4 ms·

Since the bitstream maps directly to the connections, and wrong connections can create short circuits, I wonder how often a corrupted bitstream actually destroy

Since the bitstream maps directly to the connections, and wrong connections can create short circuits, I wonder how often a corrupted bitstream actually destroys a chip?

kens 6y ago
Interesting question. The currents involved are very small, so it might not be enough to destroy the chip. One unexpected thing I noticed in the chip is a control signal to pull lines low during startup. CMOS doesn't like floating signals (since you can end up with the pull-up and pull-down transistors both on). But before the configuration is loaded, the routing lines may not be connected to anything. So the FPGA has to pull these lines low until everything is configured, and then release them.
ThrowawayR2 6y ago
Not an expert but from what I recall reading, it used to be possible on the early ones; people would put a finger on the FPGA when testing a new configuration and immediately cut power to the board if it started heating up abnormally. In modern FPGAs, there's thermal protection built in. [EDIT] See also: https://electronics.stackexchange.com/questions/445323/can-you-actually-break-an-fpga-by-programming-it-wrong https://electronics.stackexchange.com/questions/445323/can-y...
- mng2 6y ago
  Only the most modern FPGAs have thermal protection, but before that they added a CRC check to the bitstream. This leaves open the question of malicious bitstreams...
duskwuff 6y ago
Older parts could be damaged somewhat easily by putting multiple drivers onto an internal tristate bus. If one driver is high and another is low, a current flows and one of the drivers is damaged. Newer parts (Spartan-3 and later, maybe even earlier) dropped that feature -- it's no longer possible to create internal bus contention. You can still cause issues by driving an external pin that conflicts with an external driver, but that's pretty standard; even microcontrollers have that issue.

Since the bitstream maps directly to the connections, and wrong connections can create short circuits, I wonder how often a corrupted bitstream actually destroys a chip?

kens 6y ago

Interesting question. The currents involved are very small, so it might not be enough to destroy the chip. One unexpected thing I noticed in the chip is a control signal to pull lines low during startup. CMOS doesn't like floating signals (since you can end up with the pull-up and pull-down transistors both on). But before the configuration is loaded, the routing lines may not be connected to anything. So the FPGA has to pull these lines low until everything is configured, and then release them.

ThrowawayR2 6y ago

Not an expert but from what I recall reading, it used to be possible on the early ones; people would put a finger on the FPGA when testing a new configuration and immediately cut power to the board if it started heating up abnormally. In modern FPGAs, there's thermal protection built in. [EDIT] See also: https://electronics.stackexchange.com/questions/445323/can-you-actually-break-an-fpga-by-programming-it-wrong https://electronics.stackexchange.com/questions/445323/can-y...

mng2 6y ago
Only the most modern FPGAs have thermal protection, but before that they added a CRC check to the bitstream. This leaves open the question of malicious bitstreams...

duskwuff 6y ago

Older parts could be damaged somewhat easily by putting multiple drivers onto an internal tristate bus. If one driver is high and another is low, a current flows and one of the drivers is damaged. Newer parts (Spartan-3 and later, maybe even earlier) dropped that feature -- it's no longer possible to create internal bus contention. You can still cause issues by driving an external pin that conflicts with an external driver, but that's pretty standard; even microcontrollers have that issue.