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I have another curiosity question: Why is there a 10-pin AVR ISP connector and a 6-pin version?

It seems that the only difference is that the 10-pin version uses the extra pins for a heap more ground pins.

I personally prefer the 10-pin version because it is much easier to get 10-pin IDC connectors at most electronics stores. I have only seen 6-pin connectors at really specialized online stores like Digikey and Element14. But why is there the need for so many ground connectors?

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    \$\begingroup\$ I usually use the 6-pin version and not had any problems. \$\endgroup\$ Apr 22 at 0:04
  • \$\begingroup\$ @KevinWhite Yeah.. 6-pin is good when space is tight but it can be reversed which is sometimes a bit annoying. \$\endgroup\$
    – HotGlue
    Apr 22 at 0:09
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    \$\begingroup\$ With the ground pins the cable can be longer and still perform well. With only 6 pins you'll start to get errors if the cable is too long. How long I don't know of the top of my head. \$\endgroup\$
    – Aaron
    Apr 22 at 0:32
  • \$\begingroup\$ I rarely need to get any 6-pin connectors as they are already on the programmer. The boards use simple 2x3 male headers. \$\endgroup\$ Apr 22 at 0:33
  • \$\begingroup\$ In the case of STM, it is because there are two different protocols. 10-pin supports both JTAG and SWD, whereas 6-pin only supports SWD which is a bit more proprietary than JTAG. \$\endgroup\$
    – DKNguyen
    Apr 22 at 0:39
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The reason for the GND pins is crosstalk. This is not much of a problem with short cables but becomes an issue with longer ones. The crosstalk is caused by the capacitance between the wires in the cable.

With the AVR, the original 'standard' was 10 pin by Kanda. Only later did the 6 pin emerge. All my AVR based boards used the 10 pin. A 6 pin connector can be 'made' by chopping up a larger one if the need arises.

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