Service manual

Theory
3-9
These clocks are used for specific peripherals and are generated at various locations on the PCBA.
• 20-MHz clock for Ethernet
• 24.6-MHz clock for audio codec
• 16.9-MHz clock for audio codec
• 12-MHz clock for keyboard/mouse controller
• 14.3-MHz clock for the PCI/ISA bridge and SVGA hardware
• 11.354-MHz clock divided to 1.89 MHz for SDLC
• 68.5-MHz and 34.25-MHz clock for the SVGA hardware
• 8.33-MHz clock for the PCI/ISA bridge and ISA bus
Hardware Reset
The hardware reset originates from the MAX695 power supervisor IC. The supervisor IC keeps the
860 PORESET input asserted (thereby keeping the CPU from running) when the system voltage is
less than 4.65 V. Once the system voltage exceeds 4.65 V for 200 ms, the supervisor IC releases
the 860 PORESET input, allowing the CPU to boot. The 860 generates two reset outputs, HRESET
and SRESET. HRESET is buffered and is used to reset all PCI devices and most other major
devices on the PCBA, including ISA devices. (ISA devices, however, can also be individually reset
under software control by writing to one of the ISA latches.)
Figure 3-4: Reset signals
Software Reset
To cause a software reset, the 860 processor causes a unrecoverable situation by first disabling its
internal machine check interrupt and then forcing a machine check by accessing unmapped address
space, causing the CPU to reboot.
supervisor
IC chip
MPC860
ISA bus
controller
video
controller
PCI
bridge
audio
Ethernet
SRST
VCC
PCI bus reset
HRST
POreset
IRQ0
power ON/OFF reset
(output only)
pfail
pfail
input
from
display
IRTS
reset
assembly