C517A_99 Datasheet, PDF (25/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

Fundamental Structure
C517A
2.2 CPU Timing
A machine cycle of the C517A consists of 6 states (12 oscillator periods). Each state is devided into
a phase 1 half and a phase 2 half. Thus, a machine cycle consists of 12 oscillator periods,
numbererd S1P1 (state 1, phase 1) through S6P2 (state 6, phase 2). Each state lasts one oscillator
period. Typically, arithmetic and logic operations take place during phase 1 and internal register-to-
register transfers take place during phase 2.
The diagrams in figure 2-2 show the fetch/execute timing related to the internal states and phases.
Since these internal clock signals are not user-accessible, the XTAL1 oscillator signals and the ALE
(address latch enable) signal are shown for external reference. ALE is normally activated twice
during each machine cycle: once during S1P2 and S2P1, and again during S4P2 and S5P1.
Executing of a one-cycle instruction begins at S1P2, when the op-code is latched into the instruction
register. If it is a two-byte instruction, the second reading takes place during S4 of the same
machine cycle. If it is a one-byte instruction, there is still a fetch at S4, but the byte read (which would
be the next op-code) is ignored (discarded fetch), and the program counter is not incremented. In
any case, execution is completed at the end of S6P2.
Figures 2-2 (a) and (b) show the timing of a 1-byte, 1-cycle instruction and for a 2-byte, 1-cycle
instruction.
Semiconductor Group
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