用C51編寫單片機延時函數

參考了51單片機 Keil C 延時程序的簡單研究，自己也親身測試和計算了一些已有的延時函數。

這裡假定單片機是時鐘頻率為12MHz，則一個機器周期為：1us.
參考了51單片機 Keil C 延時程序的簡單研究後，我們可知道， 在Keil C中獲得最為準確的延時函數將是

voiddelay(unsignedchart)

{

while(--t);

}

反彙編代碼如下：

執行DJNZ指令需要2個機器周期，RET指令同樣需要2個機器周期，根據輸入t，在不計算調用delay()所需時間的情況下，具體時間延時如下：
tDelay Time (us)12×1+2 ＝422×2＋2＝6N2×N＋2＝2(N＋1)

當在main函數中調用delay(1)時， 進行反彙編如下：

調用delay()時，多執行了兩條指令，其中MOV R, #data需要1個機器周期，LJMP需要2個機器周期，即調用delay()需要3us.

Keil C仿真截圖與計算過程:



加上調用時間，準確的計算時間延時與Keil C仿真對比如下：(可見,仿真結果和計算結果是很接近的)
tDelay Time (us)仿真11.0592Mhz時鐘(us)13＋2×1+2 ＝7 | 7.7(實際)7.6023＋2×2＋2＝9 | 9.99.76N3＋2×N＋2＝2N＋5 | (2N+5)*1.1/311 | 12.111.941535 | 38.537.98100205 | 225.5222.44255515 | 566.5558.81
也就是說，這個延時函數的精度為2us，最小的時間延時為7us，最大的時間延時為3+255×2＋2＝515us.
實際中使用11.0592MHz的時鐘，這個延時函數的精度將為2.2us，最小時間延時為7.7us, 最大時間延時為566.5us.
這個時間延時函數，對於與DS18B20進行單總線通信，已經足夠準確了。

現在，我們將時鐘換成11.0592MHz這個實際用到的頻率，每個機器周期約為1.1us.
現在讓我們來分析一下這個之前用過的延時函數：

//延時函數,對於11.0592MHz時鐘,例i=10,則大概延時10ms.

voiddelayMs(unsignedinti)

{

unsignedintj;

while(i--)

{

for(j=0;j<125;j++);

}

}

它的反彙編代碼如下：

分析: T表示一個機器周期(調用時間相對於這個ms級的延時來說，可忽略不計)

1C:0000MOVA,R7;1T
2DECR7;1T低8位字節減1
3MOVR2,0x06;2T
4JNZC:0007;2T若低8位字節不為0,則跳到C:0007
5DECR6;1T低8位字節為0,則高8位字節減1
6C:0007ORLA,R2;1T
7JZC:001D;2T若高8位也減為0,則RET
8CLRA;1TA清零
9MOVR4,A;1TR4放高位
10MOVR5,A;1TR5放低位
11C:000DCLRC;1TC清零
12MOVA,R5;1T
13SUBBA,#0x7d;1TA=A-125
14MOVA,R4;1T
15SUBBA,#0x00;1TA
16JNCC:0000;2TA為零則跳到C:0000
17INCR5;1TR5增1
18CJNER5,#0x00,C:001B;2TR5>0,跳轉到C:000D
19INCR4;1T
20C:001BSJMPC:000D;2T
21C:001DRET

對於delayMs(1), 執行到第7行就跳到21行, 共需時12T, 即13.2us
對於delayMs(2), 需時9T＋13T＋124×10T＋7T＋12T ＝ 9T＋13T＋1240T＋7T＋12T ＝1281T ＝1409.1us.
對於delayMs(3), 需時9T×(3-1)＋(13T＋124×10T＋7T)×(3-1)＋12T
＝1269T×(3-1)＋12T＝2550T＝2805us.
對於delayMs(N),N>1, 需時1269T×(N－1)＋12T = 1269NT－1257T＝(1395.9N-1382.7)us.

利用Keil C仿真delayMs(1) = 0.00166558s = 1.67ms 截圖如下:




由分析可知具體的計算延時時間與Keil C仿真延時對比如下：
iTime Delay仿真延時113.2us1.67ms21409.1us3.31ms32805us4.96msN(1395.9N－1382.7)us1012.6ms16.50ms2026.5ms32.98ms3040.5ms49.46ms5068.4ms82.43ms100138.2ms164.84ms200277.8ms329.56ms500696.6ms824.13ms10001394.5ms1648.54ms15002092.5ms2472.34ms20002790.4ms3296.47ms55.6ms8.26ms73100.5ms120.34ms7201003.7ms = 1s1186.74ms

計算delayMs(10)得到延時時間為：12576.3us約等於12.6ms，接近我們認為的10ms。

本文引用地址：http://www.eepw.com.cn/article/201611/316096.htm計算結果和仿真結果只要delayMs(1)有很大出入, 其它都接近, 在接受範圍內.

經過以上分析，可見用C語言來做延時並不是不太準確，只是不容易做到非常準確而已，若有一句語句變了，延時時間很可能會不同，因為編譯程序生成的彙編指令很可能不同。


PS:
對於每條51單片機彙編指令的字長和所需機器周期匯總如下：轉自：http://bbs.mcustudy.com/printpage.asp?BoardID=2&ID=1454
Appendix E - 8051 Instruction Set

Arithmetic Operations

MnemonicDescriptionSizeCycles
ADD A,Rn Add register to Accumulator (ACC).11
ADD A,direct Add direct byte to ACC.21
ADD A,@Ri Add indirect RAM to ACC.11
ADD A,#data Add immediate data to ACC.21
ADDC A,Rn Add register to ACC with carry.11
ADDC A,direct Add direct byte to ACC with carry.21
ADDC A,@Ri Add indirect RAM to ACC with carry.11
ADDC A,#data Add immediate data to ACC with carry.21
SUBB A,Rn Subtract register from ACC with borrow.11
SUBB A,direct Subtract direct byte from ACC with borrow21
SUBB A,@Ri Subtract indirect RAM from ACC with borrow.11
SUBB A,#data Subtract immediate data from ACC with borrow.21
INC A Increment ACC.11
INC Rn Increment register.11
INC direct Increment direct byte.21
INC @Ri Increment indirect RAM.11
DEC A Decrement ACC.11
DEC Rn Decrement register.11
DEC direct Decrement direct byte.21
DEC @Ri Decrement indirect RAM.11
INC DPTR Increment data pointer.12
MUL AB Multiply A and B Result: A DIV AB Divide A by B Result: A DA A Decimal adjust ACC.11
Logical Operations

MnemonicDescriptionSizeCycles
ANL A,Rn AND Register to ACC.11
ANL A,direct AND direct byte to ACC.21
ANL A,@Ri AND indirect RAM to ACC.11
ANL A,#data AND immediate data to ACC.21
ANL direct,A AND ACC to direct byte.21
ANL direct,#data AND immediate data to direct byte.32
ORL A,Rn OR Register to ACC.11
ORL A,direct OR direct byte to ACC.21
ORL A,@Ri OR indirect RAM to ACC.11
ORL A,#data OR immediate data to ACC.21
ORL direct,A OR ACC to direct byte.21
ORL direct,#data OR immediate data to direct byte.32
XRL A,Rn Exclusive OR Register to ACC.11
XRL A,direct Exclusive OR direct byte to ACC.21
XRL A,@Ri Exclusive OR indirect RAM to ACC.11
XRL A,#data Exclusive OR immediate data to ACC.21
XRL direct,A Exclusive OR ACC to direct byte.21
XRL direct,#data XOR immediate data to direct byte.32
CLR A Clear ACC (set all bits to zero).11
CPL A Compliment ACC.11
RL A Rotate ACC left.11
RLC A Rotate ACC left through carry.11
RR A Rotate ACC right.11
RRC A Rotate ACC right through carry.11
SWAP A Swap nibbles within ACC.11
Data Transfer

MnemonicDescriptionSizeCycles
MOV A,Rn Move register to ACC.11
MOV A,direct Move direct byte to ACC.21
MOV A,@Ri Move indirect RAM to ACC.11
MOV A,#data Move immediate data to ACC.21
MOV Rn,A Move ACC to register.11
MOV Rn,direct Move direct byte to register.22
MOV Rn,#data Move immediate data to register.21
MOV direct,A Move ACC to direct byte.21
MOV direct,Rn Move register to direct byte.22
MOV direct,direct Move direct byte to direct byte.32
MOV direct,@Ri Move indirect RAM to direct byte.22
MOV direct,#data Move immediate data to direct byte.32
MOV @Ri,A Move ACC to indirect RAM.11
MOV @Ri,direct Move direct byte to indirect RAM.22
MOV @Ri,#data Move immediate data to indirect RAM.21
MOV DPTR,#data16 Move immediate 16 bit data to data pointer register.32
MOVC A,@A+DPTR Move code byte relative to DPTR to ACC (16 bit address).12
MOVC A,@A+PC Move code byte relative to PC to ACC (16 bit address).12
MOVX A,@Ri Move external RAM to ACC (8 bit address).12
MOVX A,@DPTR Move external RAM to ACC (16 bit address).12
MOVX @Ri,A Move ACC to external RAM (8 bit address).12
MOVX @DPTR,A Move ACC to external RAM (16 bit address).12
PUSH direct Push direct byte onto stack.22
POP direct Pop direct byte from stack.22
XCH A,Rn Exchange register with ACC.11
XCH A,direct Exchange direct byte with ACC.21
XCH A,@Ri Exchange indirect RAM with ACC.11
XCHD A,@Ri Exchange low order nibble of indirect RAM with low order nibble of ACC.11
Boolean Variable Manipulation

MnemonicDescriptionSizeCycles
CLR C Clear carry flag.11
CLR bit Clear direct bit.21
SETB C Set carry flag.11
SETB bit Set direct bit.21
CPL C Compliment carry flag.11
CPL bit Compliment direct bit.21
ANL C,bit AND direct bit to carry flag.22
ANL C,/bit AND compliment of direct bit to carry.22
ORL C,bit OR direct bit to carry flag.22
ORL C,/bit OR compliment of direct bit to carry.22
MOV C,bit Move direct bit to carry flag.21
MOV bit,C Move carry to direct bit.22
JC rel Jump if carry is set.22
JNC rel Jump if carry is not set.22
JB bit,rel Jump if direct bit is set.32
JNB bit,rel Jump if direct bit is not set.32
JBC bit,rel Jump if direct bit is set & clear bit.32
Program Branching

MnemonicDescriptionSizeCycles
ACALL addr11 Absolute subroutine call.22
LCALL addr16 Long subroutine call.32
RET Return from subroutine.12
RETI Return from interrupt.12
AJMP addr11 Absolute jump.22
LJMP addr16 Long jump.32
SJMP rel Short jump (relative address).22
JMP @A+DPTR Jump indirect relative to the DPTR.12
JZ rel Jump relative if ACC is zero.22
JNZ rel Jump relative if ACC is not zero.22
CJNE A,direct,rel Compare direct byte to ACC and jump if not equal.32
CJNE A,#data,rel Compare immediate byte to ACC and jump if not equal.32
CJNE Rn,#data,rel Compare immediate byte to register and jump if not equal.32
CJNE @Ri,#data,rel Compare immediate byte to indirect and jump if not equal.32
DJNZ Rn,rel Decrement register and jump if not zero.22
DJNZ direct,rel Decrement direct byte and jump if not zero.32
Other Instructions

MnemonicDescriptionSizeCycles
NOP No operation.11
其它可查看《單片機基礎》－李廣弟等編著，P70 「MCS-51單片機指令匯總」