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Functional Description
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3-7
3
Single-beat Reads/Writes
Single-beat cycles to and from the PowerPC 60x bus do not achieve data
rates as high as do four-beat cycles. The Falcon pair does take advantage
of the PowerPC 60x address pipelining as much as possible for single-beat
accesses.
Single-beat writes are the slowest kind of accesses because they require
that the Falcon pair perform a read cycle then a write cycle to the DRAM
in order to complete. Fortunately, in most 60x systems, single-beat
accesses can be held to a minimum especially with data cache and
copyback modes in place.
DRAM Speeds
The Falcon pair can be configured for 3 different speeds of DRAM: 50ns,
60ns, and 70ns. When the Falcon pair is configured for 50ns DRAMs, it
assumes that the devices are Extended Data Out (EDO) parts. When the
Falcon pair is configured for 70ns DRAMs it assumes that the devices are
fast page mode parts. When the pair is configured for 60ns DRAMs, it
allows the devices to be either fast page or EDO parts. Performance
summaries using the different devices are shown in
,
,
and
Table 3-1. PowerPC 60x Bus to DRAM Access Timing when Configured for
70ns Fast Page Devices
ACCESS TYPE
CLOCK PERIODS REQUIRED FOR:
Total
Clocks
1st
Beat
2nd
Beat
3rd
Beat
4th
Beat
4-Beat Read after Idle (Quad-word
aligned)
10
1 3
1
15
4-Beat Read after Idle (Quad-word
misaligned)
10
4
1
1
16
4-Beat Read after 4-Beat Read
(Quad-word aligned)
9/3
1
1 3
1
14/8
4-Beat Read after 4-Beat Read
(misaligned)
7/2
1
4
1
1
13/8
4-Beat Write after Idle
4
1
1
1
7