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74LS90功能：十進(jìn)制計(jì)數(shù)器（÷2 和÷5）
原理說(shuō)明：本電路是由4 個(gè)主從觸發(fā)器和用作除2 計(jì)數(shù)器及計(jì)數(shù)周期長(zhǎng)度為除5 的3 位2 進(jìn)制計(jì)數(shù)器所用的附加選通所組成。有選通的零復(fù)位和置9 輸入。
為了利用本計(jì)數(shù)器的最大計(jì)數(shù)長(zhǎng)度（十進(jìn)制），可將B 輸入同QA 輸出連接，輸入計(jì)數(shù)脈沖可加到輸入A 上，此時(shí)輸出就如相應(yīng)的功能表上所要求的那樣。
LS90 可以獲得對(duì)稱的十分頻計(jì)數(shù)，辦法是將QD 輸出接到A 輸入端，并把輸入計(jì)數(shù)脈沖加到B 輸入端，在QA 輸出端處產(chǎn)生對(duì)稱的十分頻方波。
真值表：
|
Reset Inputs復(fù)位輸入 |
輸出 |
|
R0(1) |
R0(2) |
R9(1) |
R9(2) |
QD |
QC |
QB |
QA |
|
H |
H |
L |
X |
L |
L |
L |
L |
|
H |
H |
X |
L |
L |
L |
L |
L |
|
X |
X |
H |
H |
H |
L |
L |
H |
|
X |
L |
X |
L |
COUNT
COUNT
COUNT
COUNT |
|
L |
X |
L |
X |
|
L |
X |
X |
L |
|
X |
L |
L |
X |
H=高電平 L=低電平 ×=不定
BCD 計(jì)數(shù)順序（注1）
|
Count |
輸出 |
|
QD |
QC |
QB |
QA |
|
0 |
L |
L |
L |
L |
|
1 |
L |
L |
L |
H |
|
2 |
L |
L |
H |
L |
|
3 |
L |
L |
H |
H |
|
4 |
L |
H |
L |
L |
|
5 |
L |
H |
L |
H |
|
6 |
L |
H |
H |
L |
|
7 |
L |
H |
H |
H |
|
8 |
H |
L |
L |
L |
|
9 |
H |
L |
L |
H |
5-2 進(jìn)制計(jì)數(shù)順序（注2）
|
Count |
輸出 |
|
QA |
QD |
QC |
QB |
|
0 |
L |
L |
L |
L |
|
1 |
L |
L |
L |
H |
|
2 |
L |
L |
H |
L |
|
3 |
L |
L |
H |
H |
|
4 |
L |
H |
L |
L |
|
5 |
H |
L |
L |
L |
|
6 |
H |
L |
L |
H |
|
7 |
H |
L |
H |
L |
|
8 |
H |
L |
H |
H |
|
9 |
H |
H |
L |
L |
注1：對(duì)于BCD（十進(jìn)）計(jì)數(shù)，輸出QA 連到輸入B 計(jì)數(shù)
注2：對(duì)于5-2 進(jìn)制計(jì)數(shù)，輸出QD 連到輸入A 計(jì)數(shù)
圖1 74LS90引腳圖
圖2 74LS90邏輯圖
Recommended Operating Conditions建議操作條件：
|
Symbol 符號(hào) |
Parameter 參數(shù) |
最小 |
典型 |
最大 |
UNIT 單位 |
|
VCC |
Supply Voltage 電源電壓 |
4.75 |
5 |
5.25 |
V |
|
VIH |
High Level Input Voltage輸入高電平電壓 |
2 |
- |
- |
V |
|
VIL |
LOW Level Input Voltage 輸入低電平電壓 |
- |
- |
0.8 |
V |
|
IOH |
HIGH Level Output Current高電平輸出電流 |
- |
- |
-0.4 |
mA |
|
IOL |
LOW Level Output Current低電平輸出電流 |
- |
- |
8 |
mA |
|
fCLK |
Clock Frequency (Note 5)時(shí)鐘頻率 |
A to QA |
0 |
- |
32 |
MHz |
|
B to QB |
0 |
- |
16 |
|
fCLK |
Clock Frequency (Note 6)時(shí)鐘頻率 |
A to QA |
0 |
- |
20 |
MHz |
|
B to QB |
0 |
- |
10 |
|
tW |
Pulse Width (Note 5)脈沖寬度 |
A |
15 |
- |
- |
ns |
|
B |
30 |
- |
- |
|
Reset |
15 |
- |
- |
|
tW |
Pulse Width (Note 6)脈沖寬度 |
A |
25 |
- |
- |
ns |
|
B |
50 |
- |
- |
|
Reset |
25 |
- |
- |
|
tREL |
Reset Release Time (Note 5)重置發(fā)布時(shí)間 |
25 |
- |
- |
ns |
|
tREL |
Reset Release Time (Note 6)重置發(fā)布時(shí)間 |
35 |
- |
- |
ns |
|
TA |
Free Air Operating Temperature工作溫度 |
0 |
- |
70 |
℃ |
|
Note 5: CL = 15 pF, RL = 2 kW, TA = 25℃ and VCC = 5V |
|
Note 6: CL = 50 pF, RL = 2 kW, TA = 25℃ and VCC = 5V |
|
Electrical Characteristics 電氣特性 |
|
over recommended operating free air temperature range (unless otherwise noted) |
|
Symbol 符號(hào) |
Parameter 參數(shù) |
Conditions 條件 |
最小 |
典型 |
最大 |
UNIT 單位 |
|
VI |
Input Clamp Voltage輸入鉗位電壓 |
VCC = 最小, II = -18mA |
- |
- |
-1.5 |
V |
|
VOH |
HIGH Level Output Voltage輸出高電平電壓 |
VCC=最小,IOH=最大 VIL=最大,VIH=最小 |
2.7 |
3.4 |
- |
V |
|
VOL |
LOW LevelOutput Voltage輸出低電平電壓 |
VCC = 最小, IOL = 最大 VIL = 最大, VIH = 最小(Note 8) |
- |
0.35 |
0.5 |
V |
|
IOL= 4mA, VCC =最小 |
- |
0.25 |
0.4 |
|
II |
Input Current @ MAX Input Voltage輸入電流@最大輸入電壓 |
VCC = 最大, VI =7V |
Reset |
- |
- |
0.1 |
mA |
|
VCC = 最大 VI =5.5V |
A |
- |
- |
0.2 |
|
B |
- |
- |
0.4 |
|
IIH |
HIGH Level Input Current輸入高電平電流 |
VCC = 最大, VI =2.7V |
Reset |
- |
- |
20 |
μA |
|
A |
- |
- |
40 |
|
B |
- |
- |
80 |
|
IIL |
LOW LevelInput Current輸入低電平電流 |
VCC = 最大, VI =0.4V |
Reset |
- |
- |
-0.4 |
mA |
|
A |
- |
- |
-2.4 |
|
B |
- |
- |
-3.2 |
|
IOS |
Short Circuit Output Current短路輸出電流 |
VCC = 最大 (Note 9) |
-20 |
- |
-100 |
mA |
|
ICC |
Supply Current電源電流 |
VCC = 最大 (Note 7) |
- |
9 |
15 |
mA |
交流電氣特性：
|
Symbol 符號(hào) |
Parameter 參數(shù) |
To (Output) |
RL=2K |
UNIT 單位 |
|
CL =15pF |
CL =50pF |
|
最小 |
最大 |
最小 |
最大 |
|
fMax |
Maximum Clock Frequency最大時(shí)鐘頻率 |
A to QA |
32 |
- |
20 |
- |
MHz |
|
B to QB |
16 |
- |
10 |
- |
|
tPLH |
Propagation Delay Time LOW-to-HIGH Level Output低到高電平輸出傳遞延遲時(shí)間 |
A to QA |
- |
16 |
- |
20 |
ns |
|
tPHL |
Propagation Delay Time HIGH-to-LOW Level Output高到低電平輸出傳遞延遲時(shí)間 |
A to QA |
- |
18 |
- |
24 |
ns |
|
tPLH |
|
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