1. PCI/PCIe TFP Hardware
Divider Mode
The divider can be used to generate low or fractional frequencies. The divide range is 1E0 through
1E7 in decades. The divider will also allow for fractional frequency outputs where the divider's input
source is decimal shifted by up to seven places.
The divider has another selectable mode, Period Mode. This mode may be desirable when the DDS
cannot be set exactly to the desired frequency but using a period value would be exact. When oper-
ating in this mode, the output is also a square wave with the period resolution equal to 2 times the
period of the input (Divider Source). If the Divider Source of 100 MHz is selected, the period res-
olution is 20nS. When using 100 MHz as the Divider Source, a period register of 0 corresponds to a
40 nS period (25 MHz) which is Period Mode's upper limit. Use the following formula to set the Period
Value Register.
Period Register = (Desired Period / (Divider Source period * 2)) - 2
Example of period mode calculation using 100 MHz Divider Source:
Desired period 59.3 ms= (59.3E-3 / 20E-9) - 2
Result= 2964998
Multiplier Mode
The DDS circuit also includes a frequency multiplier that may be used. The multiplier's input is the
DDS and the output is DDS times the multiplication factor. This circuit can multiply the DDS
frequency by 1, 2, 3, 4, 6, 8, 10 or 16. Note that the DDS frequency must be high or low enough for
use (depends on multiplication factor - see following ranges).
Input (MHz)
x1
22 - 150
x2
11 - 75
x3
8 - 56
x4
6 - 38
x6
5 - 23
x8
5 - 19
x10
5 - 15
x16
5 - 10
When using the Multiplier Mode, the resolution of the DDS is 1/32 Hz * the multiplier value. The out-
put of this circuitry is capable of creating interrupts. This circuit can generate rates that far exceed a
computer's ability to service the DDS interrupts. Note that the DDS signal is multiplexed with the
PPO signal, where one or the other signals is selected for output on the 15-pin I/O connector pin 15.
The TFP card will load previously set DDS configuration registers at power-on.
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