Power-On-Reset issue???

[Andrey]

I'm using QE128 64 pin. The MCU seems to lock up if the power is briefly removed and then re-applied. Anybody had experienced something like this before?

Voltage 1.86V. LVD module disabled. I am clearing SRS flags if a reset occurs. If I wait for the super-caps to discharge ~5 seconds it starts-up fine.

[Andrey]

I agree regarding the clock as a possible cause, but the datasheet is not clear if it is a Bus or CPU clock limitation. I'm starting up into 3.73 MHz bus clock. (32.768 internal clock * 1824) = 59 MHz (CPU). (59/8/2) = 3.7 MHz (BUS). Is this a part of the problem?

[bigmac]

Hello,

With a minimum supply voltage level of 1.86 volts, you likely do not meet the condition for POR to occur.  But this may be less than the minimum allowable operating supply voltage level, depending on the bus frequency used.  The supply voltage below which POR is next triggered is subject to a unit-to-unit tolerance, but may be as low as 0.9 volts.

This is why the LVD reset condition is so important, especially since your voltage ramp is presumably very slow due to the presence of the supercap. Even when a POR occurs, at the point where the POR is released, the supply voltage may not be high enough to support your operating bus frequency.  Enabling LVD reset will extend the reset duration until the supply voltage has risen sufficiently.

Regards,

Mac

[kef]

It is expected result with LVD disabled. Either keep LVD always enabled, or, enable it just before power removal.

[Andrey]

So if I have LVD disabled it would not reset the MCU?

What is the benefit of having the LVD disabled, if you just want to re-start when the power is re-applied?

[kef · Accepted solution]

LVD is what makes MCU stopped and reset when supply voltage goes out of proper operation limits. POR reset circuit is not enough. You need brown out protection. Brown out protection is provided by LVD, which should be always enabled in most of applications. LVD is disableable, which allows to reduce supply current. You may enable RESET pin and use external LVD. You may monitor supply voltage periodically and enable LVD when supply voltage is about to violate operating limits. Etc. But you can't just disable LVD and expect proper operation when voltage is too low.

[Andrey]

OK, found it. It is in the app note. AN4366