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COM20022I Datasheet(PDF) 21 Page - SMSC Corporation

Part # COM20022I
Description  10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
PDF  82 Pages
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Manufacturer  SMSC [SMSC Corporation]
Direct Link  http://www.smsc.com
Logo SMSC - SMSC Corporation

COM20022I Datasheet(HTML) 21 Page - SMSC Corporation

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10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
SMSC COM20022I
Page 21
Revision 02-27-06
DATASHEET
DREQ
nWR
DMAEN bit
Writing Address
Pointer Low
minimum 4TARB
TARB is the ARBITRATION Clock Period. It depends on the TOPR and
SLOW-ARB bit. TOPR is the period of operation clock frequency (output
of the clock multiplier). It depends on the CKUP1 and CKUP0 bits.
TARB = TOPR @ SLOW-ARB = 0
TARB = 2 TOPR @ SLOW-ARB = 1
Figure 5.3 - DREQ Pin First Assertion Timing for All DMA Modes
As an example of gating by cycle, in an ISA bus system, the Refresh period is 15
μS. Continuous transfer
by DMA must be less than 15
μS to prevent blocking by the Refresh cycle. A DMA cycle of consecutive
DMA cycles is approximately 1uS. The DMA overhead time is approximately 2.5
μS. The Refresh
execution time is 500nS. This computes to 15
μS - 2.5μS - 500nS = 12μS or 12 cycles. Therefore the
DREQ pin must be negated every 12 cycles. Figure 5.4 illustrates the rough timing of the Programmable-
Burst mode DMA transfer.



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