边沿触发器亚稳态现象的实验观测

Experimental Observation of Metastable Phenomena of Edge Triggered Flip Flops

摘要:

在跨时钟域数据传输中,常因违反触发器的建立时间和保持时间要求而产生亚稳态现象,导致数据误码。为对亚稳态发生过程进行直观观测,利用所搭建的亚稳态观测平台,在建立时间区域对TTL维持阻塞型集成边沿触发器74LS74芯片和CMOS传输门型集成边沿触发器74HC74芯片分别进行了误码测试。测试结果表明,两芯片性能接近,在建立时间0.9～1.6ns区间存在0%～100%误码过渡带,能观测亚稳态过程的直观波形。测试过程中,发现触发器输入电平上跳变时,误码率在以上区间存在稳定的单调变化曲线。当触发器输入电平下跳变时,误码率会从0%瞬变到100%,实验未观测到过渡带。输入下跳变时,当延迟参量单向递增或递减时,瞬变区域不一样,存在回差现象。

Abstract:

In cross-clock data transmission, metastable phenomena often occur due to violation of the setup time and hold time of flip-flops, resulting in bit error. In order to observe the metastable process directly, it uses the metastable observation platform to test the bit error of TTL block holding integrated edge flip-flop 74LS74 chip and CMOS transmission gate integrated edge flip-flop 74HC74 chip in setup time domain. Test results show that the performance of the two chips is close, and there is a 0%~100% BER transition band in the setup time of 0.9~1.6ns, in which the visual waveform of metastable process can be observed. It is found that there is a stable monotonic variation curve of BER in the above range when the input level of trigger jumps up. When the input level of the flip-flop jumps down, the BER changes from 0% to 100% instantaneously, and the transition zone cannot be observed. When the input jumps down and the delay parameter increases or decreases in one direction, the transient region will be different and presents a backlash phenomenon.