82C89 Datasheet, PDF (13/15 Pages) Intersil Corporation – CMOS Bus Arbiter

82C89
Timing Waveform
STATE
CLK
(6)
THVCH
T4
(1)
S2, S1, S0
(12)
TCLLL1
LOCK
(SEE NOTE 1)
T1
T2
T3
TCLCL
TCLCH
(2)
TCHCL
TSVCH ⁽³⁾
(4)
THVCL
(7)
TCLL2
(13)
T4
TSHCL (5)
SYSB/RESB
AEN
(SEE NOTE 3)
(SEE NOTE 2)
TCLSR1 (15)
PROCESSOR CLK RELATED
BUS CLK RELATED
BCLK
(18) TBLBRL
BREQ #2
BPRN #2
(BPRO #1)
BPRO #2
(BPRN #3)
(11)
TBHCL
TBLPOH (19)
(14)
TPNBL
(20)
TPnPO
BUSY
(25) TBLCBL
CBRQ
TBLBYH
(22)
(26) TBLCBH
(SEE
NOTE 2)
(16)
TCLSR2
(24)
(23)
TBLAEL TCLAEH
TBLBL
(10)
TBYSBL (8)
TBLBYL (21)
TCBSBL (9)
NOTES:
1. LOCK active can occur during any state, as long as the relationships shown above with respect to the CLK are maintained. LOCK inactive
has no critical time and can be asynchronous. CRQLCK has no critical timing and is considered an asynchronous input signal.
2. Glitching of SYSB/RESB is permitted during this time. After θ2 of T1, and before θ1 of T4, SYSB/RESB should be stable to maintain sys-
tem efficiency.
3. AEN leading edge is related to BCLK, trailing edge to CLK. The trailing edge of AEN occurs after bus priority is lost.
ADDITIONAL NOTES:
The signals related to CLK are typical processor signals, and do not relate to the depicted sequence of events of the signals referenced to
BCLK. The signals shown related to the BCLK represent a hypothetical sequence of events for illustration. Assume 3 bus arbiters of priori-
ties 1, 2 and 3 configured in serial priority resolving scheme (as shown in Figure 3). Assume arbiter 1 has the bus and is holding BUSY low.
Arbiter #2 detects its processor wants the bus and pulls low BREQ #2. If BPRN #2 is high (as shown), arbiter #2 will pull low CBRQ line.
CBRQ signals to the higher priority arbiter #1 that a lower priority arbiter wants the bus. [A higher priority arbiter would be granted BPRN
when it makes the bus request rather than having to wait for another arbiter to release the bus through CBRQ]. *Arbiter #1 will relinquish the
multi-master system bus when it enters a state not requiring it (see Table 1), by lowering its BPRO #1 (tied to BPRN #2) and releasing BUSY.
Arbiter #2 now sees that is has priority from BPRN #2 being low and releases CBRQ. As soon as BUSY signifies the bus is available (high),
arbiter #2 pulls BUSY low on next falling edge of BCLK. Note that if arbiter #2 didn’t want the bus at the time it received priority, it would pass
priority to the next lower priority arbiter by lowering its BPRO #2 [TPNPO].
Note that even a higher priority arbiter which is acquiring the bus through BPRN will momentarily drop CBRQ until it has acquired the bus.
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