Well, Fred suggested that a new thread might be appropritate to continue this discussion - so here it is.TonyS wrote:Fred, it is a bit hard for this old brain to understand the specifics of your observations. So you applied 12V to one of the analog input pins, measured 5.6V at the input to the cpu and measured 5.3V on the "5vc -analog" net / rail?Fred wrote:Dual reg + 5V6 zener + 470 ohm inputs + schottky diodes = bad conditions for the CPU. 5V1 might be a better choice IF it doesn't conduct too much at idle. If that doesn't solve it, we need to rethink the protection as it's currently not 100% safe. I measured 5.6 at the pin with 12v applied to outside. BUT, I measured 5.3 on the rail. This screws all of the ADC readings pretty badly and puts the pin at 0.6V over the digital supply which it's attached to.
Fred.
I have a bit more to say but perhaps this should be the start of a new thread in Hardware as I see that the topic of power supply design is a ToDo item for Ravage.
Thanks,
Huff
1. Based on Fred's observations, I would guess that the amount of current being shunted to the "5vdc-analog" bus during his test on his Jaguar board exceeded the current requirements for the bus under his test conditions. Since the 5V regulator can only source current (no sinking capability), I believe that it was effectively "off" otherwise the bus voltage would have been clamped to the regulator's 4.75 - 5.25V spec. So I believe that the 5.6V zener clamp on the bus was responsible for the 5.3v clamp voltage that was observed. I personally am not comfortable with the use of a 5.6V zener for this function (and definately not a 5.1V) as zeners are rather "sloppy" devices. Perhaps the input clamps could be connected to the "5vdc-cpu" bus instead (higher current requirements).
2. It's important to remember that the A/D inputs on the cpu are referenced to the "analog" bus voltage. So as long as the sensors are fed from the same supply, the resistive sensors and most likely the active sensors will provide a ratiometric value of the "analog" supply voltage with the actual value of the supply not being that important (within reason).
3. Another thing to remember is that a short to battery on the analog inputs should only be a temporary condition, tolerated by the hardware but recognized by the software as an "out-of-range" value and notify the user (via lights, sound, behaviour, ...) that there is a problem that must be resolved.
Thanks,
Huff