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Now, I know what you're thinking. I've been thinking it all night too.

Yes, I know that with the tiny fused for a 32 kHz crystal that I'll need the SPI clock to be 8 kHz or less. Even that, however, seems to not be working.

I started with a working board that had a 4 MHz crystal and divide-by-8 fusing. I first used a USBTiny to reset the fuses to the default using avrdude (62 DF FF). I then removed the crystal and its loading capacitors and tacked in a 32 kHz 12.5 pF crystal in its place. I then used the same USBTiny to set the fuses to E6 DF FF. And that's all she wrote.

With a scope, I can peek at the xtal pins and I can see 32 kHz. So I suspect the clock is running.

I knew that I'd be adding -B to the avrdude command line. Before I started all this, I experimented with various values of -B and a flash read. Sure enough, altering -B made the flash read happen faster or slower.

But I've tried adding -B values up to 250 (which I read on this page should result in a 4 kHz SPI clock). Nothing. I've tried using at least 6 different versions of the AVR-as-ISP-but-slowed-down concept with an Arduino Uno. None of them work. All of them work with an unmodified version of the device I started with (with its 4 MHz crystal and original fusing), so I'm confident the wiring is correct.

What else could it be?

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  • \$\begingroup\$ I had success in the past with rickety.us/2010/03/arduino-avr-high-voltage-serial-programmer Notice that the fuses are hard coded for a specific AVR, so check the source code. \$\endgroup\$
    – jippie
    Oct 3, 2014 at 12:22
  • \$\begingroup\$ I shouldn't have to resort to HV programming, though. Why doesn't normal ISP work? \$\endgroup\$
    – nsayer
    Oct 3, 2014 at 14:04
  • \$\begingroup\$ How slow did you get the clock - down into the 100Hz range? I assume when you say you slowed down the Arduino-as-ISP sketch, you did it by inserting delays in the program code (iirc in many versions the -B flag is not honored). \$\endgroup\$
    – Chris Stratton
    Oct 3, 2014 at 15:33
  • \$\begingroup\$ If you Google, there are at least a half dozen copies of the AVR ISP sketch that slow the SPI clock a number of ways. I don't know how slow it wound up, but to work it would have needed to be ~250 Hz, which is just silly. \$\endgroup\$
    – nsayer
    Oct 3, 2014 at 15:40
  • \$\begingroup\$ Hence a comment and not an answer. It is good to have a last resort, but in the mean time you solved it the proper way. \$\endgroup\$
    – jippie
    Oct 3, 2014 at 18:22

1 Answer 1

Reset to default
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I figured it out.

Fuse changes don't erase the flash. The previous sketch started with setting the system clock prescaler to divide-by-8. The clock prescaler survives RESET.

The fix was to temporarily tie RESET low. That way I had a chance to attach the programmer and power up without the prescaler setting taking place. I uploaded a new sketch without the prescaling and it works now.

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  • \$\begingroup\$ Congrats... was just wondering if the divider was still in, and if you were trying it slowly enough to allow for it. \$\endgroup\$
    – Chris Stratton
    Oct 3, 2014 at 15:34
  • \$\begingroup\$ Since it wasn't working, it must not have been slow enough. But at the time I didn't think it would have to be less than 1 kHz. \$\endgroup\$
    – nsayer
    Oct 3, 2014 at 15:46

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