C515A_9708 Datasheet, PDF (35/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

External Bus Interface
C515A
4 External Bus Interface
The C515A allows for external memory expansion. The functionality and implementation of the
external bus interface is identical to the common interface for the 8051 architecture with one
exception : if the C515A is used in systems with no external memory the generation of the ALE
signal can be suppressed. Resetting bit EALE in SFR SYSCON register, the ALE signal will be
gated off. This feature reduces RFI emissions of the system.
4.1 Accessing External Memory
It is possible to distinguish between accesses to external program memory and external data
memory or other peripheral components respectively. This distinction is made by hardware:
accesses to external program memory use the signal PSEN (program store enable) as a read
strobe. Accesses to external data memory use RD and WR to strobe the memory (alternate
functions of P3.7 and P3.6). Port 0 and port 2 (with exceptions) are used to provide data and
address signals. In this section only the port 0 and port 2 functions relevant to external memory
accesses are described.
Fetches from external program memory always use a 16-bit address. Accesses to external data
memory can use either a 16-bit address (MOVX @DPTR) or an 8-bit address (MOVX @Ri).
4.1.1 Role of P0 and P2 as Data/Address Bus
When used for accessing external memory, port 0 provides the data byte time-multiplexed with the
low byte of the address. In this state, port 0 is disconnected from its own port latch, and the address/
data signal drives both FETs in the port 0 output buffers. Thus, in this application, the port 0 pins are
not open-drain outputs and do not require external pullup resistors.
During any access to external memory, the CPU writes FFH to the port 0 latch (the special function
register), thus obliterating whatever information the port 0 SFR may have been holding.
Whenever a 16-bit address is used, the high byte of the address comes out on port 2, where it is
held for the duration of the read or write cycle. During this time, the port 2 lines are disconnected
from the port 2 latch (the special function register).
Thus the port 2 latch does not have to contain 1s, and the contents of the port 2 SFR are not
modified.
If an 8-bit address is used (MOVX @Ri), the contents of the port 2 SFR remain at the port 2 pins
throughout the external memory cycle. This will facilitate paging. It should be noted that, if a port 2
pin outputs an address bit that is a 1, strong pullups will be used for the entire read/write cycle and
not only for two oscillator periods.
Semiconductor Group
4-1
1997-08-01