  We developed an FPGA-based high-speed readout system for a complementary
metal-oxide-semiconductor (CMOS) image sensor to observe soft X-ray transients
in future satellite missions, such as HiZ-GUNDAM. Our previous research
revealed that the CMOS image sensor has low-energy X-ray detection capability
(0.4-4 keV) and strong radiation tolerance, which satisfies the requirements of
the HiZ-GUNDAM mission. However, CMOS sensors typically have small pixel sizes
(e.g., $\sim$10 ${\rm \mu m}$), resulting in large volumes of image data.
GSENSE400BSI has 2048$\times$2048 pixels, producing 6 Mbyte per frame. These
large volumes of observed raw image data cannot be stored in a satellite bus
system with a limited storage size. Therefore, only X-ray photon events must be
extracted from the raw image data. Furthermore, the readout time of CMOS image
sensors is approximately ten times faster than that of typical X-ray CCDs,
requiring faster event extraction on a timescale of $\sim$0.1 s. To address
these issues, we have developed an FPGA-based image signal processing system
capable of high-speed X-ray event extraction onboard without storing raw image
data. The developed compact system enabled mounting on a CubeSat mission,
facilitating early in-orbit operation demonstration. Here, we present the
design and results of the performance evaluation tests of the proposed
FPGA-based readout system. Utilizing X-ray irradiation experiments, the results
of the X-ray event extraction with the onboard and offline processing methods
were consistent, validating the functionality of the proposed system.
