C515A_9708 Datasheet, PDF (47/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Reset / System Clock
C515A
5.3 Hardware Reset Timing
This section describes the timing of the hardware reset signal.
The input pin RESET is sampled once during each machine cycle. This happens in state 5 phase
2. Thus, the external reset signal is synchronized to the internal CPU timing. When the reset is
found active (low level) the internal reset procedure is started. It needs two complete machine
cycles to put the complete device to its correct reset state, i.e. all special function registers contain
their default values, the port latches contain 1's etc. Note that this reset procedure is also performed
if there is no clock available at the device. (This is done by the oscillator watchdog, which provides
an auxiliary clock for performing a perfect reset without clock at the XTAL1 and XTAL2 pins). The
RESET signal must be active for at least two machine cycles; after this time the C515A remains in
its reset state as long as the signal is active. When the signal goes inactive this transition is
recognized in the following state 5 phase 2 of the machine cycle. Then the processor starts its
address output (when configured for external ROM) in the following state 5 phase 1. One phase
later (state 5 phase 2) the first falling edge at pin ALE occurs.
Figure 5-8 shows this timing for a configuration with EA = 0 (external program memory). Thus,
between the release of the RESET signal and the first falling edge at ALE there is a time period of
at least one machine cycle but less than two machine cycles.
One Machine Cycle
S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2
P1 P2
RESET
P0
P2
ALE
PCL
Inst. PCL
OUT
IN OUT
PCH
PCH
OUT
OUT
MCT01879
Figure 5-8
CPU Timing after Reset
Semiconductor Group
5-5
1997-08-01