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When setting up a circuit for the TI MSP430 - the reset line in all example circuits has a 47k Ohm resistor (R1 below) on it. How critical is that specific value and why that value?

This is what I've currently been using to do this.

Can I use something else higher or lower? 47k isn't an SMD part I have on hand typically. enter image description here

rfusca
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    What options do you have? 47k is in the E3 series, for goodness sake. Even if you have nothing else, you should have 1, 2.2, and 4.7 in decades from 10 to 1M. Are you hoping to replace this with a 56k part or a 1k? – Kevin Vermeer Dec 08 '11 at 04:31
  • @Kevin - I'm not saying that 47k is some whacky insane part that I can't believe they're asking for...its just not something I have. I've got a 100k,75k,22k,10k around that range. I'm less concerned with trying to replace it with something I have (it'd be nice, but not needed)..I can order some. But I'd like to understand why something would or wouldn't be a suitable alternative. – rfusca Dec 08 '11 at 04:58
  • @rfusca, as many others often say, if you also link to the document that is the source of your information it can be of great help. – Kortuk Dec 08 '11 at 18:29
  • @Kortuk - Updated with a link, if there's something better and more definite - I'd love to find it. – rfusca Dec 08 '11 at 18:35
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    Could you put two of the 22k resistors in series? If it's a 'hack' waiting for stock to get in, you could make a triangle and sorta fit them in. You could also double up on the 100k resistors and put two of them in parallel. – W5VO Dec 08 '11 at 21:19

2 Answers2

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I would expect that several factors will put constrains on the size of the pull up resistor:

  1. It must be big enough such JTAG driving circuit (programmer/debugger) will be able to override it,
  2. It must be small enough that leakage current/noise will not change logic value of the line,
  3. The RC constant that remove /RST after supply ramp up will have to be chosen to ensure proper reset.

Check documentation of uC and JTAG programmer for leakage and drive strength. Based on that size your pull up resistor. I would try to keep RC constant the same which would mean re-sizing capacitor. Note however that schematic you posted put maximum restriction on the capacitor to 2.2nF.

Standard disclaimer: You should follow manufacturer recommendation. There is good reason for that as good gals and guys that made the chip may have put really strange things into it (that are just waiting to bite you). What is worst such things may not be documented as describing them would reveal trade secret. If needed contact your support channel (usually not possible for small customers or DIYers - in this case experiment!). Note that I am not affiliated with TI and I do not have "inside" knowledge of the device.

mazurnification
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According to my experiences with MSP430, Atmel AVR, PIC and Megawin 8051, the reset resistor value is somewhat important. It has a lower and higher limit when programming, but when you running your sode on your card or breadboard. The reset resistor value is not that much important, you can hook up 10k 47k 22k generally doesn't make difference.

I am using Launchpad for programming, it has 47k for reset pin. In my circuit I used ~500k to decrease current comsunption of reset pull up resistor.

This suggestion is based on my experience, and it comes with no warranty!

Hasan
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