Siemens C541U, User Manual

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Semiconductor Group 5-5 1997-10-01
Reset / System Clock
C541U
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 RESET is found
active (high 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 C541U 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 program memory) in the following state 5 phase 1.
One phase later (state 5 phase 2) the first falling edge at pin ALE occurs.
Figure 5-3 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.
Figure 5-3
CPU Timing after Reset
MCT02092
S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2
P1 P2
PCL
OUT
PCH
OUT PCH
OUT
One Machine Cycle
RESET
P0
P2
ALE
Inst.
in OUT
PCL