A Dynamically Reconfigurable Readout Scheme of a Pixel Readout ASIC for Space X-Ray Imaging

Abstract

A grazing-incidence X-ray telescope is an instrument to observe X-rays emitted by solar activities such as solar flares and coronal mass ejections. A hybrid X-ray imaging detector consisting of a radiation sensor and a readout integrated circuit (ROIC) is required to measure both the energy and counting rate of incident X-ray photons. In this application, the data of interest in the images are concentrated in the region where solar activities occur, and most of the data are irrelevant. Thus, a reconfigurable readout scheme is necessary to obtain a shorter readout time and provide more accurate observational data. In this article, we propose a novel two-step dynamically reconfigurable readout scheme. First, a frame of trigger flag information is read out, and the addresses of the triggered pixels are generated through mode arbitration. Then, the data of interest are read out according to the pixel addresses. The advantage of such a scheme lies in that it can support not only region-of-interest (ROI) readout but also autonomously transitions between full-frame readout and zero-suppression readout modes based on the number of active pixels. To verify this readout method, a $32\times 32$ pixel readout ASIC prototype is designed in a 180-nm CMOS process. The data readout scheme is implemented by the on-chip digital controller and the external logic circuits on a field-programmable gate array (FPGA) board. The ASIC has been tested. At an active pixel rate of 10%, the readout time is reduced by 77.5% compared to the full-frame readout mode. The measured data show that the proposed ASIC achieves dynamic real-time data readout and a frame rate exceeding 190 frames/s at a test clock frequency of 3.125 MHz.