The signal lines that must be normal for the system to boot up
are listed here [List 1].
For signal lines other than these, even if they malfunction they
do not directly affect booting up the system.
7.5.8.1. DIGITAL BLOCK DIAGRAM
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KX-FT901CX-B / KX-FT903CX-B / KX-FT901CX-W / KX-FT903CX-W / KX-FT901CX-S / KX-FT903CX-S