[coreboot] LPCflasher Wiki Page
peter at stuge.se
Mon Nov 24 22:11:04 CET 2008
Joseph Smith wrote:
> Ok, I have finished the "The Circuit Explained" section. I need
> some feedback, bueller?
> A series of 1K ohms resistors 250mW (1/4W) on data lines D0 - D6
> convert the signals from 5V to 3.3v needed by the flash chip.
This sentence and design is not so good. A resistor can not be used
to safely lower voltage. The ST buffer has absolute max DC Input
VCC+0.5, which is clearly exceeded by your circuit when that resistor
is the only thing between the parallell port and the buffer. The
flash chips are likely to have even stricter ratings.
I would revisit this. At the very least use two resistors as a
voltage divider, but it will probably mean that you can not program
the LPC/FWH signals as quickly as with better level shifting. Ideally
use a fast buffer with 5V-tolerant input and 3V output, but diodes
work too. Just make sure they have the right voltage drop.
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