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| 型号： | S80486-DX4-75-S-V-8-B |
| 厂商： | ADVANCED MICRO DEVICES INC |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | 32-BIT, 75 MHz, MICROPROCESSOR, PQFP208 |
| 封装： | SQFP-208 |
| 文件页数： | 15/69页 |
| 文件大小： | 1070K |
| 代理商： | S80486-DX4-75-S-V-8-B |
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Enhanced Am486 Microprocessor
AMD
22
PRELIMINARY
4.8.2.2
HOLD Bus Arbitration Implementation
The HOLD/HLDA bus arbitration scheme is used prima-
rily in systems where all memory transfers are seen by
the microprocessor. The HOLD/HLDA bus arbitration
scheme permits simple write-back cache design while
maintaining a relatively high performing system. Figure
3 shows a typical system block diagram for HOLD/
HLDA bus arbitration.
Note: To maintain proper system timing, the HOLD
signal must remain active for one clock cycle after HITM
transitions active. Deassertion of HOLD in the same
clock cycle as HITM assertion may lead to unpredictable
processor behavior.
4.8.2.2.1 Processor-Induced Bus Cycles
In the following scenarios, read accesses are assumed
to be cache line fills. The cases also assume that the
core system logic does not return BRDY or RDY until
HITM is sampled. The addition of wait states follows the
standard 486 bus protocol. For demonstration purpos-
es, only the zero wait state approach is shown. Table 6
explains the key to switching waveforms.
CPU
L2 Cache
DRAM
Local Bus
Peripheral
I/O Bus
Interface
Slow
Peripheral
Address Bus
Data Bus
Address Bus
Data Bus
Figure 3. Typical System Block Diagram
for HOLD/HLDA Bus Arbitration
4.8.2.2.2 External Read
Scenario: The data resides in external memory (see
Figure 4).
Step 1 The processor starts the external read access
by asserting ADS = 0 and W/R = 0.
Step 2 WB/WT is sampled in the same cycle as BRDY. If
WB/WT = 1, the data resides in a write-back cache-
able memory location.
Step 3 The processor completes its burst read and as-
serts BLAST.
4.8.2.2.3 External Write
Scenario: The data is written to the external memory
(see Figure 5).
Step 1 The processor starts the external write access
by asserting ADS = 0 and W/R = 1.
Step 2 The processor completes its write to the core
system logic.
4.8.2.2.4 HOLD/HLDA External Access TIming
In systems with two or more bus masters, each bus
master is equipped with individual HOLD and HLDA
control signals. These signals are then centralized to
the core system logic that controls individual bus mas-
ters, depending on bus request signals and the HITM
signal.
Table 6. Key to Switching Waveforms
Waveform
Inputs
Outputs
Must be steady
Will be steady
May change from
H to L
Will change
from H to L
May change from
L to H
Will change
from L to H
Don’t care; any
change permitted
Changing;
state unknown
Does not apply
Center line is
High-impedance
“Off” state
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