FUJITSU MB91F109 FR30 - Manual (Page 158)

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CHAPTER 4 BUS INTERFACE
[bit 0] COE0 (Chip select Output Enable 0)
The C0E0 bit controls the CS0X output. When this bit is reset, output is permitted.
0: Inhibits output.
1: Permits output (initial value).
When the external bus mode is used, the C0E0 bit performs no I/O port control for the CS0X
pin. Always set this bit to "1".