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buspirate_spi: add support for hiz output with pullups=off

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When working with low-voltage chips, the internal 10k pull-ups
of the Bus Pirate might be too high. In such cases, it's necessary
to create an external pull-up using lower-value resistors. For this,
you can use the 'hiz' parameter.  This way, the Bus Pirate will
operate as an open drain with 'pullups=off'

So, why not just use the 'pullups=on' option for this scenario? I'm
trying to prevent a very typical human error in the training sessions
I conduct.

For example, in the previous session, a user might have left the VPU
(vextern) connected to 5V, and now they need to access a 1.8V chip.
If 'pullups=on' is used, the following will happen:
5V (VPU) --> CD4060 --> 2K (internal Bus Pirate) --> MOSI target chip.
By having the option to set a 'hiz=on', this can be avoided. Since the
pull-ups will remain deactivated while the Bus Pirate will function in
an open-drain mode

Return init error for invalid values of pullups, hiz, psus. Previously,
invalid values of the params pullups, hiz, psus were handled as "off"
(i.e. disabled). This created a potential human error when users were
running flashrom with e.g. pullups=1 and expected pullups to be on,
while the value 1 was handled as invalid and off.

Change-Id: I6d9909b23782ef3fcb1428fd127dae0d6b211855
Signed-off-by: David Reguera Garcia <dreg@rootkit.es>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/79299
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
This commit is contained in:
David Reguera Garcia
2023-12-07 03:20:51 +01:00
committed by Anastasia Klimchuk
parent 360d965ce7
commit 0dbf2ded28
2 changed files with 45 additions and 10 deletions
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8
doc/classic_cli_manpage.rst
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@ -772,6 +772,14 @@ where ``state`` can be ``on`` or ``off``.
More information about the Bus Pirate pull-up resistors and their purpose is available
`in a guide by dangerousprototypes <http://dangerousprototypes.com/docs/Practical_guide_to_Bus_Pirate_pull-up_resistors>`_.
When working with low-voltage chips, the internal 10k pull-ups of the Bus Pirate might be too high. In such cases, it's necessary to create an external pull-up using lower-value resistors.
For this, you can use the ``hiz`` parameter. This way, the Bus Pirate will operate as an open drain. Syntax is::
flashrom -p buspirate_spi:hiz=state
where ``state`` can be ``on`` or ``off``.
The state of the Bus Pirate power supply pins is controllable through an optional ``psus`` parameter. Syntax is::
flashrom -p buspirate_spi:psus=state