ceri clatworthy

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9 years ago.

MBED not starting at power-up

Good day,

I have a custom board, LPC1347, all working well, except at power up,

This ONLY happens from 12/24 Volt supply, Which is dropped to 5V with DC-DC converter, and then a standard 3V3 Reg.

/media/uploads/ceri/tek0000.jpg

Unfortunately, the micro NEVER starts from 12/24 Volt input.

not sure, but there is a glitch at about 2.7 Volts, dropping to 2.5 Volts during power up - BROWN-OUT ??

But ALWAYS from USB 5 Volts, which will not be available in normal use.

because 3V3, and 5V0 rise too slowly.

In the old days, with a PIC, I use to just set POR value to MAXIMUM,

Is there some way to stop the micro from starting up immediately,

I have seriously increased the reset R/C Time constant, but no effect.

Any suggestions ??

Cheers

Ceri

Have you checked to see what is happening on the 5v output of the DC-DC convertor? Should you be using an LDO 3.3v regulator?

posted by David Fletcher 17 Apr 2015

I am using a LDO Regulator. The 5 Volt rail and the 3V3 rail as well as !Reset rise the same. .. I definitely need to know how to get micro to reset later. .. Cheers Ceri

posted by ceri clatworthy 17 Apr 2015

Typically the nreset should be slower. A simple 100k/100nF RC should do it. May want to try a cpu supervisor (eg MIC6315-29D3U from Micrel) instead to get a more controlled behaviour.

See http://www.embeddedartists.com/sites/default/files/support/qsb/lpc1343/LPC1343_QuickStart_Board_Users_Guide.pdf That says: The reset generation is handled by a voltage supervisor chip, MIC6315-29D3U from Micrel. The reset signal will be held active (i.e., low) until the supply voltages, +3.3V, is within margins (above 2.93V). The reset duration is typically 200 ms (consult the MIC6315-29D3U datasheet for exact details). The output reset signal is open-drain so a pull-up resistor is needed (R15). There is a reset-LED that lights when reset is active. An external reset source can pull the reset signal low (with an open-collector/open-drain output). There is also a reset push-button (SW1) for manual triggering of a reset pulse.

posted by Wim Huiskamp 17 Apr 2015

I have looked at some reset chips, and this one sounds to be in the top 3, Although it did not come up in a Farnell search ??

I am ordering a few today. Many thanks Ceri

posted by ceri clatworthy 20 Apr 2015

1 Answer

Wim Huiskamp
9 years ago.

Does it start when you apply a hard reset after the powersupply has settled to its final value.

Could there be any voltages (eg from the 5V dc/dc) on any input or output pins before the 3V3 power has reached its final value. That could lead to some latchup and cause higher than normal currents (does the package heat up?)

Accepted Answer

Yes. .. Micro starts 99.5% of the time after a hard reset. The only source of volts is from 3V3 reg. Which is from 5V0 source. Which is from 12/24 volts.

posted by ceri clatworthy 17 Apr 2015

Every thing so far is cold, no heating up, I just looked at schematic, No 5V0 Anywhere, just straight to 3V3 LDO. But OC Driver chips have 12 Volts.

Very bizarre, - must just be a bit marginal.

Cheers

Ceri

posted by ceri clatworthy 20 Apr 2015