INPUT/OUTPUT PORTS2055.6 Port 0, 1, 2, and 3 Output and Floating Status Settings in CPU Power DownMode (PD, HPD)The port 0, 1, 2, and 3 output status can be set to either data output or floating whenMSM80C154S/MSM83C154S is in power down mode (PD, HPD).To set these ports to output status in power down mode, bit 0 (ALF) of the I/O control register(IOCON 0F8H) is reset to “0” before PD or HPD mode is activated (see Figure 5-11). The CPUis then stopped with the ports in data output status when power down mode is started.And to set the ports to floating status in power down mode, “1” is set in bit 0 (ALF) of the I/O control register (IOCON 0F8H) before PD or HPD mode is activated (see Figure 5-11). Theport output driver is disconnected from the port pins when power down mode is started.If “1” output from port becomes a power supply factor in respect to the external circuits whenPD or HPD mode is activated in port data output mode, the PD or HPD mode should beactivated after the port data is reset to “0” by software. And in the reverse case, PD or HPDmode is activated after the port data is set to “1”.When port pins are in floating status during PD or HPD mode, the port pin status of all pinsexcept pins 2 thru 5 of port 3 may be either open or undefined in the –0.5 to VCC+0.5V range.This mode is used only in battery back-up of CPU data.
