C504 Datasheet, PDF (9/49 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C504
Table 1
Pin Definitions and Functions (cont’d)
Symbol
XTAL2
XTAL1
Pin Number I/O
(P-MQFP-44) *)
14
–
15
–
P2.0-P2.7 18-25
I/O
PSEN
26
O
ALE
27
O
*) I = Input
O = Output
Function
XTAL2
Output of the inverting oscillator amplifier.
XTAL1
Input to the inverting oscillator amplifier and input to the
internal clock generator circuits.
To drive the device from an external clock source, XTAL1
should be driven, while XTAL2 is left unconnected. There
are no requirements on the duty cycle of the external clock
signal, since the input to the internal clocking circuitry is
divided down by a divide-by-two flip-flop. Minimum and
maximum high and low times as well as rise/fall times
specified in the AC characteristics must be observed.
Port 2
is a bidirectional I/O port with internal pullup resistors. Port
2 pins that have 1s written to them are pulled high by the
internal pullup resistors, and in that state can be used as
inputs. As inputs, port 2 pins being externally pulled low
will source current (I_IL, in the DC characteris-tics) because
of the internal pullup resistors. 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.
The Program Store Enable
output is a control signal that enables the external program
memory to the bus during external fetch operations. It is
activated every six oscillator periodes except during
external data memory accesses. Remains high during
internal program execution.
The Address Latch Enable
output is used for latching the low-byte of the address into
external memory during normal operation. It is activated
every six oscillator periodes except during an external data
memory access. When instructions are executed from
internal ROM (EA=1) the ALE generation can be disabled
by bit EALE in SFR SYSCON.
Semiconductor Group
9