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MM74C Key Encoder? The keyboard scan can be implemented by either an external clock or external capacitor. These encoders also have on-chip pull-up devices which permit switches with up to 50 k? No diodes in the switch array are needed to eliminate ghost switches.
The internal debounce circuit needs only a single external capacitor and can be defeated by omitting the capacitor. A Data Available output goes to a high level when a valid keyboard entry has been made. The Data Available output returns to a low level when the entered key is released, even if another key is depressed.
The Data Available will return high to indicate acceptance of the new key after a normal debounce period; this two-key roll-over is provided between any two switches. An internal register remembers the last key pressed even after the key is released. Features s 50 k? MM74C Block Diagram 3 www.
A www. A VCC? Note 4: Capacitance is guaranteed by periodic testing. Min Typ? The keyboard may be synchronously scanned by omitting the capacitor at osc. The designer can control both the keyboard scan rate and the key debounce period by altering the oscillator capacitor, COSE, and the key bounce mask capacitor, CMSK. The keyboard encoders connect to a switch matrix that is 4 rows by 4 columns MM74C or 5 rows by 4 columns MM74C The column scan rate is controlled by the oscillator input, which consists of a Schmitt trigger oscillator, a 2bit 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 combination 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. As used herein: 1. Life support devices or systems are devices or systems which, a are intended for surgical implant into the body, or b support or sustain life, and c whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user.
A critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
FAIRCHILDSEMICONDUCTORHKLTD MM74C922N DIP
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The keyboard scan can be implemented by either an external clock or external capacitor. No diodes in the switch array are needed to eliminate ghost switches. The internal debounce circuit needs only a single external capacitor and can be defeated by omitting the capacitor. A Data Available output goes to a high level when a valid keyboard entry has been made. The Data Available output returns to a low level when the entered key is released, even if another key is depressed. The Data Available will return high to indicate acceptance of the new key after a normal debounce period; this two-key roll-over is provided between any two switches.