C517A_99 Datasheet, PDF (15/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Introduction
C517A
Table 1-1
Pin Definitions and Functions (cont’d)
6\PERO
PSEN
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304)3 3/&&
22
49
ALE
23
50
EA
24
51
P0.0 - P0.7 26, 27,
30 - 35
52 - 59
*) I = Input
O = Output
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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.
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.
I
External Access Enable
When held high, the C517A executes
instructions from the internal ROM as long
as the PC is less than 8000H. When held
low, the C517A fetches all instructions from
external program memory. For the C517A-L
this pin must be tied low. For the C517A-4R,
if the device is protected (see section 4.6)
then this pin is only latched during reset.
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. Port 0 also outputs the code bytes
during program verification in the C517A-
4R. External pullup resistors are required
during program verification.
Semiconductor Group
1-9