Simple Driver for the IMX415 Sensor on Jetson Linux r32.6.1, Part 4 — Exposure

In this article, we will show an example of how to control the exposure time.
First, let’s clarify what exposure is. Strictly speaking, the exposure of an image sensor is the amount of light that determines the brightness of the image. In the case of the IMX415, exposure is controlled using the shutter time (exposure time). Correct configuration allows you to obtain a clear and detailed image without overexposure or underexposure.
The IMX415 documentation provides the following table:

The value of the SHR0 register is calculated using the following formula:
reg_shr0 = frame_length - coarse_time_shr0;
where frame_length is the value obtained at the FPS calculation stage. You can find more details in the previous article.

The coarse_time_shr0 value is calculated as:

coarse_time_shr0 = PIX_CLK_VAL * val / line_length / exposure_factor;

where:

PIX_CLK_VAL = 74250000 — the internal pixel clock of the sensor
val — exposure value passed from gstreamer. The valid range is taken from the devicetree:
min_exp_time = "60";
max_exp_time = "33200";

min_exp_time = 4 * H,  where H = 14.9 µs (value from the datasheet)
max_exp_time = 2242 * H
line_length — value taken from the devicetree, corresponds to the sensor HMAX register and depends on the selected operating mode of the sensor.
exposure_factor — value from the devicetree, used to scale the exposure range.

For the IMX415 mode, CSI-2/4 lane, 25 FPS, and gstreamer exposure val = 60, we obtain:
coarse_time_shr0 = 74250000 * 60 / 1100 / 1000000 = 4
frame_length = 2250 for 25 FPS.
Thus we get the value of reg_shr0:
reg_shr0 = frame_length - coarse_time_shr0;
reg_shr0 = 2250 - 4 = 2246
which matches the value given in the documentation.
The driver source code used in this article is available via the link

We also design custom carrier boards for Jetson Orin NX / Jetson AGX Orin / Jetson Nano and Nano Super.