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MC12429 Datasheet(PDF) 5 Page - Motorola, Inc |
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MC12429 Datasheet(HTML) 5 Page - Motorola, Inc |
5 / 12 page MC12429 TIMING SOLUTIONS 5 MOTOROLA Let N = 2, M = 133.3333 ÷ 1.0416 x 2 = 256 The value for M falls within the constraints set for PLL stability, therefore N[1:0] = 01 and M[8:0} = 10000000. If the value for M fell outside of the valid range a different N value would be selected to try to move M in the appropriate direction. The M and N counters can be loaded either through a parallel or serial interface. The parallel interface is controlled via the P_LOAD signal such that a LOW to HIGH transition will latch the information present on the M[8:0] and N[1:0] inputs into the M and N counters. When the P_LOAD signal is LOW the input latches will be transparent and any changes on the M[8:0] and N[1:0] inputs will affect the FOUT output pair. To use the serial port the S_CLOCK signal samples the information on the S_DATA line and loads it into a 14 bit shift register. Note that the P_LOAD signal must be HIGH for the serial load operation to function. The Test register is loaded with the first three bits, the N register with the next two and the M register with the final eight bits of the data stream on the S_DATA input. For each register the most significant bit is loaded first (T2, N1 and M8). A pulse on the S_LOAD pin after the shift register is fully loaded will transfer the divide values into the counters. The HIGH to LOW transition on the S_LOAD input will latch the new divide values into the counters. Figure 4 illustrates the timing diagram for both a parallel and a serial load of the MC12429 synthesizer. M[8:0] and N[1:0] are normally specified once at power–up through the parallel interface, and then possibly again through the serial interface. This approach allows the application to come up at one frequency and then change or fine–tune the clock as the ability to control the serial interface becomes available. The TEST output provides visibility for one of the several internal nodes as determined by the T[2:0] bits in the serial configuration stream. It is not configurable through the parallel interface. Although it is possible to select the node that represents FOUT, the CMOS output may not be able to toggle fast enough for some of the higher output frequencies. The T2, T1 and T0 control bits are preset to ‘000’ when P_LOAD is LOW so that the PECL FOUT outputs are as jitter–free as possible. Any active signal on the TEST output pin will have detrimental affects on the jitter of the PECL output pair. In normal operations, jitter specifications are only guaranteed if the TEST output is static. The serial configuration port can be used to select one of the alternate functions for this pin. Most of the signals available on the TEST output pin are useful only for performance verification of the MC12429 itself. However the PLL bypass mode may be of interest at the board level for functional debug. When T[2:0] is set to 110 the MC12429 is placed in PLL bypass mode. In this mode the S_CLOCK input is fed directly into the M and N dividers. The N divider drives the FOUT differential pair and the M counter drives the TEST output pin. In this mode the S_CLOCK input could be used for low speed board level functional test or debug. Bypassing the PLL and driving FOUT directly gives the user more control on the test clocks sent through the clock tree. Figure 5 shows the functional setup of the PLL bypass mode. Because the S_CLOCK is a CMOS level the input frequency is limited to 250MHz or less. This means the fastest the FOUT pin can be toggled via the S_CLOCK is 125MHz as the minimum divide ratio of the N counter is 2. Note that the M counter output on the TEST output will not be a 50% duty cycle due to the way the divider is implemented. T2 T1 T0 TEST (Pin 20) 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 SHIFT REGISTER OUT HIGH FREF M COUNTER OUT FOUT LOW MCNT FOUT/4 Figure 4. Timing Diagram S_CLOCK S_DATA S_LOAD M[8:0] N[1:0] P_LOAD T2 T1 T0 N1 N0 M8 M7 M6 M5 M4 M3 M2 M1 M0 M, N First Bit Last Bit |
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