[rom] Reading resistor straps should differentiate weak versus strong pulls about opentitan HOT 3 CLOSED

The ROM code is correct and only a single read is required.

The reason we need only a single read at that location is that the pinmux is already initialized with the internal pulldowns activated so that a weak pull-up will not be read as a logic-high.

There are tests in //sw/device/silicon_creator/rom/e2e/weak_straps that test all 64 strap combinations and confirm that only the ROM_BOOTSTRAP (111) and RMA_BOOTSTRAP (110) patterns are recognized by the ROM.

from opentitan.

The correct operation of strap detection can be independently verified on the ES chip by running an opentitantool console in one window and the teacup-strapping.sh script in another.

• For strapping value 63, you will see the ROM respond with bootstrap:1.
• For strapping value 60, you will see nothing because the ROM hangs (and bootloops) to wait for the JTAG LC-tap connection.
• For all other values, the next stages (ie ROM_EXT and owner code) will boot. If you happen to be running the test binary associated with //sw/device/silicon_creator/rom/e2e/weak_straps:sw_strap_value, it will read the straps and print the value when it starts.

from opentitan.

Ah good to know. Thank you for checking!

from opentitan.