Interrupts in 6809 microprocessor

Interrupts:
The 6809, like most other microprocessors, has instructions and the hardware to handle interrupts. Interrupts are a mechanism for allowing outside events temporarily to divert the micro from its current program. When an interrupt arrives, a call is made to a nominated routine which runs an 'interrupt service routine' ending with a 'return' instruction which returns the micro to the interrupted program. So, an interrupt is just like a subroutine call except that control is not passed explicitly to the routine by the main program, but happens independently of it.
Interrupts are used for infrequent or irregular events which must be acted upon by the processor. The use of interrupts saves having to look periodically to see whether the event has occurred or not. This is known as polling and can tie the micro up for a large proportion of the time, especially if the event is a rare one, like power failure.

Implementation of Interrupts:
Most micros have one or more pins on them devoted to interrupts. A level (usually "0") on one of these pins causes the processor to enter an interrupt service routine (ISR), whose entry address is kept in a special area in memory. This is done by means of something very like a subroutine call; the return address is pushed onto the stack. The routine completes and a return from interrupt instruction is executed, which resumes execution of the interrupted program when a return from interrupt instruction (RTI) is executed. In order to avoid chaos, further interrupts on the same pin are not usually allowed between the arrival of the interrupt and the return from the interrupt routine.
Some processors allow multiple interrupts, which means that one interrupt service routine may itself be interrupted by another ISR. This is known as nesting interrupts. Whether or not an interrupt is serviced at all may be decided by the user, who may disable and enable interrupts, so that they may be ignored if they are not relevant.

Warnings:
Interrupts can be dangerous!! Care must be taken to ensure that interrupts do not corrupt information which is being used by the interrupted routine.
Finally, the cause of an interrupt MUST be cleared (if necessary) by the micro before the RTI instruction is executed, or else the processor will be immediately interrupted again, and you get into a deadlock situation.

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Static Method

Static Method:

A method may be declared as static, meaning that it acts at the class level rather than at the instance level.

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Multimeter

Multimeter

A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance. There are two categories of multimeters, analog multimeters and digital multimeters (often abbreviated DMM or DVOM.)

Quantities Measured

Contemporary multimeters can measure many quantities. The common ones are:

Voltage in volta.

Current in amperes.

Resistance in ohms.

Digital multimeters may also include circuits for:

Continuity that beeps when a circuit conducts.

Diodes and transistors

Accuracy

Digital multimeters generally take measurements with accuracy superior to their analog counterparts. A multimeter's quoted accuracy is specified as being that of the lower (mV) DC range, and is known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current, resistance, AC and other ranges will usually have a lower accuracy than the basic DC volts figure.

Burden voltage

On both DC and AC current ranges a multimeter will cause voltage drop in the circuit under test. This is primarily due to the current shunt resistor used for measurement. This voltage drop is known as the burden voltage, specified in volts per ampere.

Digital Multimeters (DMM or DVOM)

Modern multimeters are often digital due to their accuracy, durability and extra features. In a digital multimeter the signal under test is converted to a voltage and an amplifier with electronically controlled gain preconditions the signal. A digital multimeter displays the quantity measured as a number, which prevents parallax errors.

Modern digital multimeters may have an embedded computer, which provides a wealth of convenience features.

Modern meters may be interfaced with a personal computer by IrDA links, RS-232 connections, USB, or an instrument bus such as IEEE-488. The interface allows the computer to record measurements as they are made. Some DMM's can store measurements and upload them to a computer.

The first digital multimeter was manufactured in 1955 by Non Linear Systems.

Safety

Digital meters are category rated based on their intended application, as set forth by the CEN EN61010 standard. There are four categories:

Category I: used where current levels are low.

Category II: used on residential branch circuits.

Category III: used on permanently installed loads such as distribution panels, motors, and appliance outlets.

Category IV: used on locations where current levels are high, such as service entrances, main panels, and house meters.

Each category also specifies maximum transient voltages for selected measuring ranges in the meter. Category-rated meters also feature protections from over-current faults.

DMM alternatives

A general-purpose DMM is generally considered adequate for measurements at signal levels greater than one millivolt or one milliampere, or below one gigaohm — levels far from the theoretical limits of sensitivity. Other instruments are used for accurate measurements of very small or very large quantities. These include nanovoltmeters, electrometers and picoammeters.

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