Intel Embedded Microcontroller, 8XC251SA Port 0 and Port 2 Status, Port Bit/16-bit Nonpage Mode

8XC251SA, SB, SP, SQ USER’S MANUAL

13.7 PORT 0 AND PORT 2 STATUS

This section summarizes the status of the port 0 and port 2 pins when these ports are used as the external bus. A more comprehensive description of the ports and their use is given in Chapter 7, “Input/Output Ports.”

When port 0 and port 2 are used as the external memory bus, the signals on the port pins can orig- inate from three sources:

•the 8XC251Sx CPU (address bits, data bits)

•the port SFRs: P0 and P2 (logic levels)

•an external device (data bits)

The port 0 pins (but not the port 2 pins) can also be held in a high-impedance state. Table 13-3 lists the status of the port 0 and port 2 pins when the chip in is the normal operating mode and the external bus is idle or executing a bus cycle.

Table 13-3. Port 0 and Port 2 Pin Status In Normal Operating Mode

Port	8-bit/16-bit	Nonpage Mode		Page Mode
	Addressing	Bus Cycle	Bus Idle	Bus Cycle	Bus Idle
Port 0	8 or 16	AD7:0 (1)	High Impedance	A7:0 (1)	High Impedance
Port 2	8	P2 (2)	P2	P2/D7:0 (2)	High Impedance
	16	A15:8	P2	A15:8/D7:0	High Impedance

NOTES:

1.During external memory accesses, the CPU writes FFH to the P0 register and the register con- tents are lost.

2.The P2 register can be used to select 256-byte pages in external memory.

13.7.1 Port 0 and Port 2 Pin Status in Nonpage Mode

In nonpage mode, the port pins have the same signals as those on the 8XC51FX. For an external memory instruction using a 16-bit address, the port pins carry address and data bits during the bus cycle. However, if the instruction uses an 8-bit address (e.g., MOVX @Ri), the contents of P2 are driven onto the pins. These pin signals can be used to select 256-bit pages in external memory.

During a bus cycle, the CPU always writes FFH to P0, and the former contents of P0 are lost. A bus cycle does not change the contents of P2. When the bus is idle, the port 0 pins are held at high impedance, and the contents of P2 are driven onto the port 2 pins.

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