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Semiconductor Group
17
Central Processing Unit
3.2
CPU Timing
A machine cycle consists of 6 states (12 oscillator periods). Each state is divided into a phase 1
half, during which the phase 1 clock is active, and a phase 2 half, during which the phase 2 clock is
active. Thus, a machine cycle consists of 12 oscillator periods, numbered S1P1 (state 1, phase 1)
through S6P2 (state 6, phase 2). Each state lasts for two oscillator periods. Typically, arithmetic and
logical operations take place during phase 1 and internal register-to-register transfers take place
during phase 2.
The diagrams in figure 3-1 show the fetch/execute timing related to the internal states and phases.
Since these internal clock signals are not user-accessible, the XTAL2 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.
Execution of a one-cycle instruction begins at S1P2, when the op-code is latched into the instruction
register. lf it is a two-byte instruction, the second is read during S4 of the same machine cycle. lf 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, and the program counter is not incremented. In any case, execution is completed
at the end of S6P2.
Figures 3-1 A) and B) show the timing of a 1-byte, 1-cycle instruction and for a 2-byte, 1-cycle
instruction.
Most SAB 80(C)515 instructions are executed in one cycle. MUL (multiply) and DIV (divide) are the
only instructions that take more than two cycles to complete; they take four cycles. Normally two
code bytes are fetched from the program memory during every machine cycle. The only exception
to this is when a MOVX instruction is executed. MOVX is a one-byte, 2-cycle instruction that
accesses external data memory. During a MOVX, the two fetches in the second cycle are skipped
while the external data memory is being addressed and strobed. Figures 3-1 C) and D) show the
timing for a normal 1-byte, 2-cycle instruction and for a MOVX instruction.
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Summary of Contents for SAB 80515 Series
Page 9: ...Semiconductor Group 9 Introduction Figure 1 2 Block Diagram ...
Page 12: ...Semiconductor Group 12 Fundamental Structure Figure 2 1 Detailed Block Diagram ...
Page 18: ...Semiconductor Group 18 Central Processing Unit Figure 3 1 Fetch Execute Sequence ...
Page 73: ...Semiconductor Group 73 On Chip Peripheral Components Figure 7 25 A D Converter Block Diagram ...
Page 83: ...Semiconductor Group 83 On Chip Peripheral Components Figure 7 33 a Timer 2 Block Diagram ...
Page 215: ...Device Specifications Semiconductor Group 215 ...
Page 217: ...Device Specifications Semiconductor Group 217 Pin Configuration P LCC 68 ...
Page 219: ...Device Specifications Semiconductor Group 219 Logic Symbol ...
Page 226: ...Device Specifications Semiconductor Group 226 Figure 1 Block Diagram ...
Page 229: ...Device Specifications Semiconductor Group 229 Figure 2 Memory Address Spaces ...
Page 239: ...Device Specifications Semiconductor Group 239 Figure 4 Block Diagram of the A D Converter ...
Page 241: ...Device Specifications Semiconductor Group 241 Figure 5 Interrupt Request Sources ...
Page 242: ...Device Specifications Semiconductor Group 242 Figure 6 Interrupt Priority Level Structure ...