I've got a simple circuit with an ATtiny25 on which I indend to do in system programming. I'm wondering, will the rest of my circuit interfere with the SPI programming?


So the data sheet says:

  • PB0 = MOSI, this is used as an output so should be fine
  • PB1 = MISO, this is used as an output so should be fine
  • PB2 = SCK, this is used an input, connected to a voltage divider. So the 442 ohm resistor will cause a 7.4/11.3 mA current at 3.3/5 V, respectively.

Am I screwed here, is that too much?

Edit: I'm planning to use the Atmel-ICE programmer, but can't find any docs on how much current that can source.

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    \$\begingroup\$ As long as you're not messing with the reset pin you should be fine. And 7.4/11.3 mA is nothing. You should be looking over your shoulder if the programmer needs to give 100 mA. \$\endgroup\$ – Harry Svensson Jul 20 '17 at 9:52
  • \$\begingroup\$ @HarrySvensson That's good to hear. I looked up the SPI specs and all currents in that are in the µA range, that's what made me worried. But I suppose chip-to-chip SPI is different from ISP SPI. \$\endgroup\$ – Chris Jul 20 '17 at 9:56
  • \$\begingroup\$ @HarrySvensson So what would be considered as "messing with the reset pin"? Is my 1k resistor from vcc to reset fine? \$\endgroup\$ – Chris Jul 20 '17 at 9:59
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    \$\begingroup\$ That's just a regular pull-up that's usually on the reset pin, a little bit strong (10k would suffice), If you would've used anything less than 100 ohm's it would be within the range of "messing around". \$\endgroup\$ – Harry Svensson Jul 20 '17 at 10:26

is that too much?

no. i have done worse.

  • \$\begingroup\$ Excellent. Doesn't matter that it's a clock line? \$\endgroup\$ – Chris Jul 20 '17 at 10:31

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