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Semiconductor Group
29
External Bus Interface
5.2
PSEN, Program Store Enable
The read strobe for external fetches is PSEN. PSEN is not activated for internal fetches. When the
CPU is accessing external program memory, PSEN is activated twice every cycle (except during a
MOVX instruction) no matter whether or not the byte fetched is actually needed for the current
instruction. When PSEN is activated its timing is not the same as for RD. A complete RD cycle,
including activation and deactivation of ALE and RD, takes 12 oscillator periods. A complete PSEN
cycle, including activation and deactivation of ALE and PSEN takes 6 oscillator periods. The
execution sequence for these two types of read cycles is shown in figure 5-1 a) and b).
5.3
ALE, Address Latch Enable
The main function of ALE is to provide a properly timed signal to latch the low byte of an address
from P0 into an external latch during fetches from external memory. The address byte is valid at the
negative transition of ALE. For that purpose, ALE is activated twice every machine cycle. This
activation takes place even it the cycle involves no external fetch. The only time no ALE pulse
comes out is during an access to external data memory when RD/WR signals are active. The first
ALE of the second cycle of a MOVX instruction is missing (see figure 5-1b) ). Consequently, in any
system that does not use data memory, ALE is activated at a constant rate of 1/6 of the oscillator
frequency and can be used for external clocking or timing purposes.
5.4
Overlapping External Data and Program Memory Spaces
In some applications it is desirable to execute a program from the same physical memory that is
used for storing data. In the SAB 80(C)515, the external program and data memory spaces can be
combined by AND-ing PSEN and RD. A positive logic AND of these two signals produces an active
low read strobe that can be used for the combined physical memory. Since the PSEN cycle is faster
than the RD cycle, the external memory needs to be fast enough to adapt to the PSEN cycle.
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Содержание SAB 80515 Series
Страница 9: ...Semiconductor Group 9 Introduction Figure 1 2 Block Diagram ...
Страница 12: ...Semiconductor Group 12 Fundamental Structure Figure 2 1 Detailed Block Diagram ...
Страница 18: ...Semiconductor Group 18 Central Processing Unit Figure 3 1 Fetch Execute Sequence ...
Страница 22: ...Semiconductor Group 22 Memory Organization Figure 4 3 Mapping of the Lower Portion of the Internal Data Memory ...
Страница 30: ...Semiconductor Group 30 External Bus Interface Figure 5 1 a and b External Program Memory Execution ...
Страница 38: ...Semiconductor Group 38 On Chip Peripheral Components Figure 7 3 Output Driver Circuits of Ports 1 through 5 ...
Страница 59: ...Semiconductor Group 59 On Chip Peripheral Components Figure 7 16 a Functional Diagram Serial Interface Mode 0 ...
Страница 60: ...Semiconductor Group 60 On Chip Peripheral Components Figure 7 16 b Timing Diagram Serial Interface Mode 0 ...
Страница 61: ...Semiconductor Group 61 On Chip Peripheral Components Figure 7 17 a Functional Diagram Serial Interface Mode 1 ...
Страница 62: ...Semiconductor Group 62 On Chip Peripheral Components Figure 7 17 b Timing Diagram Serial Interface Mode 1 ...
Страница 73: ...Semiconductor Group 73 On Chip Peripheral Components Figure 7 25 A D Converter Block Diagram ...
Страница 83: ...Semiconductor Group 83 On Chip Peripheral Components Figure 7 33 a Timer 2 Block Diagram ...
Страница 111: ...Semiconductor Group 111 On Chip Peripheral Components Figure 7 54 Timing Diagram System Clock Output ...
Страница 113: ...Semiconductor Group 113 Interrupt System Figure 8 1 a Interrupt Structure of the SAB 80 C 515 80 C 535 ...
Страница 114: ...Semiconductor Group 114 Interrupt System Figure 8 1 b Interrupt Structure of the SAB 80 C 515 80 C 535 cont d ...
Страница 204: ...Semiconductor Group 204 Instruction Set XCH A Ri Operation XCH A Ri Bytes 1 Cycles 1 Encoding 1 1 0 0 0 1 1 i ...
Страница 215: ...Device Specifications Semiconductor Group 215 ...
Страница 217: ...Device Specifications Semiconductor Group 217 Pin Configuration P LCC 68 ...
Страница 219: ...Device Specifications Semiconductor Group 219 Logic Symbol ...
Страница 226: ...Device Specifications Semiconductor Group 226 Figure 1 Block Diagram ...
Страница 229: ...Device Specifications Semiconductor Group 229 Figure 2 Memory Address Spaces ...
Страница 239: ...Device Specifications Semiconductor Group 239 Figure 4 Block Diagram of the A D Converter ...
Страница 241: ...Device Specifications Semiconductor Group 241 Figure 5 Interrupt Request Sources ...
Страница 242: ...Device Specifications Semiconductor Group 242 Figure 6 Interrupt Priority Level Structure ...
Страница 268: ...Device Specifications Semiconductor Group 268 AC Testing Input Output Waveforms AC Testing Float Waveforms ...