AVR Fuse bits - ATtiny12

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I'm having some pretty good luck with the ATtiny12 on the STK500
board, but after changing fuse bits, changing them back seems a bit of
a challenge.   Here's where I am...

  I can program the '12 fine with ISP serial method (I'm using Mac OS
X unix with the GNU uisp
  tool for downloading & interrogation )

  I have written several assembly programs and everything works fine.

  I can query the fuse bits, and I see the results fine  

  I can change the fuse-bits   eg. from the default 0x52 to 0x72.  

After changing the fuse-bits to turn of ISP, as expected, the ISP
interactions result in "unable to enter programming mode"

To change them, I understood that I need to use "High-Voltage
Programming" and so I followed the hookups in the STK500 doc's to wire
it for that.   I tell uisp to use hivoltage with -dhiv option.

    I can then download programs just fine still - using the high
voltage serial pgm.

    I can query the fuses - and the session goes well enough, but the
report is that the fuse bit low byte
   (the only byte in there) is at 0xFF

    I can issue a high voltage fuse-bit write and it tells me that it
has successfully written 0x52 back in
   to the part, but it still reads as 0xFF in high voltage, and when
going back to ISP programming,
   ISP seems to still be behaving as if it's 0x52 (SPI port off).

Can anyone offer any info on changing Fuse Bits after you have turned
of the SPI port? Have you ever changed it back?  Should this work...
or is the Fuse-Bit some destructive process perhaps...

Any clarification appreciated...

Ross.

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