MT9M034
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20
Synchronizing Register Writes to Frame Boundaries
Changes to most register fields that affect the size or
brightness of an image take effect on two frames after the
one during which they are written. These fields are noted as
“synchronized to frame boundaries” in the MT9M034
Register Reference. To ensure that a register update takes
effect on the next frame, the write operation must be
completed after the leading edge of FV and before the
trailing edge of FV.
Fields not identified as being frame−synchronized are
updated immediately after the register write is completed.
The effect of these registers on the next frame can be difficult
to predict if they affect the shutter pointer.
Restart
To restart the MT9M034 at any time during the operation
of the sensor, write a “1” to the Restart register (R0x301A[1]
= 1). This has two effects: first, the current frame is
interrupted immediately. Second, any writes to
frame−synchronized registers and the shutter width registers
take effect immediately, and a new frame starts (in video
mode). The current row completes before the new frame is
started, so the time between issuing the Restart and the
beginning of the next frame can vary by about t
ROW
.
Image Acquisition Modes
The MT9M034 supports two image acquisition modes:
video(master) and single frame.
Video
The video mode takes pictures by scanning the rows of the
sensor twice. On the first scan, each row is released from
reset, starting the exposure. On the second scan, the row is
sampled, processed, and returned to the reset state. The
exposure for any row is therefore the time between the first
and second scans. Each row is exposed for the same
duration, but at slightly different point in time, which can
cause a shear in moving subjects as is typical with electronic
rolling shutter sensors.
Single Frame
The single−frame mode operates similar to the video
mode. It also scans the rows of the sensor twice, first to reset
the rows and second to read the rows. Unlike video mode
where a continuous stream of images are output from the
image sensor, the single−frame mode outputs a single frame
in response to a high state placed on the TRIGGER input pin.
As long as the TRIGGER pin is held in a high state, new
images will be read out. After the TRIGGER pin is returned
to a low state, the image sensor will not output any new
images and will wait for the next high state on the TRIGGER
pin.
The TRIGGER pin state is detected during the vertical
blanking period (i.e. the FV signal is low). The pin is level
sensitive rather than edge sensitive. As such, image
integration will only begin when the sensor detects that the
TRIGGER pin has been held high for 3 consecutive clock
cycles. If the trigger signal is applied to multiple sensors at
the same time, the single frame output of the sensors will be
synchronized to within 1 PIXCLK if is PLL disabled or 2
PIXCLKs if PLL is enabled.
During integration time of single−frame mode and video
mode, the FLASH output pin is at high.
Continuous Trigger
In certain applications, multiple sensors need to have their
video streams synchronized (E.g. surround view or
panorama view applications). The TRIGGER pin can also
be used to synchronize output of multiple image sensors
together and still get a video stream. This is called
continuous trigger mode. Continuous trigger is enabled by
holding the TRIGGER pin high. Alternatively, the
TRIGGER pin can be held high until the stream bit is
enabled (R0x301A[2] = 1) then can be released for
continuous synchronized video streaming.
If the TRIGGER pins for all connected MT9M034 sensors
are connected to the same control signal, all sensors will
receive the trigger pulse at the same time. If they are
configured to have the same frame timing, then the usage of
the TRIGGER pin guarantees that all sensors will be
synchronized within 1 PIXCLK cycle if PLL is disabled, or
2 PIXCLK cycles if PLL is enabled.
With continuous trigger mode, the application can now
make use of the video streaming mode while guaranteeing
that all sensor outputs are synchronized. As long as the initial
trigger for the sensors takes place at the same time, all
subsequent video streams will be synchronous.
Temperature Sensor
The MT9M034 sensor has a built−in PTAT−based
temperature sensor, accessible through registers, that is
capable of measuring die junction temperature.
The temperature sensor can be enabled by writing
R0x30B4[0] = 1 and R0x30B4[4] = 1. After this, the
temperature sensor output value can be read from
R0x30B2[10:0].
The value read out from the temperature sensor register is
an ADC output value that needs to be converted downstream
to a final temperature value in degrees Celsius. Since the
PTAT device characteristic response is quite linear in the
temperature range of operation required, a simple linear
function in the format of listed in the equation below can be
used to convert the ADC output value to the final
temperature in degrees Celsius.
Temperature + slope R0x30B2[10 : 0] ) T
0
(eq. 5)
For this conversion, a minimum of 2 known points are
needed to construct the line formula by identifying the slope
and y−intercept “T
0
”. These calibration values can be read
from registers R0x30C6 and R0x30C8 which correspond to
value read at 70
°C and 55°C respectively. Once read, the
slope and y−intercept values can be calculated and used in
the above equation.
