Data Translation DT300 Series, User Manual

Page: 95 / 164

Principles of Operation
95
C/T Clock Sources
The following clock sources are available for the user counters:
• Internal C/T clock
• External C/T clock
• Internally cascaded clock
Refer to the following subsections for more information on these clock sources.
Internal C/T Clock
The internal C/T clock uses a 20 MHz time base. Counter/timer operations start on the rising
edge of the clock input signal.
Through software, specify the clock source as internal and the frequency at which to pace the
counter/timer operation (this is the frequency of the clock output signal). The maximum
frequency that you can specify for the clock output signal is 10 MHz. For a 16-bit counter, the
minimum frequency that you can specify for the clock output signal is 305.17 Hz. For a 32-bit
cascaded counter, the minimum frequency that you can specify for the clock output signal is
0.00465 Hz, which corresponds to a rate of once every 215 seconds.
External C/T Clock
The external C/T clock is useful when you want to pace counter/timer operations at rates not
available with the internal C/T clock or if you want to pace at uneven intervals. The rising
edge of the external C/T clock input signal is the active edge.
Using software, specify the clock source as external and the clock divider used to determine
the frequency at which to pace the operation (this is the frequency of the clock output signal).
The minimum clock divider that you can specify is 2.0; the maximum clock divider that you
can specify is 65535. For example, if you supply an external C/T clock with a frequency of 5
MHz and specify a clock divider of 5, the resulting frequency of the external C/T clock output
signal is 1 MHz. The resulting frequency of the external C/T clock output signal must not
exceed 2.5 MHz.
Connect the external C/T clock source to the board through the STP300 screw terminal panel
as follows:
• For Counter 0, connect the external C/T clock signal to screw terminal TB26 (pin 41 of
connector J1)
• For Counter 1, connect the external C/T clock signal to screw terminal TB30 (pin 7 of
connector J1)
• For Counter 2, connect the external C/T clock signal to screw terminal TB34 (pin 36 of
connector J1)
• For Counter 3, connect the external C/T clock signal to screw terminal TB38 (pin 2 of
connector J1)