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參數(shù)資料
| 型號(hào)： | 74C922 |
| 廠商： | Fairchild Semiconductor Corporation |
| 英文描述： | 16-Key Encoder 20-Key Encoder |
| 中文描述： | 16鍵編碼器20鍵編碼器 |
| 文件頁(yè)數(shù)： | 6/10頁(yè) |
| 文件大小： | 218K |
| 代理商： | 74C922 |
Typical Applications
(Continued)
Asynchronous Data Entry Onto Bus (MM74C922)
TL/F/6037–12
Outputs are in TRI-STATE until key is pressed, then data is placed on bus.
When key is released, outputs return to TRI-STATE.
Expansion to 32 Key Encoder (MM74C922)
TL/F/6037–13
Theory of Operation
The MM74C922/MM74C923 Keyboard Encoders imple-
ment all the logic necessary to interface a 16 or 20 SPST
key switch matrix to a digital system. The encoder will con-
vert
a
key
switch
closer
5(MM74C923) bit nibble. The designer can control both the
keyboard scan rate and the key debounce period by altering
the oscillator capacitor, C
OSE
, and the key bounce mask
capacitor, C
MSK
. Thus, the MM74C922/MM74C923’s per-
formance can be optimized for many keyboards.
to
a
4(MM74C922)
or
The keyboard encoders connect to a switch matrix that is 4
rows by 4 columns (MM74C922) or 5 rows by 4 columns
(MM74C923). When no keys are depressed, the row inputs
are pulled high by internal pull-ups and the column outputs
sequentially output a logic ‘‘0’’. These outputs are open
drain and are therefore low for 25% of the time and other-
wise off. The column scan rate is controlled by the oscillator
input, which consists of a Schmitt trigger oscillator, a 2-bit
counter, and a 2–4-bit decoder.
When a key is depressed, key 0, for example, nothing will
happen when the X1 input is off, since Y1 will remain high.
When the X1 column is scanned, X1 goes low and Y1 will go
low. This disables the counter and keeps X1 low. Y1 going
low also initiates the key bounce circuit timing and locks out
the other Y inputs. The key code to be output is a combina-
tion of the frozen counter value and the decoded Y inputs.
Once the key bounce circuit times out, the data is latched,
and the Data Available (DAV) output goes high.
If, during the key closure the switch bounces, Y1 input will
go high again, restarting the scan and resetting the key
bounce circuitry. The key may bounce several times, but as
soon as the switch stays low for a debounce period, the
closure is assumed valid and the data is latched.
A key may also bounce when it is released. To ensure that
the encoder does not recognize this bounce as another key
closure, the debounce circuit must time out before another
closure is recognized.
The two-key roll-over feature can be illustrated by assuming
a key is depressed, and then a second key is depressed.
Since all scanning has stopped, and all other Y inputs are
disabled, the second key is not recognized until the first key
is lifted and the key bounce circuitry has reset.
The output latches feed TRI-STATE, which is enabled when
the Output Enable (OE) input is taken low.
6
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| 74C923 | 16-Key Encoder 20-Key Encoder |
| 74F02DCQB | Quad 2-Input NOR Gate |
| 74F02DCX | Quad 2-Input NOR Gate |
| 74F02DMQB | Quad 2-Input NOR Gate |
| 74F02DMX | Quad 2-Input NOR Gate |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| 74C922N | 制造商: 功能描述: 制造商:undefined 功能描述: |
| 74C923 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:16, 20-Key Encoder |
| 74C925 | 制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:4-Digit Counters with Multiplexed |
| 74C925 WAF | 制造商:Fairchild Semiconductor Corporation 功能描述: |
| 74C926 | 制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:4-Digit Counters with Multiplexed |
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