In order to better support the MegaSquirt-II processor, the MegaSquirt^® printed circuit board has been updated. The new MegaSquirt^® Version 3 (V3) board incorporates a host of new features and improvements over the previous V1.01 and V2.2 board versions. It now features:

• Component Specifications: The first thing to note is that the circuit has been updated based on operational feedback from the hundreds of MegaSquirt^® installations. These updates offer significant improvements in both reliability and form. The selection of specific components maintain high reliability and offer easy-to-obtain device sourcing. The circuit has also implemented automotive temperature grade components (-40 to +125 degrees C); using the MS-II daughtercard and an extended-temp serial driver extends the temperature range to full automotive rating. The results of this is obvious - increased reliability.

• PCB Quality Uprating: The V3 Printed Circuit Board (PCB) has been completely redesigned from the ground-up, with the result of a easier-to-assemble board with new features targeted to the 'do-it-yourselfer'. The V3 PCBs are being produced by a commercial board house that has an extensive medical and industrial background. The V3 boards are 2-ounce copper finish trace, compared to the 1-ounce finish of the V2.2 boards. These boards are the highest quality available anywhere, and this quality is readily apparent in the finished product. In addition, these boards undergo a full electrical checkout to detect faults and failures, important for finding faulty boards before assembly.

• 4-Layer Board: The V3 PCB consists of four layers (top, bottom, and two inner layers), compared to only two layers on the V1.01 and V2.2 boards. The inner layers used for power and ground planes, and the top and bottom are used for signal routing. Considerable time was spent in the board layout design to properly separate the high-current power and ground from the sensitive digital power/ground. There are separate ground planes, one for the high-current drivers and the other for the digital supply ground, with both returning to the output connector at a single point. With this correct implementation, the noisy high-current switching grounds do not couple into the digital ground, significantly reducing noise and the resultant problems that a common ground system can create. In fact, board testing in a running vehicle with injector PWM and ignition flyback noise sources yields a digital power/ground plane noise level of 25 milliVolts.

• Larger PCB Traces: All of the high-current drivers (injectors, ignition) have very large PCB traces in order to handle the current demands for multiple low-impedance injectors. The trace widths of the older V2.2 board were sufficient for almost all applications, but for the V3 board the traces have been enlarged a factor of three. Tests of the V3 trace resistance (using a 10-amp injector load and voltage difference measurements along the traces) show that there is practically no voltage drops present (voltage drops indicate resistance which results in heat). Also, these traces are separated from the digital power and ground planes, as well as any other input signals, to eliminate cross-talk effects. This is very important in that high current switching of injectors and ignition coils generate very sharp current pulses and these can easily couple into sensitive measurement inputs and microprocessor circuits.

• Injector Driver Circuit Protection: The new V3 MegaSquirt^® board is much more resistant to wiring errors and component problems. There is new circuitry incorporated which protect the injector drivers from short circuits. This circuit, a board build-option during construction, limits the maximum current to 14 amps, at which point the driver FETs will transition into a "linear" region and start direct limiting current. In fact, the outputs can be shorted together for an short period of time without any damage to the driver or FETs.

• Fuel Pump/FIdle Circuit Protection: The same current limiting is used on the fuel pump and fast idle drivers, limiting each to around 0.7 amps. Shorts to ground and/or excessive current draw will not damage the driver components. In addition, both of these drivers now use a Zener diode for flyback clamping instead of a recirculation diode. The recirculation diode allowed a potential feedback path for back-powering the controller in certain installations, the Zener diode method eliminates this path.

• External Vref Protection: The external sensor 5 volt reference line (Vref) is protected with a resettable fuse, also known as a polyfuse. The entire 5 volt digital ground plane line is protected against overvoltage by a 5.6 volt Zener diode. If a short or application of full battery voltage is applied to the Vref output, the polyfuse will heat up and limit the current flow, while the Zener will keep the digital plane voltage within the proper value. There is also another polyfuse on the voltage regulator side feeding the power plane, so a failure of the regulator will not damage the digital components.

• PWM Flyback On-Board: The flyback and synchronous rectifier (flyback damper) circuits are now on the V3 board, so no external flyback board is required as was the case with version V2.2 MegaSquirt. This is a build option, so if you are using high-impedance injectors and no PWM current limiting these components can be omitted to reduce assembly cost.

• Substantial Heat-Sink: To handle the heat from the injector drivers, a "bus bar" heatsink arrangement is used in the V3 board. A simple aluminum flat bar is fitted on the top of the PCB and all of the heat generating components mount directly on this - this heatsink is easily constructed by drilling a few simple holes. The bus bar heatsink provides an enormous heat dissipation capability so the worry of overheated components is eliminated. The bus bar couples the heat to the side rail of the MegaSquirt^® case which provides even more extensive heat dissipation. Environmental chamber testing of the V3 board under full load indicates that there is at most a 12 degree C increase of temperature over ambient, worst-case component.

• Prototying Area: To keep in the spirit of do-it-yourself, a large prototype area is provided. This area is used to assemble various circuits in order to help customize the MegaSquirt^® for any application. There is also easy access to the digital power supply to provide power. Circuits, such as knock detectors, additional output drivers, etc. can be assembled here. In essence, this allows the MegaSquirt^® to adapt and grow in function without having to purchase an upgrade PCB - just add the circuit here!

• Variable Reluctor Input Circuit: To support variable reluctance (VR) trigger inputs, a new VR circuit has been included on the board. This circuit will sense the zero-crossing signal point of a VR sensor and provide proper triggering. In addition, the trigger threshold voltage is adjustable via. an on-board trimmer potentiometer. In addition, an adjustable hysteresis trimmer is also included in the circuit. With the adjustable threshold/hysteresis, the VR input circuit can also be used for hall sensor detection, even ignition coil primary triggering. Also, the original MegaSquirt^® tach input circuit is also included, and can be selected for use during board construction via. two jumpers.

• Direct Coil Driver Circuit: To support the ignition capability of MegaSquirt-II and MSnS-E, a direct ignition coil driver has been added to the V3 board. This circuit uses a VB921 (or BIP373) ignition coil driver to provide direct ignition coil control. This device has a over-current foldback mechanism to prevent ignition coil overcharging, which protects both board and ignition coil.

• Bluetooth Device Support: To support the use of Bluetooth or other wireless adapters, the serial connector (DB-9) has a source of 5 volts on pin 9 to provide direct power to external modules like the SocketModem Bluetooth Adapter. Using this adapter and a Bluetooth adapter on the PC end, a "Virtual Com" link can be established which will allow wireless operation of MegaTune or any other MegaSquirt^® tuning software. Zigbee wireless has also been operational using this method.

As well, the remaining two ADCs with circuits have been brought out, mirroring the MAT/CLT sensors. Having the ADC come out with the low-pass filter resistor/cap that then terminates to a jumper hole would be helpful. Users can then run a jumper to wherever they want.

For the new MegaSquirt^® V3 main board, the case will be the same as used for the other MegaSquirt^® boards (LMB 6x4 case or equivalent), however the end-plates are different as the DB connectors have moved.

However there are changes to both the front and rear panel. The driving force behind this was the use of an integrated heat sink "bus bar" setup. The heat sink is mode out of a easy to obtain 3/4" x 1/8" flat aluminum stock cut 6 inches long. This bus bar mounts across the top side of the PCB and all of the TO-220 power devices mount on this (there are 10 of 'em - the regulator, two drive FETs, two power Zener circuits, two flyback dampers, two TO-220-packaged power resistors-the 0.050-ohm for overcurrent detection, and the IGBT). The TO-220 devices mount to a simple 3/4" x 1/8" flat aluminum strip, 6 inches long, laid on the PCB top with the TO-220 packages bent over and screwed down thru the angle and PCB (much like the flyback board right now). This provides greatly increased heat sink capability, and even with a 3-injector (1.2 ohm) per bank load at 3K RPM the overall sink stays cool.

Note that the entire flyback circuit is on the board, as is a 12 amp overcurrent foldback circuit (the two large black rectangles on the heat sink w/ two leads are power resistors). The driver stage should be bulletproof to overheating and overcurrent.

The new board is a 4-layer board, meaning that there are separate power and ground planes sandwiched inside. The power and ground plane really helps in noise immunity and component routing. Also not visible is the fact that all of the high power components (injector, ignition) have a separate ground return path and traces back to the connector by itself without connecting to the digital/analog power planes. The injector driver output traces themselves are on the inner layers as copper pour areas back to the connector.

Note that there are many more components on the board, which has led to putting the silk screen labeling under many of the components. Not much that can be done due to the density of the board (less real-estate due to the heat sink and proto area and more components) - we will provide a component silkscreen map that people can follow. Same goes with part numbering, components on this board have different part designations compared to the other boards. There are so many new components on this board that it was hard to keep everything the same, so it was chosen to renumber everything, but this time the numbers are consecutive. This makes it a little harder to compare circuit mods with V2.2 and older, but this board has many of the mods and the prototype area will be the major area of focus for new circuitry which will need new documentation.

The V3 main board is now available in either 96HC908 (MegaSquirt/MSnS-E) or MegaSquirt-II partial kit form from Bowling and Grippo at:

www.bgsoflex.com/mspo1.html

Order MSPKIT for the 98HC908 processor (MegaSquirt or MSnS-E code) or MS2PKIT for MegaSquirt-II daughter cards. MegaSquirt^® distributors may sell complete V3 kits, check the 'Products and Distributors' forum at www.msefi.com

The Digi-Key BOM automated ordering page for the V3 main board is here:

The assembly guide for the V3 main board is here:

The parts orientation is here:

 

There is a copper area along the top - this is where the heat sink runs across the board. Also, this copper area is the ground plane for all of the power driver circuits - notice that it hooks over to the right and down to the connector pins. All power ground currents flow on this plane layer. Not shown is the ground plane for the processor+components, this is on an inner layer which also joins the connector (but this is on a different layer). So the ground plane for the power devices and the ground plane for the processor meet up right through the connector pins. None of the high-currents for the injectors or IGBT flow on the processor ground plane. Also, the XG2 ground pin for the optoisolator is routed to the power driver ground plane and not the processor ground plane.

Also not shown is the that below the ground plane for the power devices (below the square tube area) there are three more routing layers - each layer is used to route one of the three power circuits (two injector drivers and the IGBT) directly to the connector via wide traces.

The MAP sensor has also been moved to the bottom and should be close enough that the barb protrudes slightly beyond the rear cover, enough to easily slip on a vacuum hose.

The components have been re-numbered. The change in reference designators for the V3 board was not taken lightly. But with all of the change of the circuitry and the inclusion of the flyback board it was very hard to keep the same numbering. Plus, the original V1.01 and V2.2 main boards had gaps, etc. in the sequence of numbers. So, everything is renumbered.

This is really not just the same old MegaSquirt^® when it comes to build or support. The main difference is all of the build options that people can do - here are a few listed:

• Variable reluctor or coil/hall sensor,
• tach inverting installed/not,
• installing of the flyback damping section or not,
• installation of the over-current protection of the injector FETs or not,
• Install of ignition driver or not,
• Installation of the not-though-up-as-of-yet circuits for the prototype area or not,
• and, all combinations of the above, including everything.

People will have to get used to identifying which board they have - easy to do since the v3 main board has the prototype area and is an easy key to the board they have. This is the result of fulfilling the general request for the added options.

Metal Oxide Varistor (MOV)

The MegaSquirt-II PCB receives electrical current along from the vehicle's battery. If the voltage from the charging system surges or spikes (rises above the accepted level) the Metal Oxide Varistor (MOV) diverts just enough current into the outlet's grounding wire to keep the voltage supply to the components at the acceptable level.

A MOV forms a connection between the hot power line and the grounding line. A MOV has three parts: a piece of metal oxide material in the middle, joined to the power and grounding line by two semiconductors.

These semiconductors have a variable resistance that is dependent on voltage. When voltage is below a certain level, the electrons in the semiconductors flow in such a way as to create a very high resistance. When the voltage exceeds that level, the electrons behave differently, creating a much lower resistance to ground. When the voltage is correct, an MOV does nothing. When voltage is too high, an MOV can conduct 'excess' current to ground in order to eliminate the extra voltage.

Because the extra current is diverted into the MOV and to ground, the voltage in the PCB power supply line returns to a normal level, so the MOV's 'by-pass' resistance shoots up again. In this way, the MOV only diverts the surge current, while allowing the standard current to continue powering the components on the PCB.

MegaSquirt v3.0 PCB Schematics

Please note that a few component values may have been tweaked - especially in the VR input circuit, but possibly others as well. Follow the BOM values for the current component specifications and use these over the values listed on the following schematics.

Note 1: For MS-II daughter card implementation, the following signals apply to DIP-40 socket:

• 6 - CAN in
• 11 - CAN out
• 16 - V12 for IAC stepper
• 17 - IGN - JP1 pin ^#5
• 18 - Knk Window
• 35 - IAC2B - X5
• 36 - IAC2A - X4
• 37 - IAC1B - X3
• 38 - IAC1A - X2

Note 1: One of two possible tach input options are enabled during PCB build-up:

1. Opto-isolated for ignition coils (negative terminal), Hall, HEI, and EDIS setups, or
2. Variable-reluctance (VR) tach input for VR (can also be used for Hall, HEI and EDIS setups).

1. If you experience tach drop-outs at higher rpms when using the VR input circuit, you might need to replace the diode D24. The issue is with the reverse recovery time of a 1N4001, its in the 30µsec region. You can use a 1N4148 diode (1N4148DICT-ND from Digi-Key) which is much faster and should solve this problem. This became the 'standard build configuration' in October of 2008.
2. You can remove the capacitor C32. This is the capacitor in parallel with R47. Although removing this capacitor may cause a ever-so slight phase shift, it will probably not be noticeable.
3. You can use a 0.001µF capacitor in C31 (instead of a 0.1µF capacitor) which may help with high frequency signals (such as high tooth count wheels). Many later kits (post-October 2008) come with a 0.001µF capacitor as standard.

To select opto-isolator tach circuit, jumper TachSelect to OptoIn and jumper Tsel to OptoOut. For VR tach circuit (non-invert) jumper TachSelect to VrIn and jumper Tsel to VrOut. For inverted VR tach signaling, jumper VrOutInv to Tsel.

Note 2: Diode D1 may not be needed with Hall sensor (operating at 5 volts) - install jumper in its place.

Note 3: Diode D2 is normally a 1N4001 installed to add an additional 0.7V forward series voltage drop (i.e. D1 + D2 + Vf_opto_diode = 0.7 + 0.7 + 1.2 = 2.6V). For Hall sensor use, install jumper in place of diode. To use in ignition coil primary flyback pulse detection trigger, replace with 24V Zener (connected in reverse, operate in avalanche mode), eliminate C30, and lower C12 value to 470 pF.

Note 4: Jumper locations XG1 and XG2 normally jumpered. For extreme ignition coil noise, XG1 can be grounded directly to engine via external connection. XG1 can also be used for Hall sensor open-collector operation.

Please note that a few component values may have been tweaked - especially in the VR input circuit, but possibly others as well. Follow the BOM values for the current component specifications and use these over the values listed on the following schematics.

Note 1: Current limit circuit for driver FET protection. Clamp current set to ~14 amps. Circuit can be omitted if protection is not required or desired - in this case install jumper in place of R37 and R38

Note 2: Active flyback clamp circuit. If using avalanche-rated FETs for Q1/Q5 then circuit can be omitted. Clamp voltage equals Zener avalanche voltage.

Note 3: Flyback PWM damping circuit. Circuit can be omitted if PWM current limiting not used.

The top drawing A shows the current when the injector driver is on, during the high-time of the PWM waveform. Basically the FET is turned on, energizing the injector. This is the PWM high time, and it depends on the setting of the PWM duty cycle.

The second drawing B is the path when the driver is turned off during PWM flyback damping, or recirculation mode. This occurs during the off time of the PWM current limit. During this time the injector is trying to maintain the current that was there during the PWM on time (see Lenz law). So the injector will generate a current (from the collapsing magnetic field) that flows thru the FR302 diode, the TIP125 transistor, thru the +12V feed and back to the injector. The +12V feed to the MS is used as a path. Note that this is the closed loop path, this is not ground-referenced - the +12V is used as a return path, not as the potential source.

Note 1: High-power ignition drive, optional circuit - populate only if implementing drive.

Note 1: For MS-II implementation, the following connector signal jumpers are suggested: