Accessing External Data Memory - Siemens C500 User Manual

States
ALE
PSEN
P0
P2
Figure 3-2
External Program Memory Fetches

3.2.2 Accessing External Data Memory

The port 2 drivers use the strong pullups during the entire time that they are emitting address bits
that are 1«s. This occurs when the MOVX @ DPTR instruction is executed and when external
program fetches are executed. During this time the port 2 latch (the special function register) does
not have to contain 1«s, and the contents of the port 2 SFR are not modified. If the external memory
cycle is not immediately followed by another external memory cycle, the undisturbed contents of the
port 2 SFR will reappear in the next cycle.
Figure 3-3 and 3-4 show in detail the timings of the external data memory read and write cycles.
Semiconductor Group
S1 S2 S3 S4
P1 P2 P1 P2 P1 P2 P1
Data
Sampled
PCL
Out
PCH Out
3-4
S5 S6 S1
P2 P1 P2 P1 P2 P1 P2
Data Data
Sampled Sampled
PCL
Out
PCH Out
CPU Timing
C500 Family
S2
P1 P2
PCL
Out
PCH Out
MCD02772
1998-04-01