MT9P401 CONFIDENTIAL AND PROPRIETARY
NOT FOR PUBLIC RELEASE
www.onsemi.com
33
scans. Each row is exposed for the same duration, but at
slightly different point in time, which can cause a shear in
moving subjects.
Whenever the mode is changed to an ERS mode (even
from another ERS mode), and before the first frame
following reset, there is an anti−blooming sequence where
all rows are placed in reset. This sequence must complete
before continuous readout begins. This delay is:
tALLRESET = 16 × 2004 × tACLK
Global Reset Release
The GRR modes attempt to address the shearing effect by
starting all rows’ exposures at the same time. Instead of the
first scan used in ERS mode, the reset to each row is released
simultaneously. The second scan occurs as normal, so the
exposure time for each row would different. Typically, an
external mechanical shutter would be used to stop the
exposure of all rows simultaneously.
In GRR modes, there is a startup overhead before each
frame as all rows are initially placed in the reset state
(tALLRESET). Unlike ERS mode, this delay always occurs
before each frame. However, it occurs as soon as possible
after the preceding frame, so typically the time from trigger
to the start of exposure does not include this delay. To ensure
that this is the case, the first trigger must occur no sooner
than tALLRESET after the previous frame is read out.
Exposure
The nominal exposure time, tEXP, is the effective shutter
time in ERS modes, and is defined by the shutter width, SW,
and the shutter overhead, SO, which includes the effect of
Shutter_Delay. Exposure time for other modes is defined
relative to this time. Increasing Shutter_Delay (SD)
decreases the exposure time. Exposure times are typically
specified in units of row time, although it is possible to
fine−tune exposures in units of tACLKs (where tACLK is 2
× tPIXCLK).
tEXP = SW × tROW – SO × 2 × tPIXCLK
where:
SW = max(1, (2 × 16 × Shutter_Width_Upper) +
Shutter_Width_Lower)
SO = 208 × (Row_Bin + 1) + 98 + min(SD, SDmax) – 94
SD = Shutter_Delay + 1
SDmax = 1232; if SW < 3
1504, otherwise
The exposure time is calculated by determining the reset
time of each pixel row (with time 0 being the start of the first
row time), and subtracting it from the sample time. Under
normal conditions in ERS modes, every pixel should end up
with the same exposure time. In global shutter release
modes, or in row binning modes, the exposure times of
individual pixels can vary.
In global shutter release modes (described later) exposure
time starts simultaneously for all rows, but still ends as
defined above. In a real system, the exposure would be
stopped by a mechanical shutter, which would effectively
stop the exposure to all rows simultaneously. Because this
specification does not consider the effect of an external
shutter, each output row’s exposure time will differ by
tROW from the previous row.
Global shutter modes also introduce a constant added to
the shutter time for each row, because the exposure starts
during the global shutter sequence, and not during any row’s
shutter sequence. For each additional row in a row bin, this
offset will increase by the length of the shutter sequence.
In Bulb_Exposure modes (also detailed later), the
exposure time is determined by the width of the TRIGGER
pulse rather than the shutter width registers. In ERS bulb
mode, it is still a multiple of row times, and the shutter
overhead equation still applies. In GRR bulb mode, the
exposure time is granular to ACLKs, and shutter overhead
(and thus Shutter_Delay) has no effect.
Operating Modes
In the default operating mode, the MT9P401 continuously
samples and outputs frames. It can be put in “snapshot” or
triggered mode by setting snapshot, which means that it
samples and outputs a frame only when triggered. To leave
snapshot mode, it is necessary to first clear Snapshot then
issue a restart.
When in snapshot mode, the sensor can use the ERS or the
GRR. The exposure can be controlled as normal, with the
Shutter_Width_Lower and Shutter_Width_Upper registers,
or it can be controlled using the external TRIGGER signal.
The various operating modes are summarized in Table 15.
Table 15. OPERATING MODE
Mode Settings Description
ERS Continuous Default Frames are output continuously at the frame rate defined by tFRAME.
ERS is used, and the exposure time is electronically controlled to be tEXP
ERS Snapshot Snapshot = 1 Frames are output one at a time, with each frame initiated by a trigger.
ERS is used, and the exposure time is electronically controlled to be tEXP
ERS Bulb Snapshot = 1;
Bulb_Exposure = 1
Frames are output one at a time, with each frame’s exposure initiated by a trigger.
ERS is used. End of exposure and readout are initiated by a second trigger
GRR Snapshot Snapshot = 1;
Global_Reset = 1
Frames are output one at a time, with each frame initiated by a trigger.
GRR is used. Readout is electronically triggered based on SW
GRR Bulb Snapshot = 1;
Bulb_Exposure = 1;
Global_Reset = 1
Frames are output one at a time, with each frame initiated by a trigger.
GRR is used. Readout is initiated by a second trigger
1. In ERS bulb mode, SW must be greater than 4 (use trigger wider than tROW x 4).