Daisy Chaining Example with 2 Device Requests
An I/O bus uses a daisy chaining arbitration scheme. Three I/O devices (devices 1, 2, and 3) are
connected to this bus where device 1 has the highest priority and device 3 has the lowest priority
in obtaining a grant to use the bus. Using the attached diagram, show the timing of:
(1) the bus request line,
(2) bus grant lines issued from the arbiter, device 1 and device 2 , and
(3) the bus release line
when:
(1) device 1 makes a bus request at T=10ns,
(2) device 2 makes a bust request at T=20 ns,
(3) device 1 issues a bus release 20 ns after it receives the grant (i.e., 20 ns after it sees the
incoming grant signal)
(4) device 2 issues a bus release 30 ns after it receives the grant
(5) upon seeing a request, the arbiter issues a grant after a delay of 10ns,
(6) each I/O device in the daisy chain can pass the grant signal to the next lower priority
device 10 ns after it sees the incoming grant signal
(7) a device lowers its bus request 10 ns after it receives (and takes) the grant
(8) the arbiter lowers the grant 10 ns after it sees the bus release
(9) a device releases its bus-release signal 10 ns after it has received an acknowledgment
from the arbiter
(10) the arbiter can issue a new grant 10 ns after all handshaking is complete
A handshaking protocol for daisy chaining:
Bus
Request
Arbiter
Grant
Device 1
Grant
Device 2
Grant
Bus
Release
0
20
40
60
80
100
120
140
time (ns)