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| 型号： | DM240002 |
| 厂商： | Microchip Technology |
| 文件页数： | 36/42页 |
| 文件大小： | 0K |
| 描述： | BOARD DEV EXPLORER 16 44-PIN |
| 标准包装： | 1 |
| 系列： | Explorer 16 44 引脚 |
| 类型： | MCU |
| 适用于相关产品： | dsPIC30，dsPIC33，PIC32，PIC24FJ，PIC24HJ |
| 所含物品： | Explorer 16 开发板，PIC24FJ64GA004 和 dsPIC33FJ32GP204 PIM 模块 |
| 产品目录页面： | 659 (CN2011-ZH PDF) |
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44-Pin Demo Board User’s Guide
3.2.12
Lesson 12: Look-up Table (ROM Array)
Lesson 8 introduced function calls. Lesson 12 shows how function calls and calculated
modification of the Program Counter may be used to implement a look-up table (see
Example 3-11).
It is sometimes useful to implement a table to convert from one value to another.
Expressed in a high-level language it might look like this:
y = function(x) ;
That is for every value of x , it returns the corresponding y value.
Look-up tables are a fast way to convert an input to meaningful data because the trans-
fer function is pre-calculated and “looked up” rather than calculated on the fly.
PIC microcontrollers implement these by directly modifying the Program Counter (PC).
An example might be a function that converts hexadecimal numbers to the ASCII
equivalent. Each digit’s individual nibble can be pulled out of the number and used as
the index to the look-up table. The index advances the PC to the appropriate RETLW
instruction to load WREG with the corresponding constant and returns to the calling
program.
EXAMPLE 3-11:
LOOK-UP TABLE
;Enter with index in Wreg
LookupTable
ADDWF
RETLW
RETLW
...
RETLW
PCL,f
'0'
'1'
'F'
;jump to
;index 0
;index 1
;index 15
If the table falls across a 256 byte program memory boundary, or if somehow the
look-up table is called with an out of bounds index value, it will jump to a location out of
the table.
Good programming practices dictate a few additional instructions. First, since the table
has only sixteen entries, make sure a number no larger than 16 is passed in. The sim-
plest way to do this is to logically AND the contents of WREG before modifying PCL:
ANDLW 0x0F . More complex error recovery schemes may be appropriate, depending
on the application.
In addition, there are some nuances to be aware of should the table cross a 256 word
boundary. The Program Counter is 13 bits wide, but only the lower 8 bits are repre-
sented in PCL (see Figure 3-12). The remaining 5 bits are stored in PCLATH. However,
an overflow of the lower 8 bits is not automatically carried over into PCLATH. Instead,
be sure to check for and handle that case in the code. See the PCL and PCLATH
section in the PIC16F882/883/884/886/887 Data Sheet (DS41291) for more details of
how PCLATH is used.
DS41296B-page 32
? 2007 Microchip Technology Inc.
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相关代理商/技术参数 |
参数描述 |
|---|---|
| DM240002 | 制造商:Microchip Technology Inc 功能描述:Explorer 16 kit(44-PIN) w/ PIC24FJ64GA00 |
| DM240011 | 功能描述:开发板和工具包 - PIC / DSPIC MPLAB Starter Kit for PIC24F MCU RoHS:否 制造商:Microchip Technology 产品:Starter Kits 工具用于评估:chipKIT 核心:Uno32 接口类型: 工作电源电压: |
| DM240012 | 功能描述:开发板和工具包 - PIC / DSPIC PIC24E USB Starter Kit RoHS:否 制造商:Microchip Technology 产品:Starter Kits 工具用于评估:chipKIT 核心:Uno32 接口类型: 工作电源电压: |
| DM240013-1 | 功能描述:开发板和工具包 - PIC / DSPIC Microstick for 3V PIC24F K-series RoHS:否 制造商:Microchip Technology 产品:Starter Kits 工具用于评估:chipKIT 核心:Uno32 接口类型: 工作电源电压: |
| DM240013-2 | 功能描述:开发板和工具包 - PIC / DSPIC RoHS:否 制造商:Microchip Technology 产品:Starter Kits 工具用于评估:chipKIT 核心:Uno32 接口类型: 工作电源电压: |
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