Siemens SAB 80515 Series, User Manual, Page: 34 / 270

Semiconductor Group 34
System Reset
6.1.2 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 at pin 10) the internal reset procedure is started. lt 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 not performed
if there is no clock available at the device. The RESET signal must be active for at least two machine
cycles; after this time the SAB 80(C)515 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 6-2 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 6-2
CPU Timing after RESET
MCT01879
S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2 S3 S4 S5 S6 S1 S2
P1 P2
PCL
OUT
PCL
OUT
PCH
OUT PCH
OUT
One Machine Cycle
RESET
P0
P2
ALE
IN
Inst.
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