C509-L_97 SIEMENS [Siemens Semiconductor Group], C509-L_97 Datasheet - Page 87

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C509-L_97

Manufacturer Part Number
C509-L_97
Description
8-Bit CMOS Microcontroller
Manufacturer
SIEMENS [Siemens Semiconductor Group]
Datasheet
6.1.1.2.3 Port 0 and Port 2 used as Address/Data Bus
As shown in figure 6-5 and below in figure 6-6, 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 6-5. 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 6-6
Port 2 Circuitry
Semiconductor Group
Latch
Write to
Latch
Read
Pin
Read
Int. Bus
D
CLK
Latch
Bit
Q
Q
6-9
On-Chip Peripheral Components
MUX
Addr.
Control
=1
Arrangement
Internal
Pull Up
V
CC
MCS03228
Port
Pin
1997-10-01
C509-L

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