reset pin

[rebeltaz 36]

I know that I should have probably asked this a long time ago, but...

 

On my past projects, I have simply tied the reset pin directly to the 3.3v supply line. I am not sure where I saw that, or if I just decided that on my own. I haven't had a problem with that yet.

 

Reading the Family User's Guide today, I find that the suggested reset circuit is a 47K resistor to the positive (3.3v) supply and a .001 to .01uf capacitor to ground. I have been reading all night and most source on the net (here included) say that the capacitor is optional but that the resistor is essential. Why? What harm will befall my circuits that are currently running with out it? The User's Guide is a little vague as to the reason why.

 

Also, on the same lines, I have always used the LaunchPad as an external programmer, with a jumper on my circuits between the reset pin and 3.3v line that I remove when I want to program the chip in-circuit. If my circuit has it's own power supply, I know that I need to connect the Test , Reset and Ground lines from the LaunchPad to program it, but using the R/C reset circuit described in the User's Guide, do I still have to remove the external voltage to the reset pin? Where does the LaunchPad reset line tie? To the junction of the 47k resistor and the 3.3v line or to the junction of the 47k resistor, the 0.001 capacitor and the IC's reset line?

I hope that makes sense. Thanks.

[cubeberg 540]

The Launchpad schematic is probably a good place to look (I've attached an image of the important area).  The resistor works exactly the same way a pull-up resistor works on a button.  It keeps the pin at a positive voltage - but when it's shorted to ground to reset the chip - you're not directly shorting VCC to GND - allowing the full connection to GND to pull the pin to a low/GND value (vs the weaker connection to VCC  through the resistor).  I believe the capacitor is to keep the reset from bouncing and resetting your chip a couple of times in a row (hardware debouncing).  Probably important in terms of a hardware button that bounces - not an issue with programming from your LP since the capacitor on board is also connected to your target circuit at that point.

 

The nice thing is that if you use the pull-up resistor configuration in your board - when you connect your LP for programming - you have to pull-up resistors - but they're both fine together.  No need to remove a jumper.

 

Other than shorting VCC to GND if you've got a reset button in your board - I'm not sure if being pulled up to full VCC might damage the board - my guess is not.  However - it might result in some unnecessary current usage.

[kenyee 5]

FWIW, I can tell you that they mean it when they say the 2.2uF cap is the max you can put on.  I soldered on a 100uF one by mistake and it wouldn't boot at all :-P

 

If what you're doing works, it's probably fine.  My guess is power usage as well.  And a 4.7K resistor is a bad value to use too.  Soldered on that by mistake one time and it wouldn't boot either :-P

 

Yeah, I need to organize parts for soldering better :-)

[mbeals 74]

 

 If my circuit has it's own power supply, I know that I need to connect the Test , Reset and Ground lines from the LaunchPad to program it, but using the R/C reset circuit described in the User's Guide, do I still have to remove the external voltage to the reset pin? 

 

If your programer and the board are dual supply but running at the same voltage, then it shouldn't matter much...but I wouldn't consider it good practice.   Ideally you either power the target from the programmer or use isolation to maintain separate ground loops.

[cde 334]

The rev 1.4 launchpad doesnt have the cap, but the rev 1.5 does.

[cubeberg 540]

The rev 1.4 launchpad doesnt have the cap, but the rev 1.5 does.

Is it not populated?  The schematic I posted was from v1.4 which shows a cap on the reset line.

[rebeltaz 36]

OK, yeah I saw that schematic, but again... not much in the way of WHY (bad habit my dad taught me years ago when I was apprecticing electronics under him as a kid - always asky WHY )

 

So in essence, unless I am doing ICP or using a reset button, I shouldn't need the resistor and cap. OK, cool. Thanks.

 

I accidentally used a 0.1 cap instead of a 0.01 cap and (with the jumper removed) the LauchPad couldn't reset the board until I removed the cap. D'oh! So, yeah... they mean it

 

So, with the pull up resistor, I tie the LauchPad's reset line to the junction of the 47K and the uPC reset pin?

 

Thanks, guys.

[cde 334]

Is it not populated?  The schematic I posted was from v1.4 which shows a cap on the reset line.

Yea, it also shows c24 (The p1.3 button debouncing cap) as DNP (do not populate), on both v1.4 and v1.5 schematics, but it's present on the v1.4 but not on the v1.5.

 

c14, the Reset Cap, is not physically present on my v1.4 board, but is on my v1.5, side by side comparison. Unfortunately, I only have one v1.4 to check.

[roadrunner84 466]

The combination works as a low pass filter, or a delay circuit. If you have a slow rising power, the msp might bout up during a brown power voltage. By using the R/C network you ensure that reset will come high after the voltage is in the stable range.

If you're using a lab supply or plain battery these will probably be no issues, but it can become a problem if you're using an on-board regulator to get power from.

[rockets4kids 204]

The cap also helps debounce the reset button, if you have one.  Also, you can't go much above 0.001 uF (1 nF) if you still want to be able to program the chip.

[roadrunner84 466]

Caps do not debounce input signals, they smear out voltage change! Worst case (with non-Schmitttriggered inputs) it can cause even more problems, since you guarantee to have no clean change from one level to the other.

[JWoodrell 285]

the reason for the capacitor and resistor on the reset pin is simple... I don't remember where I read it, but it makes sense.  it is there so that when the chip is powering up it is held in reset for some predetermined amount of time, the larger the RC circuit the longer it is held in reset while the power supplies stabilize.  then once its up, the resistor holds the reset pin high so it doesn't randomly reset like if it was floating. *edit* hehe roadrunner already posted this, i didn;t read the whole thread before posting */edit*

 

you can have as large of a capacitor on the line as you want, as long as the programming input is isolated from the capacitance it doesn't care what is there.

 

this is how I set up my stuff, and it seems to work well

[rebeltaz 36]

I like that... Cool. Thanks. No Ground connection on your programming header?

[JWoodrell 285]

no I am usually space constained so I only put the test and reset explicitly together, there is almost always a vcc and ground somewhere else i can tag into to program it