AMD Geode SC1201 - Manual (Page 156)

156 AMD Geode™ SC1200/SC1201 Processor Data Book
Core Logic Module 32579B
PIC Interrupt Sequence
A typical AT-compatible interrupt sequence is as follows.
Any unmasked interrupt generates the internal INTR signal
to the CPU. The interrupt controller then responds to the
interrupt acknowledge (INTA) cycles from the CPU. On the
first INTA cycle the cascading priority is resolved to deter-
mine which of the two 8259A controllers output the inter-
rupt vector onto the data bus. On the second INTA cycle
the appropriate 8259A controller drives the data bus with
the correct interrupt vector for the highest priority interrupt.
By default, the Core Logic module responds to PCI INTA
cycles because the system interrupt controller is located
within the Core Logic module. This may be disabled with
F0 Index 40h[0]. When the Core Logic module responds to
a PCI INTA cycle, it holds the PCI bus and internally gener-
ates the two INTA cycles to obtain the correct interrupt vec-
tor. It then asserts TRDY# and returns the interrupt vector.
PIC I/O Registers
Each PIC contains registers located in the standard I/O
address locations, as shown in Table 6-46 "Programmable
Interrupt Controller Registers" on page 304.
An initialization sequence must be followed to program the
interrupt controllers. The sequence is started by writing Ini-
tialization Command Word 1 (ICW1). After ICW1 has been
written, the controller expects the next writes to follow in
the sequence ICW2, ICW3, and ICW4 if it is needed. The
Operation Control Words (OCW) can be written after initial-
ization. The PIC must be programmed before operation
begins.
Since the controllers are operating in cascade mode, ICW3
of the master controller should be programmed with a
value indicating that the IRQ2 input of the master interrupt
controller is connected to the slave interrupt controller
rather than an I/O device as part of the system initialization
code. In addition, ICW3 of the slave interrupt controller
should be programmed with the value 02h (slave ID) and
corresponds to the input on the master controller.
PIC Shadow Register
The PIC registers are shadowed to allow for 0V Suspend to
save/restore the PIC state by reading the PICs write only
registers. A write to this register resets the read sequence
to the first register. The read sequence for the shadow reg-
ister is listed in F0 Index B9h.
PCI Compatible Interrupts
The Core Logic module allows the PCI interrupt signals
INTA#, INTB#, INTC#, and INTD# (also known in industry
terms as PIRQx#) to be mapped internally to any IRQ sig-
nal with the PCI Interrupt Steering registers 1 and 2, F0
Index 5Ch and 5Dh.
PCI interrupts are low-level sensitive, whereas PC/AT inter-
rupts are positive-edge sensitive; therefore, the PCI inter-
rupts are inverted before being connected to the 8259A.
Although the controllers default to the PC/AT-compatible
mode (positive-edge sensitive), each IRQ may be individu-
ally programmed to be edge or level sensitive using the
Interrupt Edge/Level Sensitivity registers in I/O Port 4D0h
and 4D1h. However, if the controllers are programmed to
be level-sensitive via ICW1, all interrupts must be level-
sensitive. Figure 6-9 shows the PCI interrupt mapping for
the master/slave 8259A interrupt controller.
Figure 6-9. PCI and IRQ Interrupt Mapping
6.2.7 I/O Ports 092h and 061h System Control
The Core Logic module supports control functions of I/O
Ports 092h (Port A) and 061h (Port B) for PS/2 compatibil-
ity. I/O Port 092h allows a fast assertion of the A20M# or
CPU_RST. (CPU_RST is an internal signal that resets the
CPU. It is asserted for 100 µs after the negation of POR#.)
I/O Port 061h controls NMI generation and reports system
status.The Core Logic module generates an SMI for every
internal change of the A20M# state and the SMI handler
sets the A20M# state inside the GX1 module. This method
is used for both the Port 092h (PS/2) and Port 061h (key-
board) methods of controlling A20M#.
PCI INTA#-INTD# IRQ[15:14,12:9,7:3,1]
Steering Registers
F0 Index 5Ch,5Dh
ICW1
4D0h/4D1h
16
1
12
12 4
Master/Slave
8259A PIC
INTR
IRQ[13,8#,0]
3
Level/Edge
IRQ3
IRQ4
IRQ15
Sensitivity