SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
There are many other creative options for backdoors if I/O lines are very constrained. An ADC input can be set to an out of normal range value. As suggested above, an I/O line can perform a backdoor function at the beginning of the code and some other function after startup.