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參數(shù)資料
| 型號： | PIC18F4221T-I/ML |
| 廠商： | Microchip Technology |
| 文件頁數(shù)： | 70/110頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC PIC MCU FLASH 2KX16 44QFN |
| 標準包裝： | 1,600 |
| 系列： | PIC® 18F |
| 核心處理器： | PIC |
| 芯體尺寸： | 8-位 |
| 速度： | 40MHz |
| 連通性： | I²C，SPI，UART/USART |
| 外圍設備： | 欠壓檢測/復位，HLVD，POR，PWM，WDT |
| 輸入/輸出數(shù)： | 36 |
| 程序存儲器容量： | 4KB（2K x 16） |
| 程序存儲器類型： | 閃存 |
| EEPROM 大?。?/td> | 256 x 8 |
| RAM 容量： | 512 x 8 |
| 電壓 - 電源 (Vcc/Vdd)： | 4.2 V ~ 5.5 V |
| 數(shù)據(jù)轉(zhuǎn)換器： | A/D 13x10b |
| 振蕩器型： | 內(nèi)部 |
| 工作溫度： | -40°C ~ 85°C |
| 封裝/外殼： | 44-VQFN 裸露焊盤 |
| 包裝： | 帶卷 (TR) |
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PIC18F2221/2321/4221/4321 FAMILY
DS39689F-page 62
2009 Microchip Technology Inc.
6.1.2.4
Stack Full and Underflow Resets
Device Resets on stack overflow and stack underflow
conditions are enabled by setting the STVREN bit in
Configuration Register 4L. When STVREN is set, a full
or underflow will set the appropriate STKFUL or
STKUNF bit and then cause a device Reset. When
STVREN is cleared, a full or underflow condition will set
the appropriate STKFUL or STKUNF bit but not cause
a device Reset. The STKFUL or STKUNF bits are
cleared by the user software or a Power-on Reset.
6.1.3
FAST REGISTER STACK
A Fast Register Stack is provided for the STATUS,
WREG and BSR registers, to provide a “fast return”
option for interrupts. The stack for each register is only
one level deep and is neither readable nor writable. It is
loaded with the current value of the corresponding
register when the processor vectors for an interrupt. All
interrupt sources will push values into the stack regis-
ters. The values in the registers are then loaded back
into their associated registers if the RETFIE, FAST
instruction is used to return from the interrupt.
If both low and high-priority interrupts are enabled, the
stack registers cannot be used reliably to return from
low-priority interrupts. If a high-priority interrupt occurs
while servicing a low-priority interrupt, the Stack regis-
ter values stored by the low-priority interrupt will be
overwritten. In these cases, users must save the key
registers in software during a low-priority interrupt.
If interrupt priority is not used, all interrupts may use the
Fast Register Stack for returns from interrupt. If no
interrupts are used, the Fast Register Stack can be
used to restore the STATUS, WREG and BSR registers
at the end of a subroutine call. To use the Fast Register
Stack for a subroutine call, a CALL label, FAST
instruction must be executed to save the STATUS,
WREG and BSR registers to the Fast Register Stack. A
RETURN, FAST instruction is then executed to restore
these registers from the Fast Register Stack.
Example 6-1 shows a source code example that uses
the Fast Register Stack during a subroutine call and
return.
EXAMPLE 6-1:
FAST REGISTER STACK
CODE EXAMPLE
6.1.4
LOOK-UP TABLES IN PROGRAM
MEMORY
There may be programming situations that require the
creation of data structures, or look-up tables, in
program memory. For PIC18 devices, look-up tables
can be implemented in two ways:
Computed GOTO
Table Reads
6.1.4.1
Computed GOTO
A computed GOTO is accomplished by adding an offset
to the program counter. An example is shown in
A look-up table can be formed with an ADDWF PCL
instruction and a group of RETLW nn instructions. The
W register is loaded with an offset into the table before
executing a call to that table. The first instruction of the
called routine is the ADDWF PCL instruction. The next
instruction executed will be one of the RETLW nn
instructions that returns the value ‘nn’ to the calling
function.
The offset value (in WREG) specifies the number of
bytes that the program counter should advance and
should be multiples of 2 (LSb = 0).
In this method, only one data byte may be stored in
each instruction location and room on the return
address stack is required.
EXAMPLE 6-2:
COMPUTED GOTO USING
AN OFFSET VALUE
6.1.4.2
Table Reads and Table Writes
A better method of storing data in program memory
allows two bytes of data to be stored in each instruction
location.
Look-up table data may be stored two bytes per
program word by using table reads and writes. The
Table Pointer (TBLPTR) register specifies the byte
address and the Table Latch (TABLAT) register
contains the data that is read from or written to program
memory. Data is transferred to or from program
memory one byte at a time.
Table read and table write operations are discussed
further in Section 7.1 “Table Reads and Table
CALL SUB1, FAST
;STATUS, WREG, BSR
;SAVED IN FAST REGISTER
;STACK
SUB1
RETURN, FAST ;RESTORE VALUES SAVED
;IN FAST REGISTER STACK
MOVF
OFFSET, W
CALL
TABLE
ORG
nn00h
TABLE
ADDWF
PCL
RETLW
nnh
RETLW
nnh
RETLW
nnh
.
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