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PIC18F4431-I/PT 参数 Datasheet PDF下载

PIC18F4431-I/PT图片预览
型号: PIC18F4431-I/PT
PDF下载: 下载PDF文件 查看货源
内容描述: 28 /40/ 44引脚增强型闪存微控制器采用纳瓦技术,高性能PWM和A / D [28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High-Performance PWM and A/D]
分类和应用: 闪存微控制器
文件页数/大小: 392 页 / 3127 K
品牌: MICROCHIP [ MICROCHIP ]
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PIC18F2331/2431/4331/4431  
2.2.2  
TANK CAPACITORS  
2.2  
Power Supply Pins  
On boards with power traces running longer than  
six inches in length, it is suggested to use a tank capac-  
itor for integrated circuits, including microcontrollers, to  
supply a local power source. The value of the tank  
capacitor should be determined based on the trace  
resistance that connects the power supply source to  
the device, and the maximum current drawn by the  
device in the application. In other words, select the tank  
capacitor so that it meets the acceptable voltage sag at  
the device. Typical values range from 4.7 F to 47 F.  
2.2.1  
DECOUPLING CAPACITORS  
The use of decoupling capacitors on every pair of  
power supply pins, such as VDD, VSS, AVDD and  
AVSS, is required.  
Consider the following criteria when using decoupling  
capacitors:  
Value and type of capacitor: A 0.1 F (100 nF),  
10-20V capacitor is recommended. The capacitor  
should be a low-ESR device, with a resonance  
frequency in the range of 200 MHz and higher.  
Ceramic capacitors are recommended.  
2.2.3  
CONSIDERATIONS WHEN USING  
BOR  
Placement on the printed circuit board: The  
decoupling capacitors should be placed as close  
to the pins as possible. It is recommended to  
place the capacitors on the same side of the  
board as the device. If space is constricted, the  
capacitor can be placed on another layer on the  
PCB using a via; however, ensure that the trace  
length from the pin to the capacitor is no greater  
than 0.25 inch (6 mm).  
When the Brown-out Reset (BOR) feature is enabled,  
a sudden change in VDD may result in a spontaneous  
BOR event. This can happen when the microcontroller  
is operating under normal operating conditions, regard-  
less of what the BOR set point has been programmed  
to, and even if VDD does not approach the set point.  
The precipitating factor in these BOR events is a rise or  
fall in VDD with a slew rate faster than 0.15V/s.  
An application that incorporates adequate decoupling  
between the power supplies will not experience such  
rapid voltage changes. Additionally, the use of an  
electrolytic tank capacitor across VDD and VSS, as  
described above, will be helpful in preventing high slew  
rate transitions.  
Handling high-frequency noise: If the board is  
experiencing high-frequency noise (upward of  
tens of MHz), add a second ceramic type capaci-  
tor in parallel to the above described decoupling  
capacitor. The value of the second capacitor can  
be in the range of 0.01 F to 0.001 F. Place this  
second capacitor next to each primary decoupling  
capacitor. In high-speed circuit designs, consider  
implementing a decade pair of capacitances as  
close to the power and ground pins as possible  
(e.g., 0.1 F in parallel with 0.001 F).  
Maximizing performance: On the board layout  
from the power supply circuit, run the power and  
return traces to the decoupling capacitors first,  
and then to the device pins. This ensures that the  
decoupling capacitors are first in the power chain.  
Equally important is to keep the trace length  
between the capacitor and the power pins to a  
minimum, thereby reducing PCB trace  
If the application has components that turn on or off,  
and share the same VDD circuit as the microcontroller,  
the BOR can be disabled in software by using the  
SBOREN bit before switching the component. After-  
wards, allow a small delay before re-enabling the BOR.  
By doing this, it is ensured that the BOR is disabled  
during the interval that might cause high slew rate  
changes of VDD.  
Note:  
Not all devices incorporate software BOR  
control. See Section 5.0 “Reset” for  
device-specific information.  
inductance.  
DS39616D-page 26  
2010 Microchip Technology Inc.  
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