UI: Unit Interval比特数据的持续时间 UI = 1 / 数据速率 以PCI2.0为例：
PCIe2.0数据速率为5Gbps，UI = 1 / 5Gbps = 200ps 按规范定义UI=199.94~200.0ps：
UI有300ppm所以不考虑SSC情况下UI规格计算：
UI = Nominal UI ± 300ppm = 200 ± 3000.000001200 = 200 ± 0.06 = 199.94 ~ 200.06ps 5.0015 Gb/s = Nominal UI 300 ppm 4.9985 Gb/s = Nominal UI - 300 ppm 考虑SSC打开时： 规范中：SSC permits a 0, - 5000 ppm modulation of the clock frequency at a modulation rate not to exceed 33 kHz. 考虑SSC需计算-5000ppm~0的裕值 则Fmin = Nominal UI - 300ppm - 5000ppm(SSC budget) = 5Gbps - 300 * 0.000001 * 5Gbps - 5000 * 0.000001 * 5Gbps = 4.9735ps UImax = 1/4.9735 = 201.06ps UImin = UI nomal 300ppm 0 =199.94ps UI 规格为199.94 ~ 201.06ps
或者可以这样计算： UImax = 200ps 300ppm * 200ps 5000ppm * 200ps = 201.06ps UImin = 200ps - 300ppm * 200ps -0 = 199.94ps
PCIe1.0（规范：PCI_Express_Electrical_Test_Consideration1.添加链接描述： ? Data rate must be within 2.5 Gb/s 300ppm/-5300ppm. This data rate does account for SSC dictated variations. Fmax = 2.50075 Gb/s, Fmin = 2.48675 Gb/s.
- 2.50075 Gb/s = Nominal UI 300 ppm, upper frequency limit.
- 2.48675 Gb/s = Nominal UI – 5000 ppm (SSC Budget) – 300 ppm, lower frequency limit. ? SSC modulation rate must be within 30-33Khz. UI 规格为 399.88 ~ 402.13ps
(2022年2月21日更新)