Siemens EF 88H Series Manual de usuario descargar pdf (Pagina 41)

Semiconductor Group 6-7

On-Chip Peripheral Components

C501

6.1.1.1 Port 0 and Port 2 used as Address/Data Bus

As shown in figure 6-7 and below in figure 6-8, the output drivers of ports 0 and 2 can be switched

to an internal address or address/data bus for use in external memory accesses. In this application

they cannot be used as general purpose I/O, even if not all address lines are used externally. The

switching is done by an internal control signal dependent on the input level at the EA

pin and/or the

contents of the program counter. If the ports are configured as an address/data bus, the port latches

are disconnected from the driver circuit. During this time, the P2 SFR remains unchanged while the

P0 SFR has 1’s written to it. Being an address/data bus, port 0 uses a pullup FET as shown in

figure 6-7. When a 16-bit address is used, port 2 uses the additional strong pullups p1 to emit 1’s

for the entire external memory cycle instead of the weak ones (p2 and p3) used during normal port

activity.

Figure 6-8

Port 2 Circuitry

MCS02123

CLK

Bit

Latch

Control

Addr.

MUX

Internal

Pull Up

Arrangement

Port

Int. Bus

Write to

Read

Latch

Read

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Siemens EF 88H Series Manual de usuario Pagina 41