Simple reset h/w question

[gbigden]

Posted on April 24, 2015 at 12:48

On the Discovery F3 board I see in the schematics that the hardware reset (NRST) is a switch to ground in parallel with a 100n capacitor. This appears to double up as a power-on reset. Is this correct and is it sufficiently reliable that an external power control/monitor chip is not needed? 

At present I am using a MCP1316M with o/c output - can I omit it for the above scheme? #stm32f303

[Hamid.Wasti]

Posted on April 24, 2015 at 13:54

The power monitor chip is needed only when you do not have a clean power supply.

At startup does your power supply rise from zero to the final voltage cleanly and monotonically over a reasonably short period of time? At shutdown does your power supply cleanly and monotonically decrease to a very low voltage? Is your power supply susceptible to brown-outs and noise? Is your power supply voltage close to the minimum operating voltage for your operating conditions? What other voltages are being used in the system and do you need to guarantee that all voltages are up before letting the processor run? If you are using a DC-DC converter, what are the startup and soft-start characteristics of your power supply? During startup, when the voltage passes the threshold at which the processor will start running, is the DC-DC converter capable of providing all the current that will be required, or is the power supply's output current limited due to soft start? These are some of the issues you need to consider in deciding whether you need a power supply monitoring chip or not.

[grant]

Posted on April 24, 2015 at 13:59

Hi there

I have never personally used an STM32F3xx but have used F0xx, F1xx, F2xx and F4xx and have never had the need for anything other than a simple RC reset circuit. 

VCC ---[10K]--*--[1K]---NRST

              |

             === 100nF

              |

             0-V

I guess your MCP may invoke reset faster, but if you are cost constrained (which I almost always am) I have never needed anything other than this.

Hope it helps.RegardsGrant    

[grant]

Posted on April 24, 2015 at 14:01

I just read the last post and agree.  I should have added that I never really have any ''nasty'' power conditions and am not operating in any critical situations (medical).

CheersGrant

[jpeacock]

Posted on April 24, 2015 at 14:05

If you are using a switching regulator with a power good output you might consider using that for the processor reset.  That way the CPU is held in reset until power is stable.  The power monitor IC is somewhat the same but usually takes longer to come out of reset since a regulator has the advantage of looking at the output voltage feedback and can anticipate stability sooner.

Caps work if you have an extremely reliable power source with an ideal rise time but I prefer to use an active reset and not rely on the external power.   Jack Peacock

[gbigden]

Posted on April 24, 2015 at 14:47

Just using USB power, with some standard regulators for 3V working

[Tesla DeLorean]

Posted on April 24, 2015 at 15:22

Ineffective resets bite a lot of people. It's not a medical issue, it's about your device starting reliably. If you like taking calls from annoyed customers that have plug and unplug your device a couple of times for it to work properly, someone in your organization will start deeming it critical.

In systems with multiple supplies, with specific start sequences, or odd rise times, you really want devices to manage/monitor them.  Most commonly you want a device capable of thresholding against the minimum operating voltages, and holding reset low for a prescribed period of time, at reset, or brownout events.