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DVR 数据表(PDF) 33 Page - Microchip Technology |
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DVR 数据表(HTML) 33 Page - Microchip Technology |
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33 / 416 page  2016-2018 Microchip Technology Inc. DS30010118D-page 33 PIC24FJ256GA705 FAMILY 2.6 External Oscillator Pins Many microcontrollers have options for at least two oscillators: a high-frequency Primary Oscillator and a low-frequency Secondary Oscillator (refer to Section 9.0 “Oscillator Configuration” for details). The oscillator circuit should be placed on the same side of the board as the device. Place the oscillator circuit close to the respective oscillator pins with no more than 0.5 inch (12 mm) between the circuit components and the pins. The load capacitors should be placed next to the oscillator itself, on the same side of the board. Use a grounded copper pour around the oscillator circuit to isolate it from surrounding circuits. The grounded copper pour should be routed directly to the MCU ground. Do not run any signal traces or power traces inside the ground pour. Also, if using a two-sided board, avoid any traces on the other side of the board where the crystal is placed. Layout suggestions are shown in Figure 2-5. In-line packages may be handled with a single-sided layout that completely encompasses the oscillator pins. With fine-pitch packages, it is not always possible to com- pletely surround the pins and components. A suitable solution is to tie the broken guard sections to a mirrored ground layer. In all cases, the guard trace(s) must be returned to ground. In planning the application’s routing and I/O assign- ments, ensure that adjacent port pins, and other signals in close proximity to the oscillator, are benign (i.e., free of high frequencies, short rise and fall times, and other similar noise). For additional information and design guidance on oscillator circuits, please refer to these Microchip Application Notes, available at the corporate website (www.microchip.com): • AN943, “Practical PICmicro® Oscillator Analysis and Design” • AN949, “Making Your Oscillator Work” • AN1798, “Crystal Selection for Low-Power Secondary Oscillator” FIGURE 2-5: SUGGESTED PLACEMENT OF THE OSCILLATOR CIRCUIT GND ` ` ` OSCI OSCO SOSCO SOSC I Copper Pour Primary Oscillator Crystal Secondary Crystal DEVICE PINS Primary Oscillator C1 C2 Sec Oscillator: C1 Sec Oscillator: C2 (tied to ground) GND OSCO OSCI Bottom Layer Copper Pour Oscillator Crystal Top Layer Copper Pour C2 C1 DEVICE PINS (tied to ground) (tied to ground) Single-Sided and In-Line Layouts: Fine-Pitch (Dual-Sided) Layouts: Oscillator |
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