Смею вмешаться
Цитата(Сергей Борщ @ Jan 17 2008, 20:13)

AIO Analog Input/output This is the Analog Input/output to/from alternate functions. The signal is connected directly to the pad, and can be used bi-directionally.
Цитата(Busel Roman @ Jan 17 2008, 22:14)

SCL, Two-wire Serial Interface Clock: When the TWEN bit in TWCR is set (one) to
enable the Two-wire Serial Interface, pin PC5 is disconnected from the port and becomes the Serial Clock I/O pin for the Two-wire Serial Interface.
По-моему, все четко и понятно - от порта отключается, но при этом к ноге остается подцепленным