C541U Ver la hoja de datos (PDF) - Siemens AG
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C541U Datasheet PDF : 65 Pages
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C540U
C541U
Table 2
Pin Definitions and Functions (cont’d)
Symbol
Pin Numbers
I/O*) Function
P-LCC-44 P-SDIP-52
P2.0 - P2.7 24 - 31 28 - 35
I/O 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.
PSEN
32
38
O 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.
ALE
33
39
O 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.
EA
35
42
I
External Access Enable
When held high, the C540U/C541U executes
instructions from the internal ROM as long as the
PC is less than 1000H for the C540U or less than
2000H for the C541U. When held low, the C540U/
C541U fetches all instructions from external
program memory. For the C540U-L/C541U-L this
pin must be tied low.
*) I = Input
O = Output
Semiconductor Group
10
1997-10-01
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