Generation and measurement of pulses and delays with RISC-controllers

An instrument for the generation of pulses and delays has been developed for applications in laserspectroscopy experiments. The pulses from a chopped laserbeam are counted, and when a preset value is reached, two signals are generated: one delayed trigger pulse with constant length and one delayed ‘shutter pulse’ (referred to as the ‘shutter window’ below) which opens a light shutter and allows one light pulse to enter the experimental region and excite the sample. The delays of the trigger pulse and the shutter window and the width of the shutter window can all be set independently to any value from 0 up to 50 ms with 0.05 ms steps. A separate microcontroller measures the width of the shutter window with an accuracy of 1 micro second s. The pulse delays and the width of the shutter window are set with thumbwheel potentiometers connected as voltage dividers. A microcontroller reads the set values with a four-channel, 12-bit AD converter. This technique saves a lot of hardware wiring as well as software writing when compared to the alternative of using separate, BCD-coded thumbwheels for each timesetting. In total, the instrument consists of two independent RISC-controllers: one PIC16C55 for counting chopper pulses and generating pulses and delays, and one PIC16C74 for measuring (and displaying) the width of the shutter window. The width of the shutter window is measured with 1 micro second resolution by taking full advantage of two different peripherial I/O devices in the PIC16C74: the 16-bit ‘input capture’ module and the external interrupt facility