C509-L_97 Datasheet, PDF (14/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Introduction
C509-L
Table 1-1
Pin Definitions and Functions (cont’d)
Symbol
P2.0 – P2.7
PSEN / RDF
ALE
EA
PRGEN
*) I = Input
O = Output
Pin Number
14-21
22
23
24
25
I/O*) Function
I/O Port 2
is a 8-bit I/O port. Port 2 emits the high-order address
byte during fetches from external program memory and
during accesses to external data memory that use 16-bit
addresses (MOVX @DPTR). In this application it uses
strong internal pullup resistors when issuing 1s. During
accesses to external data memory that use 8-bit
addresses (MOVX @Ri), port 2 issues the contents of the
P2 special function register.
P2.0 - P2.7 A8 - A15
Address lines 8 - 15
O Program Store Enable / Read FLASH
The PSEN output is a control signal that enables the
external program memory to the bus during external
code fetch operations. It is activated every third
oscillator period. PSEN is not activated during external
data memory accesses caused by MOVX instructions.
PSEN is not activated when instructions are executed
from the internal Boot ROM or from the XRAM.
In external programming mode RDF becomes active
when executing external data memory read (MOVX)
instructions.
O Address Latch Enable
This output is used for latching the low byte of the
address into external memory during normal operation.
It is activated every third oscillator period except during
an external data memory access caused by MOVX
instructions.
I
External Access Enable
The status of this pin is latched at the end of a reset.
When held at low level, the C509-L fetches all
instructions from the external program memory. For the
C509-L this pin must be tied low.
I
External Flash-EPROM Program Enable
A low level at this pin disables the programming of an
external Flash-EPROM. To enable the programming of
an external Flash-EPROM, the pin PRGEN must be held
at high level and bit PRGEN1 in SFR SYSCON1 has to
be set. There is no internal pullup resistor connected to
this pin.
Semiconductor Group
1-7
1997-10-01