Information Technology Reference
In-Depth Information
5.1.6 Clock rates
The system board runs at several clock frequencies. These are:
•
Processor speed - the processor, TXC and SRAM run at the system frequency (such as
66 MHz),
•
PCI bus speed - TCX, PIIX3 and PCI slots.
•
24 or 48 MHz - USB (Universal Serial Bus).
•
12 MHz - keyboard.
•
24 MHz - floppy clock.
•
14 MHz - ISA bus OSC.
•
8 MHz - ISA bus clock.
5.1.7 ISA/IDE interface
The IDE and ISA buses share several data, address and control lines. Figure 5.6 shows the
connections to the buses. The IDE interface uses the DD[12:0] and LA[23:17] lines, and the
ISA uses these lines as SBHE#, SA[19:8}, CS1S, CS3S, CS1P, CS3P and DA[2:0]. A multi-
plexor (MUX) is used to select either the ISA or IDE interface lines.
5.1.8 DMA interface
The PIIX3 device incorporates the functionality of two 8237 DMA controllers to give seven
independently programmable channel (channels 0-3 and Channels 5-7). DMA channel 4 is
used to cascade the two controllers and defaults to cascade mode in the DMA channel mode
(DCM) register. Figure 5.7 shows the interface connections.
SD[15:0]
IOCS16#
MEMCS16#
MEMR#
MEMW#
AEN
IOCHRDY
IOCHK#
SYSCLK
BALE
IOR#
IOW#
SMEMR#
SMEMW#
0WS#
SA[7:0]
ISA
PIIX3
DD12/SBHE#
DD[11:0]/SA[19:8]
LA23/CS16
LA22/CS3S
LA21/CS1P
LA20/CS3P
LA[19:17]/DA[2:0]
M
U
X
DIOR#
DIOW#
DDRQ[1:0]
DDAK[1:0]#
IORDY
SOE#
SDIR
DD13
IDE
Figure 5.6
IDE/ISA interface with PIIX3
