaning {Clean Until Trigger}, and once the
External Sync pulse trigger edge is received the shutter remains open for the set exposure
time, then closes. If the vertical rows are shifted midway when the External Sync pulse
trigger edge arrives, the pulse is saved until the row shifting is completed, to prevent the
CCD from getting "out of step." As expected, the response latency is on the order of one
vertical shift time, from 1-30 µs depending on the array. This latency does not prevent the
incoming signal from being detected, since photo generated electrons are still collected
over the entire active area. However, if the signal arrival is coincident with the vertical
shifting, image smearing of up to one pixel is possible. The amount of smearing is a
function of the signal duration compared to the single vertical shift time.
Note: If EXT SYNC is still active (in Figure 27, this means that if it is still HIGH) at the
end of the readout, the hardware may interpret this as a second sync pulse, and so on.