Electronics > Projects, Designs, and Technical Stuff
DIY Modular Test Equipment Project
void_error:
At this stage I'm jumping between the (still) nearly completed UI/aux psu/waveform gen/USB-UART boards tweaking stuff and thinking of things I might want to add.
Found one: add some energy storage caps to the UI board so they can power the EEPROM, micro, oscillator, and the optional RTC for an EEPROM write after power off as there will be no standby power supply and I'd like to have the previous settings saved until the next power on. Yes, it's going to have a "big clunkin' power switch".
There are two things that come into play here:
* Detect the 3.3V rail voltage decrease
* Write to the EEPROM before the capacitor voltage drops below a certain value, about 2.7V in this case, the micro can run at 32MHz down to 2.5V
The first one will be handled by one of the comparators, input via an externally available pin.
The second will be dealt with using the same comparator but with the input connected to the internal DAC acting as an internal voltage divider (referenced to Vdd).
The other input of the comparator will be internally connected to the internal voltage reference.
void_error:
--- Quote from: void_error on September 17, 2016, 06:13:19 pm ---Found one: add some energy storage caps to the UI board so they can power the EEPROM, micro, oscillator, and the optional RTC for an EEPROM write after power off as there will be no standby power supply and I'd like to have the previous settings saved until the next power on.
--- End quote ---
Good thing I remembered to RTFM for the oscillator, which is an ILSI ISM95 or ISM97 because it doesn't work down to 2.7V so I'll have to switch to the PIC's internal RC oscillator when a power failure is detected and that's possible according to the datasheet.
Another possible issue which I'm glad I didn't overlook could have been caused by the input protection diodes within the micro but another P-channel MOSFET which disconnects the resistor divider's positive side solved the problem.
With that said, the schematic for the UI seems to be complete... except a few part values...
EDIT: Forgot to change the nets for the encoders from +3V3 to VDD... and maybe a few more pull-ups. Corrected everything. Even had some MOSFETs backwards...:palm:
max_torque:
maybe silly question, but wouldn't it be worth doing all the different modules on a standarised card, with a std backplane connector? therefore you could add extra modules, or change the allocations easily. Even better, build them into a std rack mount enclosure like this sort of thing:
and it would look pretty professional too :-+
void_error:
Someone else also suggested that, but it would be too expensive. Rack mounted enclosures aren't cheap either.
max_torque:
whilst proper eurocard rack enclosures are expensive, i bet you could do a simple DIY solution using std back plane connectors and some laser cut acrylic or simple ally frame etc??
Design you "processing" pcb as the back plane, with the IO and numerous serial bus connections, and have the various cards slot into that!
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