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Semiconductor Group
28
External Bus Interface
Timing
The timing of the external bus interface, in particular the relationship between the control signals
ALE, PSEN, RD and information on port 0 and port 2, is illustrated in figure 5-1 a) and b).
Data memory:
in a write cycle, the data byte to be written appears on port 0 just before WR is
activated, and remains there until after WR is deactivated. In a read cycle, the
incoming byte is accepted at port 0 before the read strobe is deactivated.
Program memory: signal PSEN functions as a read strobe. For further information see section 5.2.
External Program Memory Access
The external program memory is accessed under two conditions:
– whenever signal EA is active; or
– whenever the program counter (PC) contains a number that is larger than 01FFFH.
This requires the ROM-less versions SAB 80C535/80535 to have EA wired low to allow the lower
8 K program bytes to be fetched from external memory.
When the CPU is executing out of external program memory, all 8 bits of port 2 are dedicated to an
output function and may not be used for general-purpose I/O. The contents of the port 2 SFR
however is not affected. During external program memory fetches port 2 lines output the high byte
of the PC, and during accesses to external data memory they output either DPH or the port 2 SFR
(depending on whether the external data memory access is a MOVX @DPTR or a MOVX @Ri).
Since the SAB 80C535/80535 has no internal program memory, accesses to program memory are
always external, and port 2 is at all times dedicated to output the high-order address byte. This
means that port 0 and port 2 of the SAB 80C535/80535 can never be used as general-purpose I/O.
This also applies to the SAB 80C515/80515 when it is operated with only an external program
memory.
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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 ...