13-15
EXTERNAL MEMORY INTERFACE
13.6 CONFIGURATION BYTE BUS CYCLES
If EA# = 0, devices obtain configuration information from a configuration array in external mem-
ory. This section describes the bus cycles executed by the reset routine to fetch user configuration
bytes from external memory. Configuration bytes are discussed in Chapter 4, “Device Configu-
ration.”
To determine whether the external memory is set up for page mode or nonpage mode operation,
the 8XC251Sx accesses external memory using internal address FF:FFF8H (UCONFIG0). See
states 1–4 in Figure 13-16. If the external memory is set up for page mode, it places UCONFIG0
on P2 as D7:0, overwriting A15:8 (FFH). If external memory is set up for nonpage mode, A15:8
is not overwritten. The 8XC251Sx examines P2 bit 1. Subsequent configuration byte fetches are
in page mode if P2.1 = 0 and in nonpage mode if P2.1 = 1. The 8XC251Sx fetches UCONFIG0
again (states 5–8 in Figure 13-16) and then UCONFIG1 via internal address FF:FFF9H.
The configuration byte bus cycles always execute with ALE extended and one PSEN# wait state.
Figure 13-16. Configuration Byte Bus Cycles
P0
P2
ALE
PSEN#
XTAL
A7:0 = F8H
A7:0 = F8H
A7:0 = F8H
A15:8 = FFH
D7:0
D7:0
A15:8 = FFH
Nonpage Mode
State 1
A4228-01
State 2
State 3
State 4
State 5
State 6
State 7
State 8
P0
P2
A7:0 = F8H
D7:0
A15:8 = FFH
Page Mode
Содержание 8XC251SA
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Страница 3: ...May 1996 8XC251SA 8XC251SB 8XC251SP 8XC251SQ Embedded Microcontroller User s Manual...
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Страница 19: ...1 Guide to This Manual...
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Страница 31: ...2 Architectural Overview...
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Страница 41: ...3 Address Spaces...
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Страница 63: ...4 Device Configuration...
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Страница 81: ...5 Programming...
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Страница 103: ...6 Interrupt System...
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Страница 121: ...7 Input Output Ports...
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Страница 133: ...8 Timer Counters and Watchdog Timer...
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Страница 153: ...9 Programmable Counter Array...
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Страница 171: ...10 Serial I O Port...
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Страница 187: ...11 Minimum Hardware Setup...
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Страница 197: ...12 Special Operating Modes...
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Страница 207: ...13 External Memory Interface...
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Страница 239: ...14 Programming and Verifying Nonvolatile Memory...
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Страница 251: ...A Instruction Set Reference...
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Страница 391: ...B Signal Descriptions...
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Страница 401: ...C Registers...
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Страница 437: ...Glossary...
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