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ZL50062GAC Datasheet(PDF) 23 Page - Zarlink Semiconductor Inc |
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ZL50062GAC Datasheet(HTML) 23 Page - Zarlink Semiconductor Inc |
23 / 68 page ZL50062/4 Data Sheet 23 Zarlink Semiconductor Inc. by jitter. There are, however, some cases where data experience more delay than the timing signals. A common example is when multiple data lines are tied together to form bidirectional buses. The large bus loading may cause data to be delayed. If this is the case, the optimum sampling point may be 3/4 or 4/4 instead of 1/2. The optimum sampling point is dependent on the application. The user should optimize the sampling point to achieve the best jitter tolerance performance. 2.5 Input Clock Jitter Tolerance Input clock jitter tolerance depends on the data rate. In general, the higher the data rate, the smaller the jitter tolerance is, because the period of a bit cell is shorter, and the sampling point variation allowance is smaller. Jitter tolerance can not be accurately represented by just one number. Jitter of the same amplitude but different frequency spectrum can have different effect on the operation of a device. For example, a device that can tolerate 20ns of jitter of 10kHz frequency may only be able to tolerate 10ns of jitter of 1MHz frequency. Therefore, jitter tolerance should be represented as a spectrum over frequency. The highest possible jitter frequency is half of the carrier frequency. In the case of the ZL50062/64, the input clock is 8.192MHz, and the jitter associated with this clock can have the highest frequency component at 4.096MHz. For the above reasons, jitter tolerance of the ZL50062/64 has been characterized at 16.384Mbps. The lower data rates (2.048Mbps, 4.096Mbps, 8.192Mbps) will have the same or better tolerance than that of the 16.384Mbps operation. Tolerance of jitter of different frequencies are shown in the “AC Electrical Characteristics“ section, table “Input Clock Jitter Tolerance“ on page 62. The Jitter Tolerance Improvement Circuit was enabled (Control Register, bit FBDEN set HIGH, and bits FBD_MODE[2:0] set to 111B), and the sampling point was optimized. 3.0 Input and Output Offset Programming Various registers are used to control the input sampling point (delay) and the output advancement for the Local and Backplane streams. The following sections explain the details of these offset programming features. 3.1 Input Offsets Control of the Input Bit Delay allows each input stream to have a different frame boundary with respect to the master frame pulse, FP8i. Each input stream can be individually delayed by up to 7 3/4 bits with a resolution of 1/4 bit of the bit period. 3.1.1 Input Bit Delay Programming (Backplane and Local Input Streams) Input Bit Delay Registers LIDR0-31 and BIDR0-31 work in conjunction with the SMPL_MODE bit in the Control Register to allow users to control input bit fractional delay as well as input bit sample point selection for greater flexibility when designing switch matrices for high speed operation. When SMPL_MODE = LOW (input bit fractional delay mode), bits LID[4:0] and BID[4:0] in the LIDR0-31 and BIDR0-31 registers respectively define the input bit fractional delay of the corresponding local and backplane stream. The total delay can be up to 7 3/4 bits with a resolution of 1/4 bit at the selected data rate. When SMPL_MODE = HIGH (sampling point select mode), bits LID[1:0] and BID[1:0] define the input bit sampling point of the stream. The sampling point can be programmed at the 3/4, 4/4, 1/4 or 2/4 bit location to allow better tolerance for input jitter. Bits LID[4:2] and BID[4:2] define the integer input bit delay, with a maximum value of 7 bits at a resolution of 1 bit. Refer to Figure 9 and Figure 10 for Input Bit Delay Timing at 16Mbps and 8Mbps data rates, respectively. Refer to Figure 10 for Input Sampling Point Selection Timing at 8Mbps data rates. |
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