buspirate_spi: add support for hiz output with pullups=off

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

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: I8944598b266fd77ee041f0ab03724655714a3344
Signed-off-by: David Reguera Garcia <dreg@rootkit.es>
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M buspirate_spi.c
1 file changed, 15 insertions(+), 6 deletions(-)