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'[PIC]: EEPROM working without Vcc'
I got something strange here: I have a small system (momentarily as
prototype on a preliminary PCB) where (among other thing) a PIC (628A) is
communicating with an EEPROM (24CC64). For testing I added a connector to be
able to store variables into the EEPROM from an external device. When
programming the EEPROM externally I bring the DAT and CLK lines of the
onboard PIC into INPUT mode, so I should have full access from outside. Also
there are the recommended 10K pulls ups to Vcc. Well, all this works
Now, during testing I forgot to switch on my device, so the EERPOM was
without Vcc. Then started data transfer from outside and - miracle? - the
data where stored perfectly. I tried this about 20 times and it worked
without errors all the time. Is this normal? Or am I just lucky with my
charge of EEPROMs I have here?
An explanation might be, that there is enough power served by my external
device, means that by CLK and DAT via the pulls ups and some capacitors on
my PCB Vcc gets high enough to ensure operation.
Does anybody had a similar effect sometimes or is this totally normal and I
just don't know?
Any help greatly appreciated.
David Van Horn
> Now, during testing I forgot to switch on my device, so the EERPOM was
> without Vcc. Then started data transfer from outside and - miracle? -
> data where stored perfectly. I tried this about 20 times and it worked
> without errors all the time. Is this normal? Or am I just lucky with
> charge of EEPROMs I have here?
The device was powered by the protection diodes on it's input pins,
which you are NOT supposed to forward-bias.
Writing $00 to $0000 might have been interesting.
David Van Horn wrote:
He'd still have power from the clock line.
David Van Horn
> He'd still have power from the clock line.
Or drag the clock line down trying to source the power..
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