In the HDO and HDL shutter modes, an asynchronous falling edge on the VRST_INT (TTL) or
INPUT1 (LVDS) input of the camera is used to synchronize the exposure period of the camera to
the outside world (the rising edge is not significant, however, the LOW duration should last at least
1µsec). Since the falling edge is truly asynchronous, in most instances it would have the effect of
interrupting the readout of a previously exposed frame from the storage area elements of the CCD;
a residual charge from the previous exposure therefore may exist on the storage area elements. This
charge must be removed from the storage area by a “discharge” process before the next charge
transfer takes place.
7.2.3.1 One Shot high speed shutter with discharge (HDO)
This mode is also referred to as the "one-shot" or "snapshot" mode. In this mode, the camera acts
like a snapshot digital camera. The camera outputs no frames (and no Enable_Frame signals) until
a reset signal is received (see above section related to reset signals). Once a reset signal is received,
the camera immediately performs one-and-only-one exposure (with the duration determined by the
previously set EXP command) resulting in one-and-only-one valid Enable_Frame signal. Note:
there is no latency or delay between the falling edge of reset and the start of the exposure.
READOU
T
READOU
T
E
X
P
O
S
U
R
E
User defined
exposure period
1 frame =
1/12sec
E
X
P
O
S
U
R
E
NO VIDEO
VRST_INT (TTL) or INPUT1 (LVDS)
CHARGE TRANSFER (Internal Signal)
STROBE OUT (TTL)
Pixel Clock and Enable_Line (run continuously)
ENABLE_FRAME
83 ms
83 ms
Figure 7.2-3: Timing diagram—HDO Mode
In a typical frame grabber based system, the displayed image is updated only when the reset is
generated; until then, the previously captured image (resulting from the previous reset) is
displayed. Therefore this mode is referred to as the asynchronous "snapshot" mode.
NOTE: The frame grabber should be capable of sustaining long periods of time without receiving
an Enable-Frame signal.
The exposure is set, as in all shutter modes, via the serial EXP command with its 11-bit argument.
For example, EXP 018 will set up the exposure to be equivalent to 25 lines of exposure (Hex"019"
= Decimal 24 + 1); since one-line-period is 80
µ
sec, this is the same as 25 x 80
µ
sec = 0.002sec or
1/500sec.
20