A short glossary of terms which PICAXE users may encounter ...
PIC / PICmicro - A silicon chip, a microcontroller, manufactured by Microchip Technology Inc. The chip does nothing of itself until it has been programmed and requires a specialised hardware programming tool to perform that task. A PIC is not a PICAXE and they should not be confused with each other.
PICAXE - One of a range of products from Revolution Education Ltd. Each PICAXE is a pre-programmed PICmicro which contains Firmware ( which executes a downloaded program - also known as 'the interpreter' ) and a Downloader which allows programs to be loaded into it using nothing more than a simple serial or USB-to-Serial cable. The Programming Editor and AXEpad allow programs to be written in a variant of the Basic language, simulated, downloaded and debugged.
Tied-High Input - Connected to +V, directly or via a resistor. Input will always read high.
Tied-Low Input - Connected to 0V, directly or via a resistor. Input will always read low.
Pulled-Up Input - Connected by a resistor to +V. Input will read high unless something takes it low.
Pulled-Down Input - Connected by a resistor to 0V. Input will read low unless something takes it high.
Floating Input - Neither pulled-up nor pulled-down, and often not connected to anything at all. Two inputs connected together but neither pulled-up nor pulled-down would both still be floating. Input will be indeterminate and may change randomly.
TTL Input - An input which will read high when a signal above a certain voltage is present, low when a signal below another lower voltage is present. There is usually a band of voltage where the input level will be indeterminate and may vary randomly.
Schmitt / Schmitt-Trigger Input - An input which will read high only when an input signal rises above a certain voltage, will not read low until the signal drops below another lower voltage, and will not read high again until the signal rises above the higher voltage level again. Such inputs are said to have hysterisis. The input level will not vary randomly as it can with TTL inputs when between the two, high and low, voltage levels.
Output High - An output producing a high ( usually near +V ) signal. Sourcing current.
Output Low - An output producing a low ( usually near 0V ) signal. Sinking current.
Tri-Stated / High-Impedance / Hi-Z Output - An output which is neither high nor low, neither sources nor sinks current. Usually achieved by making the output a floating input under software control.
Open-Collector Output - An output which is either Tri-Stated or Output Low. Can sink current but not source current. Often used with a pulled-up input; the input will float high because of its pull-up resistor unless pulled-low by the open-collector output. Unlike Output High and Output Low outputs, multiple open-collector outputs can be joined together without risk of electrical damage. Also found as the outputs of Darlington transistor arrays ( ULN2803 etc ) where they can sink current through loads connected to voltages higher than a chip's +V power supply. Also created by using 'Transistor Buffers' and 'FET switches'.
Low-Side Driver - Closely related to an Open Collector Output, and often used to describe a transistor or FET used to switch a Load. The load is connected between +V ( or a higher voltage ) and the driver, the other side of the driver to 0V.
High-Side Driver - Similar to a Low-Side Driver but the component performing the switching has one side connected to +V ( or a higher voltage ) and the load between it and 0V. High-Side switching is most commonly found in automotive systems where the 0V is the chassis ( "negative earth" ) to which loads to be controlled have one side connected.
Maximum Source Current - The maximum current that can be handled by an Output High ( for example from an output pin, through a LED and resistor to 0V ). Exceeding the maximum may permanently damage the chip.
Maximum Sink Current - The maximum current that can be handled by an Output Low ( for example from +V, through a LED and resistor to the output pin ). Exceeding the maximum may permanently damage the chip. Where a load being controlled requires a higher sink current than available a Low-Side Driver ( transistor, Darlington transistor array or FET ) will often be used.
Clamping Diodes - Diodes which connect from an I/O pin to +V and/or 0V, either internally or externally to a chip, to help deal with voltages which are higher than +V or lower than 0V. Their presence does not mean it becomes safe to connect voltages higher than +V or lower than 0V into a chip's input; damage to the chip can still occur.
Input Impedance - The resistance of an input which determines how much loading will be put on an input signal, how much current will be drawn. Most digital inputs have a very high input impedance and will draw very little current from the input signal.
Fan-Out - The number of inputs an output can drive without the signal becoming unreliable. Usually related to input impedance of what is being driven, and, with high input impedances, fan-out is usually high.
Diode-Mixing - Using two or more diodes to combine signals. Usually used in an 'OR' configuration; a voltage on either input ( diode anodes ) will produce a voltage on the ( joined diode cathodes, pointy-ends ) output. Usually used with a pull-down resistor. Can also be used with the diodes reversed ( 'AND' configuration ), a pull-up resistor and the diode cathodes being pulled down; either taken low will produce an output low.
LDO / Low-Dropout Voltage Regulator - A voltage regulator which can produce its correct output voltage when the input voltage is not much higher. Non-LDO regulators often require the input to be a number of volts above what they output to work satisfactorily. LDO regulators are therefore suitable for battery powered systems and frequently draw less current for themselves than non-LDO regulators when operating thus extending battery life.
PIC / PICmicro - A silicon chip, a microcontroller, manufactured by Microchip Technology Inc. The chip does nothing of itself until it has been programmed and requires a specialised hardware programming tool to perform that task. A PIC is not a PICAXE and they should not be confused with each other.
PICAXE - One of a range of products from Revolution Education Ltd. Each PICAXE is a pre-programmed PICmicro which contains Firmware ( which executes a downloaded program - also known as 'the interpreter' ) and a Downloader which allows programs to be loaded into it using nothing more than a simple serial or USB-to-Serial cable. The Programming Editor and AXEpad allow programs to be written in a variant of the Basic language, simulated, downloaded and debugged.
Tied-High Input - Connected to +V, directly or via a resistor. Input will always read high.
Tied-Low Input - Connected to 0V, directly or via a resistor. Input will always read low.
Pulled-Up Input - Connected by a resistor to +V. Input will read high unless something takes it low.
Pulled-Down Input - Connected by a resistor to 0V. Input will read low unless something takes it high.
Floating Input - Neither pulled-up nor pulled-down, and often not connected to anything at all. Two inputs connected together but neither pulled-up nor pulled-down would both still be floating. Input will be indeterminate and may change randomly.
TTL Input - An input which will read high when a signal above a certain voltage is present, low when a signal below another lower voltage is present. There is usually a band of voltage where the input level will be indeterminate and may vary randomly.
Schmitt / Schmitt-Trigger Input - An input which will read high only when an input signal rises above a certain voltage, will not read low until the signal drops below another lower voltage, and will not read high again until the signal rises above the higher voltage level again. Such inputs are said to have hysterisis. The input level will not vary randomly as it can with TTL inputs when between the two, high and low, voltage levels.
Output High - An output producing a high ( usually near +V ) signal. Sourcing current.
Output Low - An output producing a low ( usually near 0V ) signal. Sinking current.
Tri-Stated / High-Impedance / Hi-Z Output - An output which is neither high nor low, neither sources nor sinks current. Usually achieved by making the output a floating input under software control.
Open-Collector Output - An output which is either Tri-Stated or Output Low. Can sink current but not source current. Often used with a pulled-up input; the input will float high because of its pull-up resistor unless pulled-low by the open-collector output. Unlike Output High and Output Low outputs, multiple open-collector outputs can be joined together without risk of electrical damage. Also found as the outputs of Darlington transistor arrays ( ULN2803 etc ) where they can sink current through loads connected to voltages higher than a chip's +V power supply. Also created by using 'Transistor Buffers' and 'FET switches'.
Low-Side Driver - Closely related to an Open Collector Output, and often used to describe a transistor or FET used to switch a Load. The load is connected between +V ( or a higher voltage ) and the driver, the other side of the driver to 0V.
High-Side Driver - Similar to a Low-Side Driver but the component performing the switching has one side connected to +V ( or a higher voltage ) and the load between it and 0V. High-Side switching is most commonly found in automotive systems where the 0V is the chassis ( "negative earth" ) to which loads to be controlled have one side connected.
Maximum Source Current - The maximum current that can be handled by an Output High ( for example from an output pin, through a LED and resistor to 0V ). Exceeding the maximum may permanently damage the chip.
Maximum Sink Current - The maximum current that can be handled by an Output Low ( for example from +V, through a LED and resistor to the output pin ). Exceeding the maximum may permanently damage the chip. Where a load being controlled requires a higher sink current than available a Low-Side Driver ( transistor, Darlington transistor array or FET ) will often be used.
Clamping Diodes - Diodes which connect from an I/O pin to +V and/or 0V, either internally or externally to a chip, to help deal with voltages which are higher than +V or lower than 0V. Their presence does not mean it becomes safe to connect voltages higher than +V or lower than 0V into a chip's input; damage to the chip can still occur.
Input Impedance - The resistance of an input which determines how much loading will be put on an input signal, how much current will be drawn. Most digital inputs have a very high input impedance and will draw very little current from the input signal.
Fan-Out - The number of inputs an output can drive without the signal becoming unreliable. Usually related to input impedance of what is being driven, and, with high input impedances, fan-out is usually high.
Diode-Mixing - Using two or more diodes to combine signals. Usually used in an 'OR' configuration; a voltage on either input ( diode anodes ) will produce a voltage on the ( joined diode cathodes, pointy-ends ) output. Usually used with a pull-down resistor. Can also be used with the diodes reversed ( 'AND' configuration ), a pull-up resistor and the diode cathodes being pulled down; either taken low will produce an output low.
LDO / Low-Dropout Voltage Regulator - A voltage regulator which can produce its correct output voltage when the input voltage is not much higher. Non-LDO regulators often require the input to be a number of volts above what they output to work satisfactorily. LDO regulators are therefore suitable for battery powered systems and frequently draw less current for themselves than non-LDO regulators when operating thus extending battery life.
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