Well, it's been a while since an update :) This is part8 of the In-Vehicle Datalogger project that's been going for some time. So, first a brief recap ...
This originally started as a project to develop a cheap datalogger for automotive use. Myself and a bunch of friends all drive rather modified Supras, pushing some obscene horsepower numbers. So engine and system tuning is pretty important. And that's the problem - you can't really know what's going on inside the engine, even with the addon boost/fuel computers etc.
So after some discussion, I decided to have a stab at building a datalogger. It went through several revisions, from an 8515 with outboard A/D, to an 8535 with onboard A/D, to the current Mega128. Heh, note, I haven't actually *built* one yet.
Basic specs are as follows :
- 16 channels analog input, muxed down to the 8 channels in the Mega128. Fully buffered inputs (high source imp, low A/D imp) with 3 jumper-selectable voltage ranges. 0-5V, 0-14V and 9-14V. Those values were chosen from the very spare specifications in the Toyota service manuals. I had wanted to include CalMicro ESD transient pretoection diode arrays on the inputs, but there just isn't any more dang room on the pcb. I'll still try, but I doubt it.
- 6 channels of opto-isolated pulse inputs. Current limited with 680 ohm resistors.
- 3 onboard remote temp sensors.
- USB and standard serial interfaces. Serial is just a 3-pin header, so will need an adapter cable. Don't know yet if this can all be powered from the USB bus - would be nice :)
- MMC card for data storage. Easy to pop a new one in for unlimited storage.
- Battery backed real-time clock for date stamping of data runs.
- Remote LCD screen interface for in car. Initially will be a simpler 4x40 lcd, but I'm planning on a 256x128 graphics screen later. Maybe.
- Onboard ADXL202 G-sensor. This means that you would need some way of *really* securely mounting the box, and as accurately perpendicular/parallel to the vehicle axis as possible. It is possible to compensate for tilt, but icky.
- Single 44-pin plug for in-car use. See question below ...
- One-button operation :) Start/stop logging, and lap marker.
There will very little actual computation going on in the box, since it's main function is to log data. Then you transfer the datasets to a PC for analysis in Excel or whatever. All out of car control is via the USB or serial connection.
Some of the methods of madness behind the design. The power supply section actually has several stages. There's an LM2676 to knock voltage down to 5.5V initially - that's a switching (and hence noisy) regulator. That feeds into 3 separate LT1121 linear LDO regulators, one 3.3V for the MMC card, one 5V for the analog supply only, and the other 5V for the rest of the circuit. Hopefully the power won't contribute to noise ...
The USB interface is the Ftdi 245BM (the new one) in parallel. Lots faster than the 232 serial emulation version. The Sleep and SND/WakeUp pins are connected, though I haven't fully decided on using that functionality yet.
I must have examined half a dozen different 5V-3.3V converter schemes :) FInally Herb Poppe directed me to the MAX3377. I could swear that all the Maxim chips I looked at earlier were only available in 2500 lots, but that one was $1.86 in singles. Cool.
** One concern I have is using the single 44-pin connector for power as well. Not the current draw, but the possibility of other pins being at a potential before the +12V/GND pins connect. How bad could would that be ? If bad, then I guess you will just have to connect power via a standard 2.5mm barrel connector first. But the single connector would be nice ....
Anyway, take a look at the schematic(s). Comments, critiques etc are all welcome - finding problems first is a good thing. You guys have been a *wealth* of information for the other earlier parts as this design has evolved. Hopefully hardware glitches/gotchas will be a minimum from all that help.
The pcb layout is 95% done, on a 6.3x2.9 board. Single board now - was using a daughterboard earlier. I'll post the layout in a couple of days when it's done, though I think I want to flip the power supply section around. It's all 10mil tracks except the ones between the pins on the 44 connector - those are 8mil.