User`s guide

Operation Theorem •35
4.6.2 Pacer Trigger Source
The timer #1 and timer #2 are cascaded together to generate the timer
pacer trigger of A/D conversion. The frequency of the pacer trigger is
software controllable. The maximum pacer signal rate is 2MHz/4=500K
which excess the maximum A/D conversion rate of the PCI-9111. The
minimum signal rate is 2MHz/65535/65535, which is a very slow
frequency that user may never use it. The output of the programmable
timer can be used as pacer interrupt source or the timer pacer trigger
source of A/D conversion. In software library, the timer #1 and #2 are
always set as mode 3 (rate generator).
4.6.3 Pre-Trigger Counter
The timer #0 is used as the pre-trigger counter. The clock source of
counter 0 is from A/D trigger source so that 8254 can count the A/D trigger
numbers after the pre-trigger signal (pin-12 of CN3) is inserted. The gate
control is set when the pre-trigger signal is change from ‘H’ to ‘L’, and
cleared when the counter is counting down to zero. In software library, the
timer #0 is always set as mode 0 (event counter).
4.6.4 I/O Address
The 8254 in the PCI-9111 occupy 4 I/O address as shown below.
BASE + 40 h LSB OR MSB OF COUNTER 0
BASE + 42 h LSB OR MSB OF COUNTER 1
BASE + 44 h LSB OR MSB OF COUNTER 2
BASE + 46 h CONTROL BYTE
The programming of 8254 is controlled by the registers BASE+0 to
BASE+3. Users can refer to 82C54 data sheet for the descriptions about
all the features of 82C54. You can download the data sheet on the
following web site:
“http://support.intel.com/support/controllers/peripheral/231164.htm”
or “http://www.tundra.com/”