Interfacing a Microprocessor To Keyboard
• When you press a key on your computer, you are activating a switch. There are many different ways of making these switches. An overview of the construction and operation of some of the most common types.
1. Mechanical key switches: In mechanical-switch keys, two pieces of metal are pushed together when you press the key. The actual switch elements are often made of a phosphor-bronze alloy with gold platting on the contact areas. The key switch usually contains a spring to return the key to the nonpressed position and perhaps a small piece of foam to help damp out bouncing.
2. Some mechanical key switches now consist of a molded silicon dome with a small piece of conductive rubber foam short two trace on the printed-circuit board to produce the key pressed signal.
3. Mechanical switches are relatively inexpensive but they have several disadvantages. First, they suffer from contact bounce. A pressed key may make and break contact several times before it makes solid contact.
4. Second, the contacts may become oxidized or dirty with age so they no longer make a dependable connection.
• Higher-quality mechanical switches typically have a rated life time of about 1 million keystrokes. The silicone dome type typically last 25 million keystrokes.
2. Membrane key switches: These switches are really a special type of mechanical switches. They consist of a three-layer plastic or rubber sandwich.
• The top layer has a conductive line of silver ink running under each key position.
The bottom layer has a conductive line of silver ink running under each column of keys.
• When u press a key, you push the top ink line through the hole to contact the bottom ink line.
• The advantages of membrane keyboards is that they can be made as very thin, sealed units.
• They are often used on cash registers in fast food restaurants. The lifetime of membrane keyboards varies over a wide range.
3. Capacitive key switches: A capacitive keyswitch has two small metal plates on the printed circuit board and another metal plate on the bottom of a piece of foam.
• When u press the key, the movable plate is pushed closer to fixed plate. This changes the capacitance between the fixed plates. Sense amplifier circuitry detects this change in capacitance and produce a logic level signal that indicates a key has been pressed.
• The big advantages of a capacitive switch is that it has no mechanical contacts to become oxidized or dirty.
• A small disadvantage is the specified circuitry needed to detect the change in capacitance.
• Capacitive keyswitches typically have a rated lifetime of about 20 million keystrokes.
4. Hall effect keyswitches: This is another type of switch which has no mechanical contact. It takes advantage of the deflection of a moving charge by a magnetic field.
• A reference current is passed through a semiconductor crystal between two opposing faces. When a key is pressed, the crystal is moved through a magnetic field which has its flux lines perpendicular to the direction of current flow in the crystal.
• Moving the crystal through the magnetic field causes a small voltage to be developed between two of the other opposing faces of the crystal.
• This voltage is amplified and used to indicate that a key has been pressed. Hall effect sensors are also used to detect motion in many electrically controlled machines.
• Hall effect keyboards are more expensive because of the more complex switch mechanism, but they are very dependable and have typically rated lifetime of 100 million or more keystrokes.
Keyboard Circuit Connections and Interfacing
• In most keyboards, the keyswitches are connecting in a matrix of rows and columns, as shown in fig.
• We will use simple mechanical switches for our examples, but the principle is same for other type of switches.
• Getting meaningful data from a keyboard, it requires the following three major tasks:
1. Detect a keypress.
2. Debounce the keypress.
3. Encode the keypress
• Three tasks can be done with hardware, software, or a combination of two, depending on the application.
1. Software Keyboard Interfacing:
• Circuit connection and algorithm : The following fig (a) shows how a hexadecimal keypad can be connected to a couple of microcomputer ports so the three interfacing tasks can be done as part of a program.
• The rows of the matrix are connected to four output port lines. The column lines of matrix are connected to four input-port lines. To make the program simpler, the row lines are also connected to four input lines.
• When no keys are pressed, the column lines are held high by the pull-up resistor connected to +5V. Pressing a key connects a row to a column. If a low is output on a row and a key in that row is pressed, then the low will appear on the column which contains that key and can be detected on the input port.
• If you know the row and column of the pressed key, you then know which key was pressed, and you can convert this information into any code you want to represent that key.
• The following flow chart for a procedure to detect, debounce and produce the hex code for a pressed key.
• An easy way to detect if any key in the matrix is pressed is to output 0’s to all rows and then check the column to see if a pressed key has connected a low to a column.
• In the algorithm we first output lows to all the rows and check the columns over and over until the column are all high. This is done before the previous key has been released before looking for the next one. In the standard keyboard terminology, this is called two-key lockout.
• Once the columns are found to be all high, the program enters another loop, which waits until a low appears on one of the columns, indicating that a key has been pressed. This second loop does the detect task for us. A simple 20-ms delay procedure then does the debounce task.
• After the debounce time, another check is made to see if the key is still pressed. If the columns are now all high, then no key is pressed and the initial detection was caused by a noise pulse or a light brushing past a key. If any of the columns are still low, then the assumption is made that it was a valid keypress.
• The final task is to determine the row and column of the pressed key and convert this row and column information to the hex code for the pressed key. To get the row and column information, a low is output to one row and the column are read.
If none of the columns is low, the pressed key is not in that row. So the low is rotated to the next row and the column are checked again. The process is repeated until a low on a row produces a low on one of the column.
• The pressed key then is in the row which is low at that time.
• The connection fig shows the byte read in from the input port will contain a 4-bit code which represents the row of the pressed key and a 4-bit code which represent the column of the pressed key.
• Error trapping: The concept of detecting some error condition such as “ no match found” is called error trapping. Error trapping is a very important part of real programs. Even in simple programs, think what might happen with no error trap if two keys in the same row were pressed at exactly at the same time and a column code with two lows in it was produced.
• This code would not match any of the row-column codes in the table, so after all the values in the table were checked, assigned register in program would be decremented from 0000H to FFFFH. The compare decrement cycle would continue through 65,536 memory locations until, by change the value in a memory location matched the row-column code. The contents of the lower byte register at hat point would be passed back to the calling routine. The changes are 1 in 256 that would be the correct value for one of the pressed keys. You should keep an error trap in a program whenever there is a chance for it.
2. Keyboard Interfacing with Hardware: For the system where the CPU is too busy to be bothered doing these tasks in software, an external device is used to do them.
• One of a MOS device which can be do this is the General Instruments AY5-2376 which can be connected to the rows and columns of a keyboard switch matrix.
• The AY5-2376 independently detects a keypress by cycling a low down through the rows and checking the columns. When it finds a key pressed, it waits a debounce time.
• If the key is still pressed after the debounce time, the AY5-2376 produces the 8- bit code for the pressed key and send it out to microcomputer port on 8 parallel lines. The microcomputer knows that a valid ASCII code is on the data lines, the AY5-2376 outputs a strobe pulse.
• The microcomputer can detect this strobe pulse and read in ASCII code on a polled basis or it can detect the strobe pulse on an interrupt basis.
• With the interrupt method the microcomputer doesn’t have to pay any attention to the keyboard until it receives an interrupt signal.
• So this method uses very little of the microcomputer time. The AY5-2376 has a feature called two-key rollover. This means that if two keys are pressed at nearly the same time, each key will be detected, debounced and converted to ASCII.
• The ASCII code for the first key and a strobe signal for it will be sent out then the ASCII code for the second key and a strobe signal for it will be sent out and compare this with two-key lockout.
Example
• Interface a 4 * 4 keyboard with 8086 using 8255 an write an ALP for detecting a key closure and return the key code in AL. The debounce period for a key is 10ms. Use software debouncing technique. DEBOUNCE is an available 10ms delay routine.
• Solution: Port A is used as output port for selecting a row of keys while Port B is used as an input port for sensing a closed key. Thus the keyboard lines are selected one by one through port A and the port B lines are polled continuously till a key closure is sensed. The routine DEBOUNCE is called for key debouncing. The key code is depending upon the selected row and a low sensed column.
The higher order lines of port A and port B are left unused. The address of port A and port B will respectively 8000H and 8002H while address of CWR will be 8006H. The flow chart of the complete program is as given. The control word for this problem will be 82H. Code segment CS is used for storing the program code.
• Key Debounce : Whenever a mechanical push-button is pressed or released once, the mechanical components of the key do not change the position smoothly, rather it generates a transient response .
These transient variations may be interpreted as the multiple key pressure and responded accordingly by the microprocessor system.
• To avoid this problem, two schemes are suggested: the first one utilizes a bistable multivibrator at the output of the key to debounce .
• The other scheme suggests that the microprocessor should be made to wait for the transient period (usually 10ms), so that the transient response settles down and reaches a steady state.
• A logic ‘0’ will be read by the microprocessor when the key is pressed.
• In a number of high precision applications, a designer may have two options- the first is to have more than one 8-bit port, read (write) the port one by one and then from the multibyte data, the second option allows forming 16-bit ports using two 8-bit ports and use 16-bit read or write operations.
Interfacing To Alphanumeric Displays
• To give directions or data values to users, many microprocessor-controlled instruments and machines need to display letters of the alphabet and numbers. In systems where a large amount of data needs to be displayed a CRT is used to display the data. In system where only a small amount of data needs to be displayed, simple digit-type displays are often used.
• There are several technologies used to make these digit-oriented displays but we are discussing only the two major types.
• These are light emitting diodes (LED) and liquid-crystal displays (LCD).
• LCD displays use very low power, so they are often used in portable, battery- powered instruments. They do not emit their own light, they simply change the reflection of available light. Therefore, for an instrument that is to be used in low- light conditions, you have to include a light source for LCDs or use LEDs which emit their own light.
• Alphanumeric LED displays are available in three common formats. For displaying only number and hexadecimal letters, simple 7-segment displays such as that as shown in fig are used.
• To display numbers and the entire alphabet, 18 segment displays such as shown in fig or 5 by 7 dot-matrix displays such as that shown in fig can be used. The 7- segment type is the least expensive, most commonly used and easiest to interface with, so we will concentrate first on how to interface with this type.
1. Directly Driving LED Displays: Figure shows a circuit that you might connect to a parallel port on a microcomputer to drive a single 7-segment , common-anode display. For a common-anode display, a segment is tuned on by applying a logic low to it.
• The 7447 converts a BCD code applied to its inputs to the pattern of lows required to display the number represented by the BCD code. This circuit connection is referred to as a static display because current is being passed through the display at all times.
• Each segment requires a current of between 5 and 30mA to light. Let’s assume you want a current of 20mA. The voltage drop across the LED when it is lit is about 1.5V.
• The output low voltage for the 7447 is a maximum of 0.4V at 40mA. So assume that it is about 0.2V at 20mA. Subtracting these two voltage drop from the supply voltage of 5V leaves 3.3V across the current limiting resistor. Dividing 3.3V by 20mA gives a value of 168Ω for the current-limiting resistor. The voltage drops across the LED and the output of 7447 are not exactly predictable and exact current through the LED is not critical as long as we don’t exceed its maximum rating.
2. Software-Multiplexed LED Display:
• The circuit in fig works for driving just one or two LED digits with a parallel output port. However, this scheme has several problem if you want to drive, eight digits.
• The first problem is power consumption. For worst-case calculations, assume that all 8 digits are displaying the digit 8, so all 7 segments are all lit. Seven segment time 20mA per segment gives a current of 140mA per digit. Multiplying this by 8 digits gives a total current of 1120mA or 1.12A for 8 digits.
• A second problem of the static approach is that each display digit requires a separate 7447 decoder, each of which uses of another 13mA. The current required by the decoders and the LED displays might be several times the current required by the reset of the circuitry in the instrument.
• To solve the problem of the static display approach, we use a multiplex method, example for an explanation of the multiplexing.
• The fig shows a circuit you can add to a couple of microcomputer ports to drive some common anode LED displays in a multiplexed manner. The circuit has only one 7447 and that the segment outputs of the 7447 are bused in parallel to the segment inputs of all the digits.
• The question that may occur to you on first seeing this is: Aren’t all the digits going to display the same number? The answer is that they would if all the digits were turned on at the same time. The tricky of multiplexing displays is that only one display digit is turned on at a time.
• The PNP transistor is series with the common anode of each digit acts as on/off switch for that digit. Here’s how the multiplexing process works.
• The BCD code for digit 1 is first output from port B to the 7447. the 7447 outputs the corresponding 7-segment code on the segment bus lines. The transistor connected to digit 1 is then turned on by outputting a low to the appropriate bit of port A. All the rest of the bits of port A are made high to make sure no other digits are turned on. After 1 or 2 ms, digit 1 is turned off by outputting all highs to port A.
• The BCD code for digit 2 is then output to the 7447 on port B, and a word to turn on digit 2 is output on port A.
• After 1 or 2 ms, digit 2 is turned off and the process is repeated for digit 3. the process is continued until all the digits have had a turn. Then digit 1 and the following digits are lit again in turn.
• A procedure which is called on an interrupt basis every 2ms to keep these displays refreshed wit some values stored in a table. With 8 digits and 2ms per digit, you get back to digit 1 every 16ms or about 60 times a second.
• This refresh rate is fast enough so that the digits will each appear to be lit all time.
Refresh rates of 40 to 200 times a second are acceptable.
• The immediately obvious advantages of multiplexing the displays are that only one 7447 is required, and only one digit is lit at a time. We usually increase the current per segment to between 40 and 60 mA for multiplexed displays so that they will appear as bright as they would if they were not multiplexed. Even with this increased segment current, multiplexing gives a large saving in power and parts.
• The software-multiplexed approach we have just described can also be used to drive 18-segment LED devices and dot-matrix LED device. For these devices, however you replace the 7447 in fig with ROM which generates the required segment codes when the ASCII code for a character is applied to the address inputs of the ROM.
-
Liquid Crystal displays are created by sandwiching a thin 10-12 µm layer of a liquid-crystal fluid between two glass plates. A transparent, electrically conductive film or backplane is put on the rear glass sheet. Transparent sections of conductive film in the shape of the desired characters are coated on the front glass plate.
• When a voltage is applied between a segment and the backplane, an electric field is created in the region under the segment. This electric field changes the transmission of light through the region under the segment film.
• There are two commonly available types of LCD : dynamic scattering and field- effect.
• The Dynamic scattering types of LCD: It scrambles the molecules where the field is present. This produces an etched-glass-looking light character on a dark background.
• Field-effect types use polarization to absorb light where the electric field is present. This produces dark characters on a silver- gray background.
• Most LCD’s require a voltage of 2 or 3 V between the backplane and a segment to turn on the segment.
• We cannot just connect the backplane to ground and drive the segment with the outputs of a TTL decoder. The reason for this is a steady dc voltage of more than about 50mV is applied between a segment and the backplane.
• To prevent a dc buildup on the segments, the segment-drive signals for LCD must be square waves with a frequency of 30 to 150 Hz.
• Even if you pulse the TTL decoder, it still will not work because the output low voltage of TTL devices is greater than 50mV.
• CMOS gates are often used to drive LCDs.
• The Following fig shows how two CMOS gate outputs can be connected to drive an LCD segment and backplane.
• The off segment receives the same drive signal as the backplane. There is never any voltage between them, so no electric field is produced. The waveform for the on segment is 180 out of phase with the backplane signal, so the voltage between this segment and the backplane will always be +V.
• The logic for this signal, a square wave and its complement. To the driving gates, the segment-backplane sandwich appears as a somewhat leaky capacitor.
• The CMOS gates can be easily supply the current required to charge and discharge this small capacitance.
• Older inexpensive LCD displays turn on and off too slowly to be multiplexed the way we do LED display.
• At 0c some LCD may require as mush as 0.5s to turn on or off. To interface to those types we use a nonmultiplexed driver device.
• More expensive LCD can turn on and off faster, so they are often multiplexed using a variety of techniques.
• In the following section we show you how to interface a nonmultiplexed LCD to a microprocessor such as SDK-86.
• Intersil ICM7211M can be connected to drive a 4-digit, nonmultiplexed, 7- segment LCD display.
• The 7211M input can be connected to port pins or directly to microcomputer bus.
We have connected the CS inputs to the Y2 output of the 74LS138 port decoder.
• According to the truth table the device will then be addressable as ports with a base address of FF10H. SDK-86 system address lines A2 is connected to the digit-select input (DS2) and system address lines A1 is connected to the DS1 input. This gives digit 4 a system address of FF10H.
• The data inputs are connected to the lower four lines of the SDK-86 data bus. The oscillator input is left open. To display a character on one of the digits, you simply keep the 4-bit hex code for that digit in the lower 4 bits of the AL register and output it to the system address for that digit.
• The ICM7211M converts the 4-bit hex code to the required 7-segment code.
• The rising edge of the CS input signal causes the 7-segment code to be latched in the output latches for the address digit.
• An internal oscillator automatically generates the segment and backplane drive waveforms as in fig . For interfacing with the LCD displays which can be multiplexed the Intersil ICM7233 can be use.
