A fast digital readout circuit for binary pixel sensors is presented. The architecture is designed to cope with matrices of the order of ten thousands of pixels and it can be involved proficiently for sparse single-particle detection at variable frame-rates, with an upper-bound of 1 Mfps in case the sensed flux is lower than few tens of pixel per frame. These characteristics make the architecture suitable for a black and white imaging sensor in a weak source environment where accurate timing and extremely high frame rates are required. In this work we describe the key features of the architecture, then we present an application case where the readout implemented in a CMOS pixel detector has been used ...