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CY28412OXCT Datasheet(PDF) 8 Page - Cypress Semiconductor |
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CY28412OXCT Datasheet(HTML) 8 Page - Cypress Semiconductor |
8 / 17 page PRELIMINARY CY28412 Document #: 38-07612 Rev. *C Page 8 of 17 Calculating Load Capacitors In addition to the standard external trim capacitors, trace capacitance and pin capacitance must also be considered to correctly calculate crystal loading. As mentioned previously, the capacitance on each side of the crystal is in series with the crystal. This means the total capacitance on each side of the crystal must be twice the specified crystal load capacitance (CL). While the capacitance on each side of the crystal is in series with the crystal, trim capacitors (Ce1,Ce2) should be calculated to provide equal capacitive loading on both sides. As mentioned previously, the capacitance on each side of the crystal is in series with the crystal. This mean the total capac- itance on each side of the crystal must be 2 times the specified load capacitance (CL). While the capacitance on each side of the crystal is in series with the crystal, trim capacitors (Ce1,Ce2) should be calculated to provide equal capacitance loading on both sides. Use the following formulas to calculate the trim capacitor values for Ce1 and Ce2. CL....................................................Crystal load capacitance CLe......................................... Actual loading seen by crystal using standard value trim capacitors Ce..................................................... External trim capacitors Cs ..............................................Stray capacitance (terraced) Ci .......................................................... Internal capacitance (lead frame, bond wires etc.) PD (Power-down) Clarification The VTT_PWRGD# /PD pin is a dual function pin. During initial power-up, the pin functions as VTT_PWRGD#. Once VTT_PWRGD# has been sampled low by the clock chip, the pin assumes PD functionality. The PD pin is an asynchronous active high input used to shut off all clocks cleanly prior to shutting off power to the device. This signal is synchronized internal to the device prior to powering down the clock synthe- sizer. PD is also an asynchronous input for powering up the system. When PD is asserted high, all clocks are driven to a low value and held prior to turning off the VCOs and the crystal oscillator. PD (Power-down) – Assertion When PD is sampled high by two consecutive rising edges of CPUC, all single-ended outputs will be held low on their next high to low transition and differential clocks must be held high or Hi-Z (depending on the state of the control register drive mode bit) on the next diff clock# high to low transition. When the SMBus PD drive mode bit corresponding to the differential Figure 1. Crystal Capacitive Clarification XTAL Ce2 Ce1 Cs1 Cs2 X1 X2 Ci1 Ci2 Clock Chip Trace 2.8pF Trim 33pF Pin 3 to 6p Figure 2. Crystal Loading Example Load Capacitance (each side) Total Capacitance (as seen by the crystal) Ce = 2 * CL – (Cs + Ci) Ce1 + Cs1 + Ci1 1 + Ce2 + Cs2 + Ci2 1 ( ) 1 = CLe [+] Feedback |
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