Intel MCS 51 User Manual page 316

i~.
83C152 HARDWARE DESCRIPTION
versionof the Hold/Hold Acknowledgefeature is se-
ea are done only through DMA operations, not by
lectedby
setting the control
bit ARB in PCON. MOVXinstructions.
The functions of the ARE and REQ bits in PCON, One CPU is pro-cd to be the Arbiter and the
then, are
other, to be the Requester. The ALE Switch selects
ARB REQ
Hold/Hold Acknowledge Logic
whichCPU'sALE signalwillbe directedto the address
latch. The Arbiter's ALE is selectedif HLDA is high,
o 0
Disabled
and the Requester's
ALE is selected if= is low.
o
1
C152 generates~, detects HLDA
1 0 C152 detects ~, generatesHLDA
1 1 Invalid
'k~m-
4.3.3 USING
THE HOLD/HOLDACKNOWLEDGE
The ~~~ logic ordy affects DMA opera-
tion withexternalRAM and doesn'taffectother opera-
tions with external RAM, such as MOVXinstruction.
Figure 4.6 shows a system in which two 83C152Sare
sharinga dobal RAM. In this svstem.both CPUSare
,+DJ
270427-34
Figure 4.7. ALE Switch Select
execu~g ~om internal ROM. Neith~ CPU usea the
bus exceptto accessthe shared RAM, and such access-
The ALE Switchlogic csn be implementedby a single
74HCO0, a s shownin Figure 4.7.
L-kL
Ws
ALE
SE
tmmz
.-
-~
7
4
L
j
7
s
rP
AM
miim
ALE
5X352
REQ
270427-33
Figure 4.6. Two 83C152S Sharing External RAM
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