[MAGEEC] Chip variations: tests, results, and graphs

Steve Kerrison Steve.Kerrison at bristol.ac.uk
Sun Jul 27 17:25:58 BST 2014


Is JTAG connected during test runs? Interested to know how much difference
that makes, although it should be constant thus irrelevant.
On 27 Jul 2014 17:13, "James Pallister" <James.Pallister at bristol.ac.uk>
wrote:

>  Hi All,
>
> Some preliminary findings on variations in the ATMEGA328 chips.
>
> I've tested 28 chips. For each chip 9 tests were ran:
>
>   flash0
>  Test the area of flash 0x0800-0x0810
>   flash1
>  Test the area of flash 0x080C-0x081C  flash2
>  Test the area of flash 0x08FC-0x090C  flash3
>  Test the area of flash 0x0FFC-0x100C  ram0
>  Repeatedly access 0x0000 - 0x0020
>   ram1
>  Repeatedly access 0xF007 - 0xF017
>   alu
>  Perform combinations of mul, fmul, inc and dec
>   nop
>  Lots of nops
>   branch
>  Repeatedly branch randomly in low flash
>
> Each test was run 8 times, and outliers excluded. All were run in the same
> harness, with the same crystal, resistors, etc.
>
> Results across each test. These show the distributions of the measurements
> taken, where the distribution consists of the 28 chips.
>
>
> From the first graph: there is a significant different, even on a day to
> day basis. I'm not sure what causes this - I'd expect the power to be lower
> at lower temperatures, which we don't see.
>
> For each chip:
>
> Mean power for each chip:
>
>
>
> The average power changes quite a lot, going from < 80mW to > 100mW. In
> almost all cases, the average power was higher on Saturday - not sure why
> this is, the temperature difference was only 3-4 degrees (and I'd expect
> the temperature to go down with lower temperature).
>
> The chips tend to vary as a whole, e.g. rather than the ALU varying
> significantly in one part rather than another. The following graph marks
> the average power for each test, divided by test type (different
> color/marker combinations) per chip. This should allow hopefully allow us
> to do a calibration run first.
>
> Legend for the above graph:
>   Blue cross
>  ram1
>   Red star
>  ram0
>   Green plus
>  alu
>   Black plus
>  flash3
>   Green cross
>  flash2
>   Blue star
>  flash1
>   Red plus
>  flash0
>   Black star
>  nop
>
>
> Any thoughts / ideas for more tests are welcome.
>
>
> James
>
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