PWM Clock Frequencies
The base clock frequency for the PWM outputs is 24.81 MHz on the 7340 and 7330 controllers, 24.58 MHz on the 7350 controller, and 20.48 on 7334/42/44 controllers. This base clock frequency is divided down depending on the clock value selected.
The following table describes the PWM clock frequency settings:
|512||0||40.00 kHz||48.46 kHz||48.01 kHz|
|1K||1||20.00 kHz||24.23 kHz||24.00 kHz|
|2K||2||10.00 kHz||12.11 kHz||12.00 kHz|
|4K||3||5.00 kHz||6.06 kHz||6.00 kHz|
|8K||4||2.50 kHz||3.03 kHz||3.00 kHz|
|16K||5||1.25 kHz||1.51 kHz||1.50 kHz|
|33K||6||625.00 Hz||757.14 Hz||750.09 Hz|
|External Clock/256 Hz||7||External Clock/256 Hz||External Clock/256 Hz||External Clock/256 Hz|
|65K||8||312.50 Hz||378.57 Hz||375.05 Hz|
|131K||9||156.25 Hz||189.29 Hz||187.52 Hz|
|262K||10||78.13 Hz||94.64 Hz||93.76 Hz|
|524K||11||39.06 Hz||47.32 Hz||46.88 Hz|
|1048K||12||19.53 Hz||23.66 Hz||23.44 Hz|
|2097K||13||9.77 Hz||11.83 Hz||11.72 Hz|
|4194K||14||4.88 Hz||5.92 Hz||5.86 Hz|
|External Clock/ 32,768 Hz||15||External Clock/ 32,768 Hz||External Clock/ 32,768 Hz||External Clock/ 32,768 Hz|
|* These clock value numbers are used for purposes of discussion only.|
When you configure a PWM output, the clock frequency applies to both outputs (PWM 1 and PWM 2). If you configure one PWM output for a clock value of 3, and then the second PWM output for a clock value of 4, the value of 4 applies to both PWM outputs.
The only exception is when the clock settings for the two PWM outputs are 0 and 8, 1 and 9, 2 and 10, and so on, in which case each output has a different frequency. This is because clock values 0–7 and 8–15 are paired. When the clock settings for the two PWM outputs are 0 and 8, 1 and 9, 2 and 10, and so on, each output has a different frequency.
Clock values 0 and 8 use the common time base of 10.24 MHz (12.8 MHz on the 7350). The frequency of clock value 0 is found by dividing the time base by 256, while clock value 8 is found by dividing the time base by 32768. If you have an NI 7344 controller and you switch channel B to clock value 10, the time base changes to 2.56 MHz. This change also switches channel A to 2.56/256 = 10 kHz.
If the value in one set changes, it switches the other channel to the pairing clock value. If the two values are in the same group, the two channels have the same frequency.