Influence of the gate location on the ring oscillator frequency

W artykule przedstawiono wyniki analizy zmian częstotliwości oscylatora pierścieniowego, uzyskane poprzez zmianę miejsca lokowania pojedynczych bramek w różnych częściach struktury reprogramowalnej. Przeprowadzono symulacje komputerowe oraz wykonano badania eksperymentalne układu oscylatora pierścieniowego złożonego z N inwerterów, dla kilku wariantów rozmieszczenia bramek i wykorzystania linii połączeniowych pomiędzy obszarami CLBs. Układ oscylatora implementowano w strukturze FPGA (Spartan-3).This paper presents the results of investigations how the inverter location in the area of a reconfigurable FPGA chip influence the properties of a ring oscillator. Ring oscillators are very often implemented in FPGA structures, even in the very advanced projects. They are used both as a single element or an array of sensors for measuring the chip temperature and thermal verification on reconfigurable systems [1, 2], as well as for measuring the propagation delay on the internal wires of the FPGA chip [3, 4]. In our investigation the ring oscillator composed of 11 inverters was implemented in the Spartan-3 structure (Fig. 1). There were performed simulations and experiments. We tested whether and how the location of the single inverter and the delay of lines influenced the ring oscillator frequency (Figs. 2 and 3). The properties of different connections between CLBs in the FPGA structure are described (Figs. 4 and 5). The ring oscillator was located in different areas of the chip to minimize or specially increase the length of lines between the inverters (Figs. 6, 7 and 8). The simulation and experiment results are presented in Tab. 1 and discussed. In conclusion we can state that when one wants to use a ring oscillator as a sensor and to analyze the frequency or delay times, there should be considered not only the influence of temperature or voltage supply of the chip core [8] but also the location of the sensor. In the case of an array of sensors, each ring oscillator should be analyzed and calibrated independently