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CY28416OXCT Datasheet(PDF) 8 Page - SpectraLinear Inc |
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CY28416OXCT Datasheet(HTML) 8 Page - SpectraLinear Inc |
8 / 14 page CY28416 Rev 1.0, November 22, 2006 Page 8 of 14 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 needs to be synchronized internal to the device prior to powering down the clock synthesizer. PD is also an asynchronous input for powering up the system. When PD is asserted HIGH, all clocks need to be 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 must be held LOW on their next HIGH-to-LOW transition and differential clocks must held high or tri-stated (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 (CPU, SRC, and DOT) clock output of interest is programmed to ‘0’, the clock output must be held with “Diff clock” pin driven high at 2 x Iref, and “Diff clock#” tri-state. If the control register PD drive mode bit corresponding to the output of interest is programmed to “1”, then both the “Diff clock” and the “Diff clock#” are tri-state. Note the example in Figure 3 shows CPUT = 133 MHz and PD drive mode = ‘1’ for all differential outputs. This diagram and description is appli- cable to valid CPU frequencies 100, 133, 166, 200, 266, 333, and 400 MHz. In the event that PD mode is desired as the initial power-on state, PD must be asserted high in less than 10 s after asserting VTT_PWRGD#. PD Deassertion The power-up latency needs to be less than 1.8 ms. This is the time from the deassertion of the PD pin or the ramping of the power supply until the time that stable clocks are output from the clock chip. All differential outputs stopped in a tri-state condition resulting from power down must be driven high in less than 300 s of PD deassertion to a voltage greater than 200 mV. After the clock chip’s internal PLL is powered up and locked, all outputs are to be enabled within a few clock cycles of each other. Figure 4 is an example showing the relationship of clocks coming up. Unfortunately, we can not show all possible combinations, designers need to insure that from the first active clock output to the last takes no more than two full PCI clock cycles. Figure 3. Power-down Assertion Timing Waveform PD USB, 48MHz DOT96T DOT96C SRCT 100MHz SRCC 100MHz CPUT, 133MHz PCI, 33 MHz REF CPUC, 133MHz |
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