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WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2948
Posted: 08:58am 25 Aug 2016
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Hi David,
Personally I would not install the MCP option as it potentially resets the MicroMite whenever an SD card is inserted.
The MCP is there to reset the MM in case the voltage drops too low. Typically 3v3 should be on the +v line, however as more things are added to the power rail, this voltage can drop. If it drops too low then the MM can become unstable. But with the MCP device, this monitors the power rail and will reset the MM before it becomes unstable.
The issue with SD cards is that some makes/models cause a significant 'blip' on the +v line. With the MCP in place, this blip is enough to trigger the reset. This can be really annoying in a development environment where you may be inserting an SD card on a regular basis.
With a decent power supply, and with no MCP installed, you should have a very stable MicroMite.
WW
WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2948
Posted: 09:03am 25 Aug 2016
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Continuing with my previous post; there are two more things to point out regarding an MCP reset supervisory chip:
1> On the Explore64 with PCB version 1C, there is space for an MCP reset device. If installed, then the same 'reset when inserting SD card' issue may be observed. This is not an issue with PCB version 1B as there is no provision for the MCP device
2> This is something that Geoff experience back in the MaxiMite days and his solution was to install a 2R resistor in the SD cards +v line. This does indeed work on the 'custom' MM's I have built, but there is no way to add onto the E64 (1C) or E100 modules without cutting a track.
No doubt Grogster and/or Geoff may chime in here . . .
dhester Newbie Joined: 17/07/2016 Location: United KingdomPosts: 11
Posted: 09:43am 25 Aug 2016
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WW
Thanks for the quick reply.
Since fixing the lcd problem the board has run great, so I won't bother fitting the reset circuit.
Have just got a mini ps/2 keyboard I ordered from amazon, so this weekends project will be making a case and getting the expolore & lcd to be a "stand alone" computer.
MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209
Posted: 09:44am 25 Aug 2016
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I have a different opinion and that is that a supervisory chip can prevent a lot of headache.
A voltage drop can cause lots of different things to go wrong.
Internally in the microcontroller RAM, registers, PC counter etc can loose their integrity and cause a running program to misbehave.
And when a program misbehaves lots of trouble can happen especially when machinery or equipment is controlled. A supervisory chip is a cheap and good insurance against problems.
When a SD card causes a voltage dip then that should be solved. A supervisory chip that resets in that case is a good indicator that something is wrong.
Not ignore it but fix it.
A better power supply or more capacitance can be a solution.
Edited by MicroBlocks 2016-08-26Microblocks. Build with logic.
WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2948
Posted: 10:32am 25 Aug 2016
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Totally agree - but with a decent power supply in the first place then the 'headache' doesn't exist.
It is only 'peripherals' running from the same power supply as the MM that can potentially cause the 'dip' in voltage. The MM+ by itself will not 'fluctuate' the power supply to cause any issue whatsoever.
The SD card can very much be regarded as a 'peripheral' and you are correct to say it is better to fix the issue. The 2R does fix this issue and hence no need for the MCP!
However, that said, without the MCP on an E64 (v1C) then I would not have realised there was a voltage dip that as you say, may have caused unseen 'internal' issues with the PIC.
Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9642
Posted: 03:55pm 25 Aug 2016
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Silicon Chip elected to use a 2.7v version of the MCP120 supervisory IC, whereas I used a 3.15v version. It is possible that they discovered this problem and changed the MCP120 to a lower-voltage version. I don't have anything other then 3.15v versions, and 2.7v reset voltage threshold seemed to ME to be getting to low with respect to the minimum for the PIC32 to run at, meaning that you could introduce a power-blip problem in the PIC32, but the MCP120 never resets.
I could be totally wrong here. Next time I order from Microchip(damn - I just did that yesterday!), I will order some 2.7v versions - that may well totally solve the problem.
If it proves to be a major problem - resets caused by inserting the SD card, then the easiest solution for right now is to do as WW suggests, and remove the supervisory IC - for now.
If anyone has a 2.7v version they can try in their board that has this issue already - WW? - then that would be incredibly useful information.
Version 1D of the E64 and 1C of the E100 will BOTH have 2R2 and 100n caps in an R-C filter like on the Maximite series, which cures this problem.
It does not happen with ALL uSD cards, which is why it was not picked up before now. Smoke makes things work. When the smoke gets out, it stops!
WhiteWizzard Guru Joined: 05/04/2013 Location: United KingdomPosts: 2948
Posted: 07:05pm 25 Aug 2016
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Happens with 270, 300, and 315. This is on E64 & E100
Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9642
Posted: 07:16pm 25 Aug 2016
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Useful information. Thanks. No point in changing the 3.15v ones I have for the 2.7v ones SC are using then......Smoke makes things work. When the smoke gets out, it stops!
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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