C515A_9708 Datasheet, PDF (44/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Reset / System Clock
C515A
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.
Figure 5-6
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 5 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, 4 and 5) output a one (1). Port 2 lines output a zero (or one) after reset, if
EA is held low (or high). Port 6 is an input-only port. It has no internal latch and therefore the
contents of the special function registers P6 depend on the levels applied to port 6.
The content of the internal RAM of the C515A 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
1997-08-01