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At system reset, the 21164 microprocessor's irq_h<3:0> pins are driven by the
clock divisor values set by four jumpers on J27. During normal operation, these sig-
nals are used for interrupt requests. The pins are either switched to ground or pulled
up in a specific combination to set the 21164 microprocessor's internal divider.
The 21164 microprocessor produces the divided clock output signal sys_clk_out1
that drives the CY2308 PLL clock-driver chip. This clock provides the references to
synchronize the 21164 microprocessor and the 21174 chip. The 21174 provides the
system memory and I/O (PCI) clock references. It also provides system-level clock-
ing to DIMMs, PCI slots, the PCI-ISA bridge, the PCI ID controller, and the PCI
arbiter.
A 14.3-MHz crystal produces the signal 14mhz_out. This signal is delivered to the
37C935 combination controller for the diskette data separator and other I/O clocks.
The combination controller produces output clock osc, which is then delivered to the
two ISA slots and the PCI-to-ISA bridge for synchronization.
4.6 Reset and Initialization
A TL7702B power monitor senses the +3.3-V rail to ensure that it is stable before
+2.5 V is applied to the 21164 microprocessor. In normal operation, should the
+3.3-V rail fall below 2.5 V, the power monitor enables shdn_l, which turns off the
+2.5-V regulator (pc164lx.32).
An external reset switch can be connected to J21 (pc164lx.28). The reset function
initializes the 21164 microprocessor and the system logic. The p_dcok signal pro-
vides a full system initialization, equivalent to a power-down and power-up cycle.
In addition, the fan sense signal (fan_ok_l) is logically ORed with the reset switch
output and, when enabled, drives rst_l, indicating a fan failure.
The rst_l signal is buffered and drives a set of sys_reset signals to reset the remain-
der of the system, including PCI and ISA devices through the 21174 chip.
Figure 4–8 shows the system reset and initialization logic.
22 January 1998 – Subject To Change
Reset and Initialization
Functional Description
4–17
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