1、嵌入式系统硬件设计嵌入式系统硬件设计SI基础基础前言 现代电路设计不断朝高速、高密度、低电压、大电流趋势发展，信号完整性（Signal Integrity，SI）、电源完整性（Power Integrity，PI）和电磁兼容（Electromagnetic Compatibility，EMC）问题日益突出。传统设计方法显得力不从心，需综合三者间相互影响进行协同设计.SI挑战n SI问题是电路高速化的产物，电路高速化指电路中信号的有效带宽增加，主要表现在系统工作频率提高和数字信号边沿率加快两方面。高速电路常见的SI问题包括：反射（Reflection）、串扰（Crosstalk）、同时开关噪声（
2、SSN，Simultaneous Switching Noise）、时序错位与抖动（Timing Skew&Jitter）等SI挑战l2outout10.0n10.0n20.0n30.0ntime sec-1.0-0.50.00.51.01.5Y Axis Title VoltA digital signalout0.0n10.0n20.0n30.0ntime sec-1.01.03.05.06.0Voltage Volt Frequency 50MHz Risetime 1ns(10%-90%)Falltime 1ns(10%-90%)Bandwidth 350MHzSpectrum of
3、a single pulseFFT(V(1)0.0M50.0M100.0M150.0MFREQ Hz0.00.10.20.30.4Y Axis Title VoltFFT(V(1)0.0M50.0M100.0M150.0M200.0MFREQ Hz0.00.10.20.30.4Aplitude Volt10 ns risetime1 ns risetimeSpectrum of a pulse trainFFT(V(1)0.0G0.2G0.4G0.6G0.8G1.0GFREQ Hz0.00.51.01.5Amplitude VoltFFT(V(1)0.0G0.2G0.4G0.6G0.8G1.0
4、GFREQ Hz0.00.51.01.5Y Axis Title VoltThe effect of missing harmonics1st harmonic only1st and 3rd harmonic only1st to 5th harmonic1st to 21st harmonicn反射Signal integrity(SI)issueInterconnect+-I(z,t)I(z,t)VGZGZLTransmission lineLoadThevenin generatorz0LvztVZvztVZtzIvztVvztVtzVCC11,nThe voltage and cur
5、rent on each point of the line is superposition of a forward and backward voltage,travelling in opposite directions.11lcvclZCnThe voltage at each point of the line depends on the reflection coefficient at each line terminals:CLCLLZZZZvLtVvLtVnTransient behavior of voltage at each line terminals:LtLV
6、vLtVvLtVtLzV1,CGCGGZZZZtVtV GtVtVtVtzV1,0,0At generator side(input):At generator side(input):Complex transient behavior related to the reflection coefficient on each extremities and transmission line discontinuities 反射解决方法n反射问题主要通过端接匹配解决，使信号在传输过程中感觉不到瞬态阻抗突变。n抖动-实际信号与理想信号间的时序波动，由系统内在噪声和外在噪声共同引起。串扰n串扰
7、-串扰是指多信号网络间边缘场耦合效应导致的噪声干扰现象。串扰n 高速电路中需要控制串扰的布线网络主要包括：高速并行总线、高速时钟总线、高速接插件等。常用的减小串扰措施包括：减小平行走线长度、增大线间距、采用平面作返回路径、减小互连特性阻抗以及采用防护布线等。Basic transmission lineDriverTransmission lineTermination resistorThe four describing parametersnR Resistance per unit length ohm/mnCCapacitance F/m nLInductance H/mnG Con
8、ductance S/m Impedance(Z0)nImpedance is measured in OhmsnImpedance describes the AC resistance a driver will see when driving a signal into a indefinitely long transmission line.Z0=R+L j G+C j For a loss less line this simplifies to:Z0=LCohm ohmLoss(a)nLoss a is measured in dB/mnLoss is the reductio
9、n of signal voltage along a line due to resistance and leakagenThe resistive losses is due to resistance in conductor and ground plane.nThe dielectric losses is due to the energy needed to change polarization of the dielectric material.nThe radiation losses is the energy sent from the conductor acti
10、ng as an antenna.Resistance at higher frequenciesWhen the conductor is much thicker than the skin depth one have to substitute the skin depth for the thickness in the resistance formula:ohmThe resistance has now become frequency dependent!R=1 d dsR=1 1 f Skin depth(ds)At higher frequencies only a th
11、in layer of the conductor transport the current.The thickness where the current density is reduced to 1/e is called the skin depth ds.d ds=1 fm Skin depth(ds)人有了知识，就会具备各种分析能力，明辨是非的能力。所以我们要勤恳读书，广泛阅读，古人说“书中自有黄金屋。”通过阅读科技书籍，我们能丰富知识，培养逻辑思维能力；通过阅读文学作品，我们能提高文学鉴赏水平，培养文学情趣；通过阅读报刊，我们能增长见识，扩大自己的知识面。有许多书籍还能培养我们的道德情操，给我们巨大的精神力量，鼓舞我们前进。
