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TI486 Microprocessor - Al Kossow's Bitsavers

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Functional Timing<br />

As shown in Figure 4-21, the <strong>TI486</strong>DLC/E samples the A20M input on the<br />

rising edge of CLK2 corresponding to the beginning of phase 2 of the internal<br />

processor clock. If A20M is asserted and paging is not enabled, the<br />

<strong>TI486</strong>DLC/E masks the A20 signal internally starting with the next cache<br />

access and externally starting with the next bus cycle. If paging is enabled, the<br />

A20 signal is not masked regardless of the state of A20M. A20 remains<br />

masked until the access following detection of an inactive state on the A20M<br />

pin. A20M must be asserted for a minimum of two CLK2 periods and must<br />

meet specified setup and hold times to be recognized on a specific CLK2 edge.<br />

An alternative to using the A20M pin is provided by the NCO bit in the CCRO<br />

configuration register. The <strong>TI486</strong>DLC/E automatically does not cache<br />

accesses, to the first 64 KBytes and to 1 MByte + 64 KBytes, if the NCO bit is<br />

set. This prevents data within the wraparound memory area from residing in<br />

the internal cache and thus eliminates the need for masking A20 to the internal<br />

cache.<br />

Figure 4-21.<br />

Masking A20 Using A20M During Burst of Bus Cycles<br />

1 Cycle 1 1 Cycle 2 1 Cycle 3 1 Cycle 4<br />

Idle 1 Non-Pipelined 1 Non-Pipelined 1 Pipelined 1 Pipelined<br />

1 (Write) 1 (Read) I. (Write) 1<br />

~4 .1 4 .r .1 4 (Write)<br />

Ti 1 T1 1 T2 1 T1 1 T2 1 T2P 1 T1 P 1 T2P 1 T1 P 1 T21<br />

1<br />

1 Ti<br />

CLK2<br />

A19-A2, 1 1<br />

:~~~6:~~~~~ ____ ~_a~ilid_1 __ ~;)( ____ ~Va_li_d_2 __ ~ __ ~ ____ ~ ____ ~ __ ~~~~~~<br />

M/IO,D/C~<br />

I<br />

W/R~~rur 1<br />

4-44 <strong>TI486</strong>DLCIE Bus Interface

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