How Far Negative Do Interest Rates Have to Go Before My Mortgage Payment Goes to $0?

5/12/16 Bloomberg Markets:

Federal Reserve Chair Janet Yellen didn’t rule out using negative rates in a future crisis but emphasized that they would be adopted as a last resort.
In written responses Thursday to questions from Representative Brad Sherman, Yellen said that “while I would not completely rule out the use of negative interest rates in some future very adverse scenario, policy makers would need to consider a wide range of issues before employing this tool in the United States, including the potential for unintended consequences.”

While it would be cool to see a -25% APR on my house mortgage (which is adjustable) that's not likely.  And negative interest rates prettty well require abolishing cash.