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I'm attempting to make my own ISP programmer for an Atmega168-20PU. For the sake of learning I want to have an LED flash as bits are being flashed to the chip. If I use an SMD LED 0603 size, in series with 220 ohm resistor in between the 5V and MOSI line will I risk corrupting data due to voltage drop across LED? What are some other concerns I should be aware of.

I want to make a similar use-case as this: TX LED

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    \$\begingroup\$ Depending on your data rate, you might not be able to see anything happening with the LED becuase the data gets transferred too fast. At 250kbps, if you program the whole Atmega168 it will only take about 0.5 seconds. \$\endgroup\$
    – BeB00
    Apr 2 '18 at 19:54
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    \$\begingroup\$ Use a transistor to drive your LED. \$\endgroup\$ Apr 2 '18 at 19:54
  • \$\begingroup\$ @BeB00 - thats a good point, SPI can be pretty quick ~MHz why have an LED limit the speed? I didn't think about that. \$\endgroup\$
    – Niko_Jako
    Apr 2 '18 at 19:56
  • \$\begingroup\$ Most programmers are quick, but usually all have a LED to tell you something is happening. The actual data is meaningless. \$\endgroup\$ Apr 2 '18 at 20:01
  • \$\begingroup\$ @Niko_Jako You may want to consider dividing the frequency down so that even at high rates it blinks at a reasonable rate for a human. Personally, I'd probably just program an MCU to blink the LEDs at a slow rate while the communications are taking place. Much easier. I like blinking LEDs because they will get attention even from the corner of your eye, where a solid light may not. So it is good user interface practice to try for blinking, if you can manage it. I like the idea. It's just implementation details as an issue for you. \$\endgroup\$
    – jonk
    Apr 3 '18 at 3:36

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