
PRELIMINARY
23
A
82454KX/GX (PB)
INIT#
O,
CMOS
INITIALIZATION. INIT is asserted by the PB (Compatibility PB in an 82454GX dual
PB system) to generate a soft reset to the processor. If INIT is asserted on the
falling edge of reset, BIST executes in the CPU before the processor boots from
ROM.
In an 82454GX dual PB system, this signal is only available on the Compatibility
PB and is not available on the Auxiliary PB.
PCIRST#
O,
CMOS
PCI RESET. PCIRST# is asserted by the PB to reset PCI bus devices for power-on
reset, programmed hard reset (TRC Register), and programmed PCI reset
(PCIRST Register).
PCLK
O,
CMOS
PCI CLOCK. This signal is an output that is derived from the processor clock
(derived frequency is 1/2 the host bus frequency). The derived PCI clock should be
externally buffered with a low skew clock driver. An external pull-down resistor is
required on this signal.
PCLKIN
I,
CMOS
PCI CLOCK INPUT. PB reference clock for all PCI bus transactions in both PB PCI
clock modes. All PCI timing is referenced to the rising edge of this clock. PCLKIN is
provided by an external low skew clock driver and should be coincident with PCLK
at the PCI slots. This can be achieved by adjusting trace lengths.
PWRGD
I,
CMOS
POWER GOOD. PWRGD provides a power-on reset to the PB (see Section 3.7).
The PB asserts PCIRST# when PWRGD is not asserted to tri-state the busses to
prevent contention of active output buffers on the PCI bus.
In an 82454GX dual PB system, all PBs assert PCIRST# when PWRGD is not
asserted. Only the Compatibility PB uses PWRGD as a power-on reset.
RESET#
I/O,
GTL+
RESET. The PB resets the host bus devices (asserts RESET#) on power-up or
when programmed through the TRC Register. The PB initializes its internal
registers to the default values, except for the Bridge Device Number Register and
the Configuration Driven on Reset Register.
In an 82454GX dual PB system, only the Compatibility PB drives this signal. For
Auxiliary PBs, this signal is an input.
Table 4. Clock, Reset, and Support Signals (Continued)
Signal
Type
Description