Click on our menu buttons immediately below to find MegaSquirt® information quickly:

ADC - stands for "analog-digital converter". In this case, it is part of the conversion circuitry in the CPU that translates the varying voltage signal to a digital value that the CPU can understand and operate on. All of the sensors (TPS, MAP, CLT, IAT) send their signal to a particular ADC pin on the processor. The ADC result is used by the processor as a 'count'. MS-I has an eight bit ADC, so the counts can be from 0 to 255. MS-II has a more precise 10-bit ADC, so the count can be from 0-1024. Both of these are mapped over a 0-5 Volt range (so the voltage into the ADC = ADC count * 5.0/1024 for MS-II, for example); however, higher external voltage may be brought down to a 0-5 volt range with a 'voltage divider' or boosted with an amplifier (such as the AD595 EGT integrated circuit).

AE - Acceleration Enrichment, the enriched mixture provided when the throttle position sensor signal changes at various rates.

AFR - Air Fuel Ratio, the mass ratio of air to fuel in the combustion chamber. See NB- and WB-EGO sensors, below.

ASE - After Start Enrichment, the enriched mixture provided for a number of engine cycles when MegaSquirt® detects that the engine has transitioned from cranking to running.

AMC - Automatic Mixture Control, the method MegaSquirt-II (V2.6+) uses to update the VE table in RAM and FLASH using feedback from the oxygen sensor. See the AMC page for more information.

ASE - After Start Enrichment, the enriched mixture provided when engine has just been started.

ATDC - After Top Dead Center, the crankshaft position with respect to the piston being at the top of its travel, meaning it has passed it's highest position and is descending.

Bit-bang - 'Bit banging' means the port is turned off and on by specific instructions in the code that occur within normal program flow. In MS-II™ based systems, the chance to turn these outputs off or on occurs every 1/8th of a millisecond. The alternative is to use timer channels. These are built in processor hardware devices that allows the user to set the PWM frequency and duty cycle, and the hardware handles the PWM switching independently of the program code. This not only reduces computational overhead, but can be switched an much higher (or lower speeds) and is also is much more accurate. MS-II™ code from B&G uses the timer channels for fuel and ignition; other codes may uses the more crude bit-banging to control the ignition and fuel (check with the code's developer to be sure).

BTDC - Before Top Dead Center, the crankshaft position with respect to the piston being at the top of its travel, meaning it has NOT passed it's highest position and is rising. Most normal spark event occur BTDC.

CAN - (Controller Area Network) - a dedicated automotive networking system to allow different automotive processor to communicate and share inputs and calculated results. Used for MegaSquirt-II™ and the GPIO board, router board, etc..

Carbon Monoxide (CO): in an automotive context, generally refers to the regulated carbon monoxide (CO) tail pipe emissions. See:

CAS Crank Angle Sensor.

Catalytic converter: a chemical device in the vehicle's exhaust system that can reduce the amount of regulated emissions emitted by converting NOx to N2 + 02, CO to CO2 and HC to CO2 + H20. Catalytic means the converter active substrate facilitates the reactions, but is not consumed (and thus has a long life with no replenishment requirements). See:

CID Cubic Inch Displacement The imperial measure of the swept volume of the pistons. 61.02 cubic inches = 1 litre.

Closed loop - refers to those times when an EFI computer is using the feedback on the mixture provided by the oxygen sensor to effectively control the injected fuel amounts.

CHT - Cylinder Head Temperature, used instead of coolant temperature (CLT) on air-cooled engines.

CLT - CooLant Temperature sensor (aka. CTS). Usually the CLT sensor is an NTC (Negative Temperature Coefficient) thermistor, or a resistor whose resistance varies with temperature (NTC means the resistance goes down as the temperature goes up).

CM - Contract Manufacturer, the factory that builds MegaSquirt® products for Bowling and Grippo.

CNP - (Coil-Near-Plug) Each spark plug has its own coil attached by a short high tension lead. See:

Compiler - a program used to generate other programs using a source code written in a computer language (assembly language, C, C++, etc.). People often use the term to refer to the program used to compile, link and locate to create a usable program. Example of Compilers are Microsoft's Visual Studio C++ .NET or Freescale's Codewarrior.

COP - (Coil-on-Plug) Each spark plug has its own coil attached directly to it. See:

CPS - Crankshaft position sensor (aka. CKP) or camshaft position sensor (aka. CMP).

CPU - Central Processing Unit, aka. "processor" or "microprocessor" the computational engine that performs the calculations to operate the injection and ignition function in MegaSquirt®. It has a number of support circuits, like the power circuit, the clock circuit, the serial and CAN communications circuits, and various input and output conditioning circuits.

Cross-compiler - refers to a compiler (see) used on one platform (for example a Pentium processor running Windows OS) for use on another processor (MegaSquirt-II™'s HSC12 processor, for example).

CTS - Coolant Temperature Sensor (aka. CLT). Usually the CTS is an NTC (Negative Temperature Coefficient) thermistor, or a resistor whose resistance varies with temperature (NTC means the resistance goes down as the temperature goes up.

Datalog - a record of the engine parameters returned over the serial connection and recorded on a laptop. MegaLogViewer can be used to view these, as well as generic text and spreadsheet programs. Older tuning sopftware tended to use an .XLS extension for dataklog, bnewer software uses the .MLS extension (Megasquirt LogS).

Datasheet - A full listing of all the specifications on a particular component, often including sample circuits and other useful info.

Diode - A two lead electrical device that allows current to flow in only one direction (as long as the voltage is low enough - see Zener diodes).

DIY - Do-It-Yourself.

DMM (digital multi meter) electronic current/resistance/potential measuring tool.

DT (dual table) the dual table embedded code that has a number of additional features over the standard B&G embedded code.

Duty Cycle (DC)– A number indicating the amount of time that some signal is at full power. In the context of MegaSquirt® EFI Controller, duty cycle is used to describe the amount of time that the injectors are on, and to describe the “hold” part of the peak and hold injector drivers (see Low Impedance Injectors, below).

EasyTherm (ET) - A Windows program that simplifies configuring your MegaSquirt® to accept the substitution of non-standard temperature sensors and to upload software revisions.

ECU - (Electronic Control Unit) is the general term for a fuel injection controller, of which MegaSquirt® is an example.

EDIS - Electronic Distributorless Ignition System is Ford's wasted-spark computer-controlled ignition module, which has been made to work with modified versions of MegaSquirt®.

EGO Sensor - Exhaust Gas Oxygen sensor, used to describe the sensor in the exhaust that measures the lean/rich state of the intake mixture. Used to control the fuel via a feedback algorithm called “closed loop”.

EGR Exhaust Gas Recirculation

EGT (Exhaust Gas Temperature) is the temperature of the exhaust gases, typically measured with a thermocouple (and a GPIO board - which has 4 dedicated EGT circuits).

Errata A list of all the errors currently known in specific documentation, board layout, etc..

FCCM - Acronym for field-programmable custom computing machine, a computing device in which programmable logic devices replace traditional general purpose processors, or fixed logic, in order to achieve superior performance or other capabilities.

FET (field effect transistor) - In MegaSquirt® EFI Controller, the transistors used to control the activation of the injectors.

FPGA - field-programmable gate array. This is a flexible, programmable switching network that will be used in UltraMegaSquirt.

FIdle - Fast Idle. A device used to control idle speed with additional air supplied by a vacuum solenoid. MegaSquirt has a simple on-off fast idle control, and does not have the ability to drive a PWM IAC (Idle Air Control) device.

Gamma - Used to indicate the change in a fuel amount from the calculated amount.

Gego - Gego is short for 'Gamma - Exhaust Gas Oxygen'. It is the change applied to the fuelling equation based on the EGO O2 sensor feedback. This feedback can (and is) done in a number of different ways, depending on the type of sensor, etc. The important thing is that this is a result of an external measurement, not a pure calculation, so it appears a bit mysteriously in the equation.

GPIO - (General Purpose Input/Output Board) - A CAN enabled processor equipped expansion board for MegaSquirt-II.

GM - General Motors, the manufacturer for the default coolant and air temperature sensors used with MegaSquirt®.

Hall sensor - an "active", magnetic field presence sensor. It is based on the Hall effect. The Hall effect is the change of resistance in a semiconductor in a magnetic field. The Hall effect sensor consists of semiconductor material which will conduct current when the material is subject to a magnetic field. These types of sensors require a "flying magnet", wheel. Instead of teeth on the wheel, as in a variable reluctor sensor, you must have small magnet and a shutter wheel.

HEI - High Energy Ignition, the distributor based electronically controlled ignition system from General Motors. There are a number of variant, identified by the module they use:

High Impedance Injectors - (a.k.a. hi-Z) Fuel injectors designed to work with a simple switch in a 12 volt circuit, no special signal conditioning is required to drive them. The resistance of a high impedance injector is about 10-15 ohms.

HR - (high resolution) the high resolution embedded code that gives higher injector pulse width resolution than the standard B&G embedded code.

Hydrocarbon (HC) - in an automotive context, generally refers to regulated unburned hydrocarbon (HC) tail pipe emissions. See:

Hz (Hertz) - the measurement of the frequency of a cyclical event, it represents to number of times per second the cycle is completed.

IAC - Idle Air Controller, though it term is sometimes used more generally, it usually refers to GM's stepper motor controller for additional idle air (and hence engine speed) during warm-up.

IAT sensor - Intake Air Temperature sensor, same as MAT, see below.

IGBT - Insulated Gate Bipolar Transistor a particular kind of transistor especially suitable for driving ignition coils.

kPa (kiloPascals) - the measurement of air pressure used in MegaSquirt® computations. It ranges from 0 (vacuum) to 101.3 kPa (standard atmospheric pressure at sea level) to 250 kPa (21psi of boost) or higher.

LIW - Acronym for long instruction word, an instruction set architecture which explicitly encodes several independent instructions, to be executed concurrently, in each "instruction word".

logic cell - A unit of FPGA "area" equivalent to one 4-input lookup table and one D flip-flop.

LUT - Acronym for 'Look-Up Table'. A small RAM (e.g. flip-flops with output multiplexer tree), usually 16 bits (sometimes 8 bits), that implements an arbitrary combinational logic function of 4 (respectively, 3) inputs. Often found with a number prefix that indicates precise number of inputs. Thus a 4-LUT is a 4-input (16 bit RAM) lookup table.

Low Impedance Injectors - (a.k.a low-Z) Fuel injectors that are designed to run at a much lower current than would be supplied by a direct 12 volt connection. They require a special signal that is initially at full current (4-6 amps, a.k.a. “peak current”) for about 1.0-1.5 ms, but then drops down to about 1 amp (“hold current”) for the rest of the opening pulse. The resistance of a low-impedance injector is typically 1-3 ohms.

LSU-4 - Bosch wide-band oxygen sensor.

MAF - Mass Air Flow sensor. A sensor used to measure the air flow through the intake tract (which must be sealed at the senor and downstream).

MAP sensor - Manifold Absolute Pressure sensor. Measure of the absolute pressure in the intake manifold (related to the engine vacuum), to determine the load on the engine and the consequent fueling requirements. The standard MAP sensor in MegaSquirt® is the MPX4250 (2.50 BAR, or 15 psi (vacuum) + 21 psig (boost)).

MAT Sensor - Manifold Air Temperature sensor, the same as IAT. The MAT circuit is identical to the CTS circuit, see CTS, above.

MegaTune - The tuning software from Eric Fahlgren, based on Bowling and Grippo's PC Configurator software. MegaTune was developed extensively from 2002 through to about 2006, but has not been updated in years. Phil Tobin's TunerStudioMS is now the tuning software for MegaSquirt® and MicroSquirt® controllers.

MIMD - Acronym for multiple instruction, multiple data. A parallel computer with multiple independent threads of control. Conventional symmetric multiprocessors are MIMD architectures.

MOV - (Metal Oxide Varistor) A MOV will shunt to ground the current from a short-term (<20 milliseconds) high voltage spike. A MOV is non-conductive during normal operation when the voltage across it remains well below its rated 'clamping voltage'. When the clamping voltage is exceed, the MOV allows current to flow to ground, protecting devices and components 'downstream' from excessive voltage surges.

.MLS - file extension used by the MegaSquirt® fuel injection controller for storing datalogs (.XSL might also be used for datalogs with older versions of tuning software).

MPX4250AP - the internal manifold absolute pressure (MAP) sensor used in MegaSquirt. It will measure intake pressure from a total vacuum to ~21psi boost. Very early MegaSquirt® controllers (from the first group buy in 2001) may have had the MPX4115 MAP sensor (vacuum to 1 atmosphere only).

MS - MegaSquirt, used in this document to refer to the MegaSquirt® fuel injection controller or its embedded software.

.MSQ, file extension used for saving MegaSquirt® fuel injection controller user settings (aka. "parameters") on a PC computer. Since approximately 2005, these have been readable XML format, so you can open these in Notepad or similar text editors.

MT - MegaTune, Eric Fahlgren's Windows-based configuration program for the MegaSquirt® EFI controller.

NB-EGO Sensor - Narrow Band EGO sensor, gives a switch at the stoichiometric ratio (the chemically correct mixture of air and fuel), but unreliable for AFR other than stoichiometric.

NOx (Oxides of Nitrogen) - in an automotive context, generally refers to the regulated oxides of nitrogen (NOx) tail pipe emissions. See:

Ohm's Law A fundamental law of electricity, that states that the current flow (I) is equal to the voltage (V) divided by the resistance (R), or:

I = V/R

Oxides of Nitrogen (NOx) - in an automotive context, generally refers to regulated 'mono-nitrogen oxides' (NO and NO2) tail pipe emissions. See:

OEM (original equipment manufacturer) - refers to parts produced for initial assembly of a new vehicle.

Open Loop - refers to those times when MegaSquirt® ignores the feedback from the oxygen sensor.

PCB (printed circuit board) - the fiberglass board that has the MegaSquirt® component layout and circuits imprinted on it.

PC Configurator (PCC) - The original tuning software from Bowling and Grippo, it has fewer features than MegaTune, and doesn't work with MSnS-E or MegaSquirt-II.

Pull-up Circuit - A very simple circuit consisting of a voltage supply and a current limiting resistor designed to prevent a signal from floating, it forces the signal to either be high (equal to the pull up voltage) or low (grounded). See the GPO output circuit for an example.

P&H Injectors - Peak and hold injectors; see Low Impedance injectors.

PIP - Profile Ignition Pick-up is the term used for the signal sent from Ford's Electronic Distributorless Ignition System (EDIS) to the electronic control unit. This is a digitally modified alternating current (AC) signal that originates from a crank angle sensor. The PIP signal into the ECU is a square wave switched at 12 volts. It provides information about both the engine speed and position.

PW (Pulse Width) - is the amount of time a signal is applied during each period. For example, is the amount of time (in milliseconds) an injector is pulled low (grounded) to inject fuel.

Pulse Width Modulation (PWM) - A signal with a fixed pulse width (frequency), which is turned on for part of the pulse.

The percent of time that the signal is on is called its duty cycle. PWM is used to limit current to low impedance fuel injectors, as well as for some idle air control valves.

Required Fuel – (Req_Fuel) The injector pulse width, in milliseconds, required to supply the fuel for a single injection event at stoichiometric combustion, 100% volumetric efficiency and standard temperature.

RPM - In common parlance, "revolutions per minute" a measure of engine speed; it is the number of times the crankshaft completes a complete revolution in 60 seconds. In FPGA, an acronym for relatively placed macro. Xilinx specific. A group of device primitives to which RLOC= attributes have been applied, in order to constrain the placement of the group and thereby floorplan the group. For example, attributing a with RLOC=R0C0, b with RLOC=R0C0 and c with RLOC=R0C1 constrains a and b to be placed in the same CLB/slice and c to be placed in the adjacent slice in the next column. Wherever a, b, and c may be placed in the device, they'll be placed together.

SAW - Spark Advance Word is the 'returning' signal to a Ford EDIS ignition unit from the ECU that sets the amount of ignition advance requested. It is in the form of a 5 volt square wave.

SIMD - Acronym for single instruction, multiple data. A parallel computer with a single thread of control broadcast across the plurality of execution units. Example: Thinking Machines CM-1.

SoC - Acronym for system-on-a-chip, an integrated embedded computer system on a single chip. In the context of SRAM-based FPGAs, this usually discounts an external FPGA configuration ROM and external RAM.

Stim (MegaStimulator) - the Stimulator is a small board which plugs into the connector of the MegaSquirt® controller. It simulates all the sensor the inputs the MegaSquirt® controller would normally see and provides power to the MegaSquirt® controller. The Stimulator also allows you to monitor the MegaSquirt® controller's injection pulses [actual], fuel pump relay operation, and fast idle solenoid output with four LEDs.

SPOUT - Spark Out is the spark advance signal sent from MS-II™ to the Ford TFI module to set the timing advance.

TBI - Throttle Body Injection is a form of injection is which the fuel is injected above the throttle(s). It was typically used on older engines since it can be a simpler system, but is also found on some very high output racing engines because the vaporization time is longer than with port injection.

TPI - Tuned Port Injection is General Motors bank-fire port fuel injection system. It was widely used on 305 and 350 cid V8 in the mid to late 1980's.

TPS - Throttle Position Sensor, a voltage divider that gives information to a MegaSquirt® controller about throttle opening, from which it computes rate of throttle opening for acceleration enrichment.

Transistor A solid-state (no moving parts) electronic device that uses a small voltage (on the base pin) to control a larger current (across the emitter and collector pins). There are two basic types of transistors, PNP and NPN. A PNP transistor allows current to flow if there is no signal on the base, and restricts the current as the base current increases. An NPN transistor allows current to flow in proportion to the signal on the base, and restricts the current as the base current decreases. For information on selecting transistors see this web page.

TunerStudioMS Phil Tobin's alternative tuner software for tuning MegaSquirt EFI controllers. It is is 100% compatible with MegaTune INI and MSQ file's. Get it here

VAF - Vane (or Volumetric) Air Flow meter (aka. MAF = Mass Airflow Sensor)

Volatile Organic Compounds (VOC) - in an automotive context, generally refers to organic components or precursors to smog formation. Organic compounds are all chemical compounds containing carbon-hydrogen (C-H) bonds of covalent character. The U.S. Environmental Protection Agency defines a VOC as any organic compound that participates in a photoreaction. An example of a VOC is PAN (peroxyacetyl nitrate), CH3COOONO2. See:

Voltage divider A simple circuit that uses two resistors in series to reduce the output voltage Vo proportionally to the input voltage Vi. Two resistors, R1 and R2, are arranged in series, with one end of R1 connected to the external signal, and the other end of R2 connected to ground. The signal is taken from between the two resistors. From Ohm's Law we can calculate that the total current will be I = Vi/(R1+R2). The same current flows in each resistor because they are in series. The voltage drop across each resistor is: V1 = I*R1 and V2 = I*R2 = Vo. V2 is output voltage Vo, which we can write as:

Vo = Vi* R2/(R1+R2)

VB921 - a particular IGBT designed for use with automotive ignition coils, it is used with the V3 main board, for example.

VE - Volumetric Efficiency. The actual amount of air being pumped by the engine as compared to its theoretical maximum. A 200 cubic inch motor will theoretically move 200 cubic inches of air in one cycle at 100% efficiency. If the engine is actually running at 75% VE, then it will move 150 cubic inches of air on each cycle.

Vref - a 5 Volt supply used to power the TPS sensor (and sometimes other external components needing a 5 Volt supply, like ignition modules or pull-ups).

VR sensor - variable reluctor sensor is an induction type sensor, it is "passive", i.e. it does not require a power source, and has a small magnet built in.

Wasted Spark - A method of firing spark plugs in which one 'double-ended' coil simultaneously fires two coils on different cylinders. One of the cylinders is the intended 'target', and is near TDC on its compression stroke, the other cylinder is offset by 360° in the firing order (of the 720° 4-stroke cycle), so it is near TDC on its exhaust stroke. The second spark is said to be 'wasted' because it does not ignite a mixture. On the other hand, the hot ionized exhaust gases require little energy to create a spark, so nearly all the energy goes to the 'target' cylinder. An example of a wasted spark system, is Ford's EDIS. The advantage is that while a missing tooth crank wheel is required, no cam sync signal (and the corresponding wheel and sensor) are needed.

WB-EGO Sensor - Wide Band EGO sensor, can be used to derive real AFR data with mixtures from 10:1 to 20:1, i.e. anything you are likely to be interested in.

WOT - Wide open throttle.

WUE - Warm Up Enrichment, the enriched mixture applied when the coolant temperature is low.

.XLS - file extension used by the MegaSquirt® fuel injection controller for storing datalogs (.MSL might also be used for datalogs with more recent versions of tuning software).

Zener diode A diode that allows current to flow freely in only one direction, unless the reverse voltage is higher than its rated avalanche voltage, in which case it will flow in the reverse direction. Often used to limit excessive voltage to certain circuits by shunting current either to ground or the power supply.

MegaSquirt® and MicroSquirt® controllers are experimental devices intended for educational purposes.
MegaSquirt® and MicroSquirt® controllers are not for sale or use on pollution controlled vehicles. Check the laws that apply in your locality to determine if using a MegaSquirt® or MicroSquirt® controller is legal for your application.
©2004, 2012 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks. This document is solely for the support of MegaSquirt® boards from Bowling and Grippo.