User’s Manual
5-2
05.99
Reset / System Clock
C513AO
The time required for a reset operation is the oscillator start-up time plus 2 machine cycles. Under
normal conditions, this must be at least 10 - 20 ms for a crystal oscillator. This requirement is
typically met using a capacitor of 4.7 to 10
µ
F. The same considerations apply if the reset signal is
generated externally (see 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 5-1
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, 1, 2, and 3 default in 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 to 3) output “1”s.
The contents of the internal RAM and XRAM of the C513AO are not affected by a reset. After
power-up, the contents are undefined, while it remains unchanged during a reset if the power supply
is not turned off.
C513AO
RESET
V
DD
+
C513AO
RESET
C513AO
RESET
&
+
V
DD
a)
b)
c)
MCS03291