Course of Microprocessor

Software dan Hardware Design Embedded Controller

Tri-State Logic

28 Januari 2015 6:16

Tri-state logic does not refer to orderly thinking in a three state geographic region! When we speak of binary (base two number) values, we mean that a given bit or logic signal can take on either one of two valid states (zero or one) at any instant in time.
A logic gate that is not forcing its output to be either one or zero is said to be tri-stated. Tri-state logic does not refer to base three numbers, but rather to a third invalid logic state when the output of a logic device is neither sinking nor sourcing current. This so-called third state is really an undefined condition, because the device output is not forcing a logic level on its output. It is said to be in a floating, high impedance, passive, or Hi-Z state, since the output circuits are effectively disconnected. A tri-state driver connected to one signal wire of the bus is shown in figure above.

On the left is an inverting buffer with an enabled tri-state output. On the fight side is an example showing two of the same type of buffers, with the top device in the disabled or passive state, and the lower device is enabled or actively driving the data bus to a logic one level. The control signal determines whether the output is passive or active, and is called the output enable or OE signal. The device shown above is actively driving the bus whenever the OE control line is at a logic one level, and is passive when the OE line is at a logic zero level. Most of the time, output enable signals are active low, meaning that the output is enabled when the/OE signal is low, and passive when the/OE signal is high. This is shown on the logic symbol with an inversion bubble where the enable signal enters the logic device.

CMOS logic

28 Januari 2015 5:51

CMOS logic (complementary symmetry MOS) is another form of MOS logic. It has advantages over NMOS logic for low power circuitry and for very complex integrated circuits. NMOS logic is relatively simple, but it has one serious drawback: it consumes a significant amount of power. In fact, it would be impossible to manufacture the largest ICs using NMOS logic, as the power dissipated by the chip would cause it to overheat.

This is the main reason CMOS logic has become the dominant form of logic used for large, complex I Cs. Instead of using a resistor to source current when the output is high, a CMOS device uses a P-channel MOSFET to pull the output high. CMOS logic is based on the use of two complementary FETs that switch the output between the power supply and ground. A simple CMOS inverter is shown in figure bellow.

CMOS logic uses two switches: one P-channel pull-up transistor, and one N-channel pull-down device to pull the output low or high, one at a time. CMOS logic is designed with an N-channel device that turns on and conducts when the gate voltage is at logic one (positive voltage), and the P-channel device turns on when the gate is at ground voltage. A CMOS inverter is comprised of a pair of FETs, one device of each type, as shown in figure above. When the transistor gate inputs are at logic one (positive voltage), the P-channel device is off, and the N-channel device is on, effectively connecting the output to ground, or logic zero. Likewise, when the input is grounded, the P-channel device turns on and the N-channel device turns off, effectively connecting the output to the positive supply voltage, or logic one. Gates and more complex logic functions can be constructed by using series and parallel-connected MOSFETs in circuits similar to the one above. The gate of a MOSFET, as implied by the symbol, is essentially an open circuit. In fact, the gate of a MOSFET does have an extremely high resistance. The operation of the MOSFET's channel is controlled by the voltage of the gate, unlike the bipolar NPN transistor we examined in the inverter, which is controlled by input (base) current. Bipolar transistors are current amplifiers, with their output current being controlled by their base current. FET outputs, on the other hand, are dependent on the gate voltage.

NMOS Logic

28 Januari 2015 4:43

The conductive state of the FET's channel is what allows or prevents current from flowing in the device. For a typical logic N-channel MOSFET, the channel becomes conductive when the gate has a positive voltage with respect to the source, allowing current to flow between the drain and source terminals. When the gate is at the same voltage as the source, no current flows. The design of MOS logic circuits can be almost exactly

equivalent to the bipolar inverter we saw earlier, substituting an N-channel MOSFET for the bipolar NPN transistor. In fact, the most of the early microcontroller integrated circuits were manufactured using variations of this method, and are referred to as NMOS logic. As can be seen from figure bellow, the NMOS FET circuit behaves in an equivalent way to the NPN transistor inverter. When the gate (control input) of the NMOS FET is at a positive voltage, the FET is ON, effectively shorting the source and drain pins. When the gate is at 0 volts, the FET is OFF, opening the circuit between the source and drain. Older NMOS logic ICs use this type of circuit. The original 8051 microcontroller was an NMOS processor.

The FET as a Logic Switch

27 Januari 2015 5:05

Most of the logic devices used in highly integrated circuits do not use bipolar transistors. Instead, they use field effect transistors. FETs perform a similar function to the bipolar transistors discussed earlier, but they are voltage controlled.
While the current flowing in the base controls bipolar transistors, the voltage between the gate and source controls field effect transistors. The gate voltage of a field effect transistor controls the current flowing in the drain-source circuit. The symbol for the FET shows the gate to be insulated from the source-drain circuit, as shown in figure above.

This type of FET is referred to as a MOSFET (metal oxide semiconductor FET), since the insulating material is silicon dioxide (SiO2), commonly known as glass (for early devices, the gate was made of metal). Like bipolar NPN and PNP transistors with opposite polarity, FETs come in N- and P- channel varieties. The N- and P- channels refer to the polarity of the source drain element of the device. A cross-section view of a FET is shown in figure bellow.

Transistor Switch OFF

27 Januari 2015 4:42

When the input is connected to logic zero (ground voltage), no current flows into the base of the transistor, since its base and emitter terminals are at the same voltage. When there is no current flowing in the base, the transistor will not allow current to flow in the collector emitter circuit either. As a result, the circuit behaves as if the transistor was removed from the circuit.
The output resistor will source current to any potential load. The output is pulled up to the supply voltage, resulting in a logic one at the output. Once again, there is a limit to the resistor's ability to source current, resulting in a limit to the number of loads that can be attached to this circuit's output. Notice these two limits are defined by the ability of the transistor to pull down the output, and the resistor's ability to pull up the output become the main limits to its ability to drive other devices. Gates can be constructed by adding diodes or transistors to the inverter circuit in figure above.

Transistor Switch ON

27 Januari 2015 4:26

Transistors can be configured to function as switches. As can be seen in figure bellow, an NPN transistor operating as a current controlled switch can be used to build a simple inverter. It changes a logic one on its input to a logic zero at its output, and vice versa. In this case, logic one is represented as a positive voltage, and a logic zero is represented by zero volts. The logic one input (positive input voltage) is supplied through a resistor from the power supply voltage to the transistor base terminal, resulting in a small base control current into the base.

The transistor is used because it has gain allowing a larger output current to flow as controlled by a weaker input. When the transistor is turned on as much as it can be, the collector emitter circuit looks almost like a short circuit, effectively connecting the output to ground or zero volts. This gives a logic zero on the collector output. When the transistor collector is shorted to ground, current flows from the supply through the resistor and into the transistor collector to ground. The transistor is said to sink the resistor current into ground. If there is an external load, such as another inverter or gate, connected to the collector output, the transistor can also sink current from the load. This is also referred to as pulling down the output voltage. The current sinking capacity of the transistor limits the number of devices this inverter can drive.

Digital Hardware Concepts

27 Januari 2015 4:26

In addition to the CPU, memory, and I/O building blocks, other logic circuits may also be required. Such logic circuits are frequently referred to as glue logic because they are used to connect the various building blocks together. The most difficult and important task the hardware designer faces is the proper selection and specification of this "glue logic." Devices such as registers, buffers, drivers and decoders are frequently used to adapt the control signals provided by the CPU to those of the other devices. While TTL gate level logic is still in use for this purpose, the programmable logic device (PLD) has become an important device in connecting the building blocks. Contemporary microcontroller designers need to acquire the following skills:

• Interpretation of manufacturers specifications
• Detailed, worst case timing analysis and design
• Worst case signal loading analysis
• Design of appropriate signal and level conversion circuits
• Component evaluation and selection
• Programmable logic device selection and design

The glue logic used to join the processor, memories, and I/O is ultimately composed of logic gates, which are themselves composed almost entirely of transistors, diodes, resistors, and interconnecting wires. In order to understand the basic operation of the glue logic, we are going to begin at the component level with a review of basic electronics concepts. These concepts will be presented as fluid flow analogies.

The Design and Development Process

27 Januari 2015 4:26

Structured design of a microcomputer requires the ability to do the system design and partitioning from the top down while implementing the system from the bottom up. The hardware design and development process should consist of the following steps:

1. Defining the requirements.
2. Collecting information on potential components.
3. Evaluate the components with respect to the requirements.
4. Do a block diagram preliminary design and component selection.
5. Perform a preliminary timing and loading analysis.
6. Define the functions of the "glue logic."
7. Schematic entry using CAD (computer-aided design) software.
8. Programmable logic device design and simulation.
9. Detailed timing analysis and simulation, adjusting the design as required.
10. Check the signal loading, buffering signals as needed.
11. Document the design and generate a net list and bill of materials.
12. Begin the design and layout of a printed circuit board.
13. Implement the design in breadboard or prototype form.
14. Program the memories and programmable logic as required for testing.
15. Debug and verify operation using oscilloscope, logic analyzer, and in-circuit emulator.
16. Update and complete documentation as the design changes.

The order of tasks shown is variable, and some of the tasks may be performed in parallel. Software design is also frequently done in parallel with hardware design, and sometimes even before the hardware design. This is frequently a result of the fact that the cost and time required to develop the software exceeds that of the hardware development. In some cases the cost of modifying existing programs may be so high as to be impractical. In these cases, it is the designer's responsibility to maintain software compatibility with previous hardware designs.

Logic Symbols

27 Januari 2015 4:26

Logic symbols are used to represent the logic functions in a more abstract way, allowing the designer to specify the logical function of a circuit without getting into the details of the underlying components (such as the transistors and resistors). The logic symbols used in this text represent those that are most commonly used in commercial documentation.
There are other standards, such as the ANSI/IEEE standard gate level symbols, but they are not encountered as frequently in practice. Figure bellow shows the logic symbols for different gates, and their functions are described in the truth tables.

The logic symbols in figure above show the shapes and Boolean logic functions for the most common gate configurations. The buffer device is a triangle--the symbol for an amplifier because it amplifies the input signal, allowing an increase in the number of loads that can be driven. Note that a small circle, often referred to as a "bubble," on an input or output terminal designates a logical inversion. Thus the inverter is shown as a triangle (amplifier) with a bubble on the output to signify the logic level inversion on the output. The logic voltage levels for TTL logic are:
Positive Logic
0 = false - lowest voltage level

1 = true = highest voltage level

Corresponding TTL Logic Voltages
0 - input voltages 0 to 0.8 volts (low)
1 - input voltages 2 to 5 volts (high)
This means that a TTL compatible logic input is guaranteed to respond to an input signal between 0 and 0.8 volts as a logic zero, and input voltages from 2 to 5 volts as a logic one. Note that voltages between 0.8 and 2 volts are not valid logic levels.
Logic voltage levels are different for different types of logic, but the most common logic levels are those corresponding to the original TTL (transistor-transistor logic), using a 5 volt power supply CMOS levels, using 3 or 5 volt power, are also common. TTL and CMOS logic--like almost every other type of logic in common use--are called positive logic because the most positive voltage corresponds to the logic one value.

Microcomputer and Microcontroller Architectures

27 Januari 2015 4:26

Microprocessors are generally utilized for relatively high performance applications where cost and size are not critical selection criteria. Because microprocessor chips have their entire function dedicated to the CPU and thus have room for more circuitry to increase execution speed, they can achieve very high-levels of processing power. However, microprocessors require external memory and I/O hardware. Microprocessor chips are used in desktop PCs and workstations where software compatibility, performance, generality, and flexibility are important.

By contrast, microcontroller chips are usually designed to minimize the total chip count and cost by incorporating memory and I/O on the chip. They are often "application specialized" at the expense of flexibilit 7 In some cases, the microcontroller has enough resources on-chip that it is the only IC required for a product. Examples of a single-chip application include the key fob used to arm a security system, a toaster, or hand-held games. The hardware interfaces of both devices have much in common, and those of the microcontrollers are generally a simplified subset of the microprocessor. The primary design goals for each type of chip can be summarized this way:

• microprocessors are most flexible
• microcontrollers are most compact

There are also differences in the basic CPU architectures used, and these tend to reflect the application. Microprocessor based machines usually have a yon Neumann architecture with a single memory for both programs and data to allow maximum flexibility in allocation of memory. Microcontroller chips, on the other hand, frequently embody the Harvard architecture, which has separate memories for programs and data. Figure 1-1 illustrates this difference.

One advantage the Harvard architecture has for embedded applications is due to the two types of memory used in embedded systems. A fixed program and constants can be stored in non-volatile ROM memory while working variable data storage can reside in volatile RAM. Volatile memory loses its contents when power is removed, but non-volatile ROM memory always maintains its contents even after power is removed.
The Harvard architecture also has the potential advantage of a separate interface allowing twice the memory transfer rate by allowing instruction fetches to occur in parallel with data transfers. Unfortunately, in most Harvard architecture machines, the memory is connected to the CPU using a bus that limits the parallelism to a single bus. A typical embedded computer consists of the CPU, memory, and I/O. They are most often connected by means of a shared bus for communication, as shown in Figure 1-2.

Diode

27 Januari 2015 4:26

The diode is a simple semiconductor device acting as a "one way" current valve. It only lets current flow in one direction. Figure above illustrates how the diode operates like a "one-way" fluid valve. All electrical current flow will be "positive" or "conventional" current flow, meaning current always flows from the most positive terminal to the most negative terminal of a component. The use of positive current flow follows the intuitive direction of the arrows inherent in the component drawings for diodes, transistors, etc.

Transistors

27 Januari 2015 4:26

The flow analogy can also be used to model how a transistor operates in a logic circuit. The transistor is an amplifier. It uses a small amount of energy to control a larger energy source, just as a valve controls a high-pressure water source. There are two kinds of transistors: bipolar and field-effect transistors (FETs).
We will look at bipolar transistors first; these amplify current. A small amount of current flows in the control circuit (the transistor base emitter circuit) to turn the transistor on. This control current is amplified (multiplied by the gain or beta of the transistor) and allows a larger current to flow in the output circuit (the collector emitter circuit). Once again, the device is not perfect because of the resistance, current, gain, and leakage limitations of real transistors. Bipolar transistors come in two polarities, NPN and PNP, with the difference being the direction in which current flows for normal operation. A bipolar PNP transistor is shown and modeled in Figure 1-7.

For most of the illustrative circuit examples in this site, we will be using NPN transistors, as shown in Figure 1-8.

Voltage, Current, and Resistance

27 Januari 2015 4:26

In Figure 1-3, a battery provides a voltage source for electricity, much like a pump provides a pressure source for a fluid. Voltage, or pressure, is required to produce current flow in the circuit. The voltage source provides the pressure "motivation," if you will, for current flow.
Resistance provides a limiting constraint on the amount of current that will actually flow. The resistor will allow a current to flow through it that is proportional to the voltage across it, and inversely proportional to the resistance value. Higher resistance is like a smaller aperture for the fluid to flow through. The resistance results in a voltage, or pressure drop, across the resistance as long as current is flowing in the resistor. Figure 1-4 illustrates this.

The wiring connecting the components in a circuit is like the piping connecting plumbing components that let a fluid flow. The flow of current in the circuit is controlled by the magnitude of the voltage (pressure) and the resistance (pressure drop) in the circuit. In Figure 1-5, the battery provides a voltage to force current through the resistor. The magnitude of the voltage (V) generated by the battery is developed across the resistor, and the magnitude of the resistance (R), determine the current (I). Note the "return" current path is often shown as "ground," which is the reference voltage used as the "zero volts" point. In this case, current flows from the positive battery terminal, through the wire, then the resistor, then through the "ground" connection to the minus terminal of the battery. This is usually not the same as earth ground, which provides a connection to a stake or pipe literally stuck in the ground. The magnitude of the current in this case is I = V / R by re-arranging the equation V : I * R, as shown in Figure 1-5. This is known as Ohm's law. Another way to look at it is that whenever current flows through a resistor, there is a drop in voltage across the resistor due to the restriction in current.

Real components are not the perfect voltage sources, resistances, etc. we have discussed so far. They have parasitic values that limit their performance in the real world and are subject to other limitations, such as operating temperature, power limits, etc. Current flows only through a complete circuit, and in most cases (for a positive power supply) current flows from the power source through the circuitry and returns to the power supply through the common "ground" connection. Current flowing through any resistance results in the dissipation of power as heat. The power dissipated is P = I~R = V*I = VZ/R. Note that voltage is sometimes denoted by the variable V and sometimes by E, for electromotive force.

All practical components have some resistance. Real batteries have an internal resistance, for example, which provides an upper limit to the current the battery can supply to an external circuit. Real wires have resistance as well, so the actual performance of a circuit will deviate somewhat from the ideal. These effects are obvious in some cases, but not in others. In an automobile starting circuit, it's not surprising that the battery, supplying 12 volts to a starter with internal resistance on the order of 0.01 to 0.1 ohms, will result in currents of hundreds of amperes in order to start the engine. On the other hand, while consulting with a prominent notebook computer manufacturer, I uncovered a design error resulting in an internal current of hundreds of amperes flowing in the circuit for a few nanoseconds. Obviously, this wreaked havoc on the operation of the computer, and generated a great deal of electromagnetic noise!

Timing Diagrams

27 Januari 2015 4:26

The timing diagram is the standard "language" of illustrating timing relationships between different parts of a design. In order to understand the relationship of different signals with respect to time, it is necessary to learn how to read and interpret timing diagrams.
Figure 1-20 shows examples of asynchronous (un-clocked or combinatorial gates) and synchronous (clocked flip-flop) logic. The notation used in this site is representative of that used in most component specifications. Timing specifications, such as delay, setup, and hold times, specify the limits under which the device is guaranteed to operate as intended. If those specifications are violated, the device may very well operate correctly most of the time. However, a change in temperature, voltage, or variations from unit to unit may make the circuit unreliable. The most undesirable result of timing violations is that the circuit makes very infrequent errors, perhaps one error in hundreds of hours of operation. If you have ever wondered why your PC crashes mysteriously for no apparent reason, timing specification violations may well be the cause!
Timing relationships are particularly important for signals that are "time shared" on a single wire. A group of wires that carries different information at different times is also called a bus.