Sun Microelectronics
88
UltraSPARC User’s Manual
UltraSPARC has a mode that keeps its request asserted on the bus until it sees an-
other request on the bus, even if it has no more pending requests. This eliminates
one cycle of arbitration latency. This mode is enabled by hard-wiring any of the
unused Node_RQ<N> lines to logical ‘1’. UltraSPARC detects this condition dur-
ing Power-On Reset processing.
Once UltraSPARC gives up the bus to another device, it gets it back only when it
initiates another bus request. Since the UltraSPARC is the most active device on
the bus in a uniprocessor system, it is highly probable that it will be parked on
the bus.
The arbitration cycle for the SC and I/O device is delayed until UltraSPARC
drops its request when it sees the new request. Thus, these devices pay a latency
penalty to access the bus.
7.4.3.3 Rules for Addr_Valid
Addr_Valid is a radial bidirectional signal between each UltraSPARC and SC, as
shown in Figure 7-10. It is driven by the CURRENT DRIVER. Addr_Valid tells the
SC when the CURRENT DRIVER is driving a valid packet; it is needed because the
CURRENT DRIVER may keep its request asserted for longer than the minimum time
required to deliver a packet or packets.
When the SC is CURRENT DRIVER, Addr_Valid informs a port that it should re-
ceive a packet from the SYSADDR bus.
Rules for the assertion/deassertion of Addr_Valid:
1. During reset, SC drives all Addr_Valid signals to a deasserted state and
releases them when RESET_L is deasserted. This initializes the holding
amplifiers to a known state.
2. Addr_Valid is asserted for the first cycle of each two-cycle packet; it is
deasserted for the second cycle.
3. The value of Addr_Valid must be maintained by holding amplifiers in the
SC when there is no active driver. Any UltraSPARC that drives Addr_Valid
always drives it low (deasserted) before releasing it. Thus, the holding
amplifier holds it in the low state.
4. UltraSPARC drives Addr_Valid during the entire time it is CURRENT
DRIVER.
5. The UltraSPARC or SC must have driven Addr_Valid low in or before the
last cycle it is CURRENT DRIVER. See Figure 7-14 on page 90.
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