C517A_99 Datasheet, PDF (58/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Reset / System Clock
C517A
The time required for a reset operation is the oscillator start-up time plus 2 machine cycles, which,
under normal conditions, must be at least 10 - 20 ms for a crystal oscillator. This requirement is
typically met using a capacitor of 4.7 to 10 µF. The same considerations apply if the reset signal is
generated externally (figure 5-1 b). In each case it must be assured that the oscillator has started
up properly and that at least two machine cycles have passed before the reset signal goes inactive.
a)
b)
&
+
RESET
C517A
c)
RESET
C517A
+
RESET
C517A
MCS03323
Figure 5-1
Reset Circuitries
A correct reset leaves the processor in a defined state. The program execution starts at location
0000H. After reset is internally accomplished the port latches of ports 0 to 6 are set to FFH. This
leaves port 0 floating, since it is an open drain port when not used as data/address bus. All other I/
O port lines (ports 1,3 to 6) output a one (1). Port 2 lines output a zero (or one) after reset, if EA is
held low (or high). Port 7 and 8 are input-only ports. They have no internal latch and therefore the
contents of the special function registers P7 and P8 depend on the levels applied to port 7 or 8.
The content of the internal RAM of the C517A is not affected by a reset. After power-up the content
is undefined, while it remains unchanged during a reset if the power supply is not turned off.
Semiconductor Group
5-2