Open Import Fuel Tuner - Fascination Software

Core Component Selection

The "Freedom-16 mite" SS16v2T from Intec Automation Inc is lacking two crucial systems necessary to achieve the OIFT goals - a D/A converter and a power supply.

Power Supply
Although a car battery is rated for 12 volts, the typical automotive systems voltage is often in the range of 13-14 volts. If the OIFT used a linear voltage regulator (7805 or similar) to supply the microcontroller system power, the regulator would have to drop 8-9 volts through itself. At around 500mA, the regulator would be heating itself by 4-5 watts. That seems to be quite a bit for a TO-220 device, so we decided to use a switching regulator instead.

Maxim makes several such regulators in an 8-pin DIP case which require few external components, and one of them happens to fit the OIFT application. The MAX744A produces up to 750mA of regulated 5 volt power at efficiencies of 85-95%. It requires only a few external components including several capacitors and an inductor. This device will accept an input voltage of 6v-16v and costs $6.48 for single quantities. This regulator seems well-suited for this application and will likely be included in the initial hardware design.

Digital-To-Analog Converter
There are several different contenders which may be used for a D/A converter. The minimum qualifications for this project are: ease of interface, voltage output range, resolution, and price. We have compiled a list of several DACs which may fit the bill:

Analog Devices
AD7249

TI
TLV5614

Linear
LTC1664

Maxim
MAX525

Number of DACs

Resolution

12 bits

10 bits

12 bits

Package

16 pin DIP

20 pin DIP

Frequency

125kHz

100kHz

50kHz

80kHz

Output Range

0-5v

0-4.6v

0-5v

Vcc Voltage

+12v @ 180mW

+5v @ 8mW

+5v @ 1.5mW

+5v @ 4mW

Price

$18.63

$10.00

$4.00

$33.01

Out of these four DACs, the Linear LTC-1664 looks the most promising. Not only is this device the cheapest, but its only disadvantage is that it is a 10-bit converter. The other converters have more serious drawbacks. The Analog Devices part requires a regulated +12v source and only has two converters, while the TI part doesn't have a full +5v output swing. The Maxim part requires a separate precision voltage reference (8 pin DIP) and is rather expensive. So unless we find a more suitable component soon, we'll probably wind up using the Linear DAC.

Interface Component Selection

It will be necessary to use extra interface chips and circuits to properly connect the "Steroid Stamp" microcontroller to the automotive signals. For instance the ignition timing and Karman sensor signals may require peak detectors for input, and/or monostable multivibrators for ouput. We will post more information regarding these component selections after the signals have been identified and researched.

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