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ADAU1761BCPZ-R7 数据表(PDF) 25 Page - Analog Devices |
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ADAU1761BCPZ-R7 数据表(HTML) 25 Page - Analog Devices |
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25 / 93 page  ADAU1761 Rev. C | Page 24 of 92 STARTUP, INITIALIZATION, AND POWER This section describes the procedure for properly starting up the ADAU1761. The following sequence provides a high level approach to the proper initiation of the system. 1. Apply power to the ADAU1761. 2. Lock the PLL to the input clock (if using the PLL). 3. Enable the core clock. 4. Load the register settings. See the Startup section for more information about the proper start-up sequence. POWER-UP SEQUENCE The ADAU1761 uses a power-on reset (POR) circuit to reset the registers upon power-up. The POR monitors the DVDDOUT pin and generates a reset signal whenever power is applied to the chip. During the reset, the ADAU1761 is set to the default values documented in the register map (see the Control Registers section). Typically, with a 10 μF capacitor on AVDD, the POR takes approximately 14 ms. AVDD POR PART READY POR ACTIVE POR ACTIVE DVDDOUT 1.35V 0.95V 1.5V POR FINISHED Figure 29. Power-On Reset Sequence The PLL lock time is dependent on the MCLK rate. Typical lock times are provided in Table 11. The DSP can be enabled immediately after the PLL is locked. Table 11. PLL Lock Times PLL Mode MCLK Frequency Lock Time (Typical) Fractional 8 MHz 3.5 ms Fractional 12 MHz 3.0 ms Integer 12.288 MHz 2.96 ms Fractional 13 MHz 2.4 ms Fractional 14.4 MHz 2.4 ms Fractional 19.2 MHz 2.98 ms Fractional 19.68 MHz 2.98 ms Fractional 19.8 MHz 2.98 ms Fractional 24 MHz 2.95 ms Integer 24.576 MHz 2.96 ms Fractional 26 MHz 2.4 ms Fractional 27 MHz 2.4 ms POWER REDUCTION MODES Sections of the ADAU1761 chip can be turned on and off as needed to reduce power consumption. These include the ADCs, the DACs, the PLL, and the DSP core. In addition, the control registers can be used to configure some functions for power saving, normal, or enhanced performance operation. See the Control Registers section for more information. The digital filters of the ADCs and DACs can each be set to over- sampling ratios of 64× or 128× (default). Setting the oversampling ratios to 64× for these filters lowers power consumption with a minimal impact on performance. See the Digital Filters section for specifications; see the Typical Performance Characteristics section for graphs of these filters. DIGITAL POWER SUPPLY The digital power supply for the ADAU1761 is generated from an internal regulator. This regulator generates a 1.5 V supply internally. The only external connection to this regulator is the DVDDOUT bypassing point. A 100 nF capacitor and a 10 μF capacitor should be connected between this pin and DGND. INPUT/OUTPUT POWER SUPPLY The power for the digital output pins is supplied from IOVDD, and this pin also sets the highest input voltage that should be seen on the digital input pins. IOVDD should be set between 1.8 V and 3.3 V; no digital input signal should be at a voltage level higher than the one on IOVDD. The current draw of this pin is variable because it depends on the loads of the digital outputs. IOVDD should be decoupled to DGND with a 100 nF capacitor and a 10 μF capacitor. CLOCK GENERATION AND MANAGEMENT The ADAU1761 uses a flexible clocking scheme that enables the use of many different input clock rates. The PLL can be bypassed or used, resulting in two different approaches to clock manage- ment. For more information about clocking schemes, PLL configuration, and sampling rates, see the Clocking and Sampling Rates section. Case 1: PLL Is Bypassed If the PLL is bypassed, the core clock is derived directly from the MCLK input. The rate of this clock must be set properly in Register R0 (clock control register, Address 0x4000) using the INFREQ[1:0] bits. When the PLL is bypassed, supported external clock rates are 256 × fS, 512 × fS, 768 × fS, and 1024 × fS, where fS is the base sampling rate. The core clock of the chip is off until the core clock enable bit (COREN) is asserted. If a clock slower than 1024 × fS is directly input to the ADAU1761 (bypassing the PLL), the number of available SigmaDSP processing cycles is reduced and the DSPSR bits in Register R57 (Address 0x40EB) should be adjusted accordingly. |
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