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MC68302FC20 Datasheet(PDF) 95 Page - Motorola, Inc

Part No. MC68302FC20
Description  Integrated Multiprotocol Processor User’s Manual
Download  480 Pages
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Maker  MOTOROLA [Motorola, Inc]
Homepage  http://www.freescale.com
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MC68302FC20 Datasheet(HTML) 95 Page - Motorola, Inc

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System Integration Block (SIB)
MOTOROLA
MC68302 USER’S MANUAL
3-45
SCR), will not activate the chip-select lines. Thus, it is conve-
nient to use one of the chip-select lines to select external ROM/
RAM that overlaps these register addresses, since, in this way,
bus contention is completely avoided during a read access to
these addresses. If, in a given application, it is not possible to
use the chip-select lines for this purpose, the IAC signal may be
used externally to prevent bus contention.
NOTE
The chip-select logic does not allow an address match during in-
terrupt acknowledge cycles.
A special case occurs when the locked read-modify-write test and set (TAS) instruction is
executed in combination with the chip selects. The assertion of wait states on the write por-
tion of the cycle will only occur if the RMCST bit in the SCR is set. Refer to 3.8.3 System
Control Bits for more details.
3.6.1 Chip-Select Logic Key Features
Key features of the chip-select logic are as follows:
• Four Programmable Chip-Select Lines
• Various Block Sizes: 8K, 16K, 32K, 64K, 128K, 256K, 512K, 1M, 2M, 4M, 8M, and 16M
Bytes
• Read-Only, Write-Only, or Read-Write Select
• Internal DTACK Generation with Wait-State Options
• Default Line (CS0) to Select an 8K-Boot ROM Containing the Reset Vector and Initial
Program
3.6.2 Chip-Select Registers
Each of the four chip-select units has two registers that define its specific operation. These
registers are a 16-bit base register (BR) and a 16-bit option register (OR) (e.g., BR0 and
OR0). These registers may be modified by the M68000 core. The BR should normally be
programmed after the OR since the BR contains the chip-select enable bit.
3.6.2.1 Base Register (BR3–BR0)
These 16-bit registers consist of a base address field, a read-write bit, and a function code
field.
FC2–FC0 —Function Code Field
This field is contained in bits 15–13 of each BR. These bits are used to set the address
space function code. The address compare logic uses these bits to determine whether an
15
13
12
210
FC2 –FC0
BASE ADDRESS (A23–A13)
RW
EN


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