C541U Datasheet, PDF (9/63 Pages) Siemens Semiconductor Group – 8 BIT CMOS MICROCONTROLLER

C541U
Table 1
Pin Definitions and Functions (cont’d)
Symbol
Pin I/O*) Function
Numbers
P-LCC-44
P2.0 - P2.7 24 - 31
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
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 three oscillator periods except during external data memory
accesses. The signal remains high during internal program
execution.
ALE
33
O The Address Latch enable
output is used for latching the address into external memory
during normal operation. It is activated every three oscillator
periods except during an external data memory access.
EA
35
I
External Access Enable
When held high, the C541U executes instructions from the
internal OTP program memory as long as the PC is less than
2000H for the C541U. When held low, the C541U fetches all
instructions from external program memory. For the C541U-L this
pin must be tied low.
P0.0 - P0.7 43 - 36
I/O Port 0
is an 8-bit open-drain bidirectional I/O port. Port 0 pins that have
1’s written to them float, and in that state can be used as high-
impedance inputs. Port 0 is also the multiplexed low-order
address and data bus during accesses to external program and
data memory. In this application it uses strong internal pullup
resistors when issuing 1’s.
*) I = Input
O = Output
Semiconductor Group
7