9.8.5
LDT / PIT Status & Control Register
This register is at I/O address 21Ch. It controls the operation of the LDT.
The LDT clock frequency is selectable from two sources, namely the Local Super I/O HF Clock
output or LF Clock output. The HF Clock is derived from an internal 48MHz signal. The
frequency is selectable via a programmable divider and is divided by 4 externally before being
fed to the LDT. The LF Clock is derived from the 32kHz RTC clock. The following clock
frequencies are available:
HF Clock / 4:
12MHz, 6MHz, 4MHz, 3MHz, 2MHz, 1.5MHz, 1MHz, 0.75MHz.
LF Clock:
32.768kHz, 1Hz.
NOTE
The BIOS will configure the Local Super I/O and LDT for a 1MHz clock frequency.
7
6
5
4
3
2
1
0
|________|________|_________|________|_________|_________|_________|_________|
|
|
|
|
|
|
|
|
INTERRUPT
RFU
CLOCK
INTERRUPT
MODE
MODE
MODE
RUN
ENABLE
SELECT
FLAG
2
1
0
Bit 0: LDT / PIT Run (Read/Write)
This bit controls whether the LDT / PIT runs or is stopped.
0 = Stop (default)
1 = Run.
Bits 3 - 1: LDT / PIT Mode (Read/Write)
These bits set the mode of the timer as follows:
0 0 0 = LDT
0 0 1 = PIT 100Hz
0 1 0 = PIT 200Hz
0 1 1 = PIT 500Hz
1 0 0 = PIT 1,000Hz
1 0 1 = PIT 2,000Hz
1 1 0 = PIT 5,000Hz
1 1 1 = PIT 10,00Hz
NOTE
The above PIT rates require LDT CSR bit 5 = 0 and the HF Clock output from the
Local Super I/O controller to be 4MHz. This results in an LDT clock frequency of
1MHz.
Bit 4: LDT / PIT Interrupt Flag (Read/Clear)
The interrupt flag is active whenever the timer is running. It is set if the LDT RUN bit is set AND
either the LDT rolls over or the PIT interval expires. It can be cleared by writing to the register
with a zero in its bit position. This should be done in the LDT / PIT interrupt service routine.
0 = LDT / PIT interrupt has not occurred
1 = LDT / PIT interrupt has occurred
Bit 5: LDT / PIT Clock Source (Read/Write)
This bit selects the clock source for the LDT / PIT as follows:
0 = Super I/O HF Clock / 4 (1MHz)
1 = Super I/O LF Clock (32.768kHz)
Bit 6: Reserved
Bit 7: LDT / PIT Interrupt Enable
This bit enables the LDT / PIT interrupt output. It has no effect on the interrupt flag.
0 = interrupt disabled
1 = interrupt enabled
9-14
PP 110/01x
Additional Local I/O Functions
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
