C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 43

• Manufacturer Part Number: C505_9708
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

4.1.2 Timing
The timing of the external bus interface, in particular the relationship between the control signals
ALE, PSEN, RD, WR and information on port 0 and port 2, is illustated in figure 4-1 a) and b).
Data memory:
Program memory: Signal PSEN functions as a read strobe.
4.1.3 External Program Memory Access
The external program memory is accessed under two conditions:
When the CPU is executing out of external program memory, all 8 bits of port 2 are dedicated to an
output function and must not be used for general-purpose I/O. The content of the port 2 SFR
however is not affected. During external program memory fetches port 2 lines output the high byte
of the PC, and during accesses to external data memory they output either DPH or the port 2 SFR
(depending on whether the external data memory access is a MOVX @DPTR or a MOVX @Ri).
4.2
The read strobe for external program memory fetches is PSEN. It is not activated for internal
program memory fetches. When the CPU is accessing external program memory, PSEN is
activated twice every instruction cycle (except during a MOVX instruction) no matter whether or not
the byte fetched is actually needed for the current instruction. When PSEN is activated its timing is
not the same as for RD. A complete RD cycle, including activation and deactivation of ALE and RD,
takes 6 oscillator periods. A complete PSEN cycle, including activation and deactivation of ALE and
PSEN, takes 3 oscillator periods. The execution sequence for these two types of read cycles is
shown in figure 4-1 a) and b).
4.3
In some applications it is desirable to execute a program from the same physical memory that is
used for storing data. In the C505 the external program and data memory spaces can be combined
by the logical-AND of PSEN and RD. A positive result from this AND operation produces a low
active read strobe that can be used for the combined physical memory. Since the PSEN cycle is
faster than the RD cycle, the external memory needs to be fast enough to adapt to the PSEN cycle.
Semiconductor Group
- whenever signal EA is active (low); or
- whenever the program counter (PC) content is greater than 3FFF H
PSEN, Program Store Enable
Overlapping External Data and Program Memory Spaces
in a write cycle, the data byte to be written appears on port 0 just before WR is
activated and remains there until after WR is deactivated. In a read cycle, the
incoming byte is accepted at port 0 before the read strobe is deactivated.
4-3
External Bus Interface
C505 / C505C
1997-08-01