Częstościomierz z układem CPLD i mikroprocesorem AVR

摘要

In this paper a simple frequency meter based on CPLD and 8-bit microprocessor is presented. This is an example, where typical functions were divided into hardware and software parts (so-called Software Hardware Co-Design Structure). The hardware part (counters, registers, multiplexer) was implemented inside a programmable device XC9572XL - Xilinx (Fig. 1), the software part was made with an 8-bit AVR ATmega88 microprocessor (Atmel). Cooperation of the CPLD device and microprocessor gives a very flexible structure (Fig. 2) which can be easily modified both in hardware and software. An example of modification inside CPLD is shown in Fig. 3. In this case the other structure of a counter and register is proposed. There is one 28-bit counter instead of a cascade of decimal counters and the necessary changes can be easily reprogrammed both inside the CPLD structure and in the program code for a microprocessor, without any changes in connections between these devices. The frequency meter can work from 1 Hz to 99 999 999 Hz and presents results on an LCD display. The measurement accuracy depends mainly on the quality of the clock signal STROBE. This signal - for counters and registers - comes from a microprocessor and can be from different sources such as eg. a microprocessor internal clock or an external temperature compensated crystal oscillator (TCXO). The measurement accuracy depends on a kind of a source. This frequency meter is programmed by students, as one of the laboratory exercises.%W artykule przedstawiono układ częstościomierza, zrealizowanego w strukturze reprogramowalnej CPLD i 8-bitowym mikroprocesorze. Część zadań związana ze zliczaniem impulsów implementowana jest w układzie programowalnym (układy liczników, rejestrów, multipleksera), część zadań związana z obróbką danych i prezentacją wyniku, realizowana jest programowo w mikroprocesorze. Przedstawiono przykład zmiany koncepcji realizacji układów licznikowych, pozwalający na optymalne wykorzystanie zasobów obu układów.