A programmable nanosecond digital delay and trigger system

A programmable digital delay and trigger system has been designed and constructed in VME format for the STACEE [1] ground-based gamma ray astrophysics project. The system is required to accommodate channel-to-channel timing differences that change as an astronomical source is tracked across the sky. The system is modular, with each VME module containing eight channels. The delay for each channel can be programmed in real time in one nanosecond steps with a range of 2048 nanoseconds. Each VME module contains trigger multiplicity logic, featuring selectable coincidence widths, and a multiplicity level programmable from one to eight. Up to eight modules can be combined to generate a global trigger derived from 64 channels. The global trigger is formed by requiring a multiplicity of module triggers, again with a programmable level from one to eight. The global trigger stops the system in order to allow the readout of the time coordinates of each channel over the VME bus. This feature can be used for the initial setup in coarse resolution mode, or in general to further refine the firmware trigger decision. Without matching any of the critical timing components, preliminary results show that the timing resolution of the system is substantially better than one nanosecond.