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SL28773ELCT Datasheet(PDF) 10 Page - Silicon Laboratories |
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SL28773ELCT Datasheet(HTML) 10 Page - Silicon Laboratories |
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10 / 22 page  SL28773 ......................Document #: 001-08400 Rev ** Page 10 of 21 . . The SL28773 requires a Parallel Resonance Crystal. Substi- tuting a series resonance crystal causes the SL28773 to operate at the wrong frequency and violates the ppm specifi- cation. For most applications there is a 300-ppm frequency shift between series and parallel crystals due to incorrect loading. Crystal Loading Crystal loading plays a critical role in achieving low ppm perfor- mance. To realize low ppm performance, use the total capac- itance the crystal sees to calculate the appropriate capacitive loading (CL). Figure 1 shows a typical crystal configuration using the two trim capacitors. It is important that the trim capacitors are in series with the crystal. It is not true that load capacitors are in parallel with the crystal and are approximately equal to the load capacitance of the crystal. Calculating Load Capacitors In addition to the standard external trim capacitors, consider the trace capacitance and pin capacitance to calculate the crystal loading correctly. Again, the capacitance on each side is in series with the crystal. The total capacitance on both side is twice the specified crystal load capacitance (CL). Trim capacitors are calculated to provide equal capacitive loading on both sides. 1 0 0 0 27M_NSS 27M_SS -1.75% 1 0 0 1 27M_NSS 27M_SS +/-0.5% 1 0 1 0 27M_NSS 27M_SS +/-0.75% 10 11 N/A N/A N/A 11 00 N/A N/A N/A 11 01 N/A N/A N/A 11 10 N/A N/A N/A 11 11 N/A N/A N/A B1b4 B1b3 B1b2 B1b1 Pin7 Pin 8 Spread (%) Table 5. Output Driver Status during CPU_STP# CPU_STP# Asserted SMBus OE Disabled Single-ended Clocks Stoppable Running Driven low Non stoppable Running Differential Clocks Stoppable Clock driven high Clock driven low Clock# driven low Non stoppable Running Table 6. Output Driver Status All Single-ended Clocks All Differential Clocks w/o Strap w/ Strap Clock Clock# PD# = 0 (Power down) Low Hi-z Low Low Table 7. Crystal Recommendations Frequency (Fund) Cut Loading Load Cap Drive (max.) Shunt Cap (max.) Motional (max.) Tolerance (max.) Stability (max.) Aging (max.) 14.31818 MHz AT Parallel 20 pF 0.1 mW 5 pF 0.016 pF 35 ppm 30 ppm 5 ppm Figure 1. Crystal Capacitive Clarification |
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