Quote:
Originally Posted by mluckham
splitting the functions apart and connecting them again with I2C... that's excessive too.
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Well, without the expander we'll have what? One digital I/O pin free?. Per-pin, digital I/O is cheap. That 8-bit port costs about the same as a single channel I2C ADC. Also, some of the digital lines on the Arduino have PWM functionality as seen with the display backlight dimming... The expander can't do that.
If your focus is on what we already have, then yes everything is fine and dandy the way it is right now. If you focus on making the system more adaptable, adding new features, functionality and such, some hardware upgrades and reworks would be highly conducive to that outcome and therefore seem perfectly reasonable to me.
The second interrupt line could be used to measure some other PWM-based sensor (I seem to recall hearing that some cars used a PWM-based VSS?), a second injector on dual-stage setups such as the DPFI TBI used on '88-91 Civics/CRXs, or could be hooked up to the interrupt output from the port expander, triggering an input-update routine.
Personally, I would like to get both the character display and buttons on the I2C bus. That way the Arduino "brain" could be located where ever is convenient (close to the ECM, somewhere central behind the dash, whatever) while the display and buttons are situated in easy view/reach, with a 5 wire umbilical (2 power, 2 I2C, 1 backlight control) between them.
Why? The display unit could be put pretty much anywhere, including being built into the OEM clock housing:
Yeah, you can do that without I2C but the umbilical ends up being a 1/2", 13 wire mass.
What about the clock functionality? An I2C real time clock chip could keep track of the time and date, running off a coin cell when the rest of the system is switched off.