Electronics > Microcontrollers

Poweron problems using PIC and automotive application

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deephaven:

--- Quote from: mtechmatt on July 10, 2012, 04:11:02 pm ---
--- Quote from: deephaven on July 10, 2012, 03:38:37 pm ---PICs don't normally need a capacitor on their MCLR pin as they have a built-in reset timer. I would ditch the capacitor and both diodes, just have a 10K pull up resistor and the programming pin connected straight to MCLR.

--- End quote ---

Could this cause issues with the 13v being back fed into the circuit when programming?

--- End quote ---

If you are using MCLR for I/O as well, then yes it could cause possible issues with what else you've attached to it. In that case you would need the diodes.

mtechmatt:
Hi,

No there is no other usage of the MCLR line other than for programming.

I have found loads of ICs to do this job on the reset pin for me which will probably simplify things greatly, is the PIC active low(I think it is as I tie to VCC) rather than active high?

I am confused to which way round the active goes. The PIC needs 5v on the MCLR line to be 'running'... can someone please clarify?

Would this be a suitable device, it appers to put out 5v once the trip of 4.65v has been reached..
.
http://uk.farnell.com/microchip/mcp100t-475i-tt/supervisory-circuit-sot-23-3-100/dp/9758380

Cheers,
Matt

mtechmatt:
after looking at the datasheet, it appears that microchip recommend this device (MCP100) using an active ow, to drive their standard MCLR (which has a line above, meaning inverted, or reset is NOT active when low.)

If I am wrong can someone point it out please.... Cheers

All very confusing!! :)

Short Circuit:
Reset is active when MCLR is low, the PIC is active when reset is not active, so when MCLR is not low. :)
If the MCLR from the PIC is also an output (many CPU reset pins work bidirectional), then you'd want a
reset device with OpenCollector/OpenDrain output and use the external pullup resistor.

mtechmatt:
SC,

So, I know the PIC is active when MCLR=5v (high)
so if my MCP100 is an ACTIVE HIGH type, does this mean its output is HIGH when it is finished, or only HIGH, when its RESET feature is active. Obviously I want the one that outputs 0v when the power first comes on, and after it reaches its threshold, it needs to go HIGH. I think it is the MCP100T as I have chosen in that datasheet, though I really dont want to get it round the wrong way at this point :)

Thanks,
Matt

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