If restricted to write(addr, data) and read(addr, &data) APIs, how would you approach identifying a shorted internal signal in a device?

Question

Anonymous · Accepted Answer

A short circuit in an internal signal implies that this signal is electrically
connected to another signal, possibly VCC or GND. This will cause unexpected
behavior when either of the shorted signals is driven.
Algorithm
 1. Initialization
    Gather information about the device:Addressable memory range.
    Data width at each address.
    Any known 'safe' addresses (e.g., read-only registers that won't affect
    device operation when written to).
    If no safe addresses are known, start with the highest address and work
    downwards to minimize the risk of disrupting critical functionality early in
    the process.
    
 2. Write-Read-Verify Loop
    For each address addr in the memory range:read(addr, &original_data): Read
    the original data at the address.
    new_data = ~original_data: Invert the original data.
    write(addr, new_data): Write the inverted data to the address.
    read(addr, &read_back_data): Read back the data from the same address.
    
    Compare:If read_back_data == new_data, the write was successful, move to the
    next address.
    
    If read_back_data != new_data:write(addr, original_data): Restore the
    original data to minimize disruption
    Potential Short: There's a potential short circuit involving a signal
    affected by writing to this address.
    
    Further Investigation:Try writing different patterns to the address and
    observe the read-back data.
    If the read-back data consistently reflects an unexpected pattern or is
    stuck at a particular value regardless of the written data, it strengthens
    the possibility of a short circuit.
    Log the address and the observed behavior for further analysis.
    
 3. Analysis

* Correlate the addresses where unexpected behavior was observed.
 * Look for patterns:Are the problematic addresses contiguous or spread out?
   Do specific bit patterns in the read-back data suggest a connection to VCC,
   GND, or another signal?
   
 * Use your understanding of the device's functionality (even if limited) to
   hypothesize which internal signals might be shorted based on the affected
   addresses.

If restricted to write(addr, data) and read(addr, &data) APIs, how would you approach identifying a shorted internal signal in a device?

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