C517A Datasheet, PDF (8/67 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C517A
Table 2
Pin Definitions and Functions (cont’d)
Symbol
XTAL2
Pin Number
P-MQFP-100
12
XTAL1
13
P2.0 - P2.7 14 - 21
PSEN
22
ALE
23
*) I = Input
O = Output
I/O*)
–
–
I/O
O
O
Function
XTAL2
is the input to the inverting oscillator amplifier and input to
the internal clock generator circuits.
To drive the device from an external clock source, XTAL2
should be driven, while XTAL1 is left unconnected.
Minimum and maximum high and low times as well as rise/
fall times specified in the AC characteristics must be
observed.
XTAL1
is the output of the inverting oscillator amplifier. This pin is
used for the oscillator operation with crystal or ceramic
resonator.
Port 2
is an 8-bit quasi-bidirectional I/O port with internal pullup
resistors. Port 2 pins that have 1's 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
characteristics) 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 1's. 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 periods except during
external data memory accesses. The signal remains high
during internal program execution.
The Address Latch enable
output is used for latching the address into external
memory during normal operation. It is activated every six
oscillator periods except during an external data memory
access.
Semiconductor Group
8
1997-10-01