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2.9.1.2 - PWM Output (8-Bit, Mode 1)

Outputs a pulse width modulated rectangular wave output. Value passed should be 0-65535, and determines what portion of the total time is spent low (out of 65536 total increments). The lower byte is actually ignored since this is 8-bit PWM. That means the duty cycle can be varied from 100% (0 out of 65536 are low) to 0.4% (65280 out of 65536 are low).

The overall frequency of the PWM output is the clock frequency specified by TimerClockBase/TimerClockDivisor divided by 28. The following table shows the range of available PWM frequencies based on timer clock settings.

Table 2.9.1.2-1. 8-bit PWM Frequencies

   PWM8 Frequency Ranges 
TimerClockBase Divisor=1 Divisor=256
0 4 MHz 15625 N/A
1 12 MHz 46875 N/A
2 48 MHz (default) 187500 N/A
3 1 MHz /Divisor 3906.25 15.259
4 4 MHz /Divisor 15625 61.035
5 12 MHz /Divisor 46875 183.105
6 48 MHz /Divisor 187500 732.422

Note that the clocks above apply to the U3 hardware revision 1.21. With hardware revision 1.20 all clocks are half of those values.

The same clock applies to all timers, so all 8-bit PWM channels will have the same frequency and will have their falling edges at the same time.

PWM output starts by setting the digital line to output-low for the specified amount of time. The output does not necessarily start instantly, but rather has to wait for the internal clock to roll. For 8-bit PWM output, the start delay varies from 0.0 to TimerClockDivisor*256/TimerClockBase. For example, if TimerClockBase = 48 MHz and TimerClockDivisor = 256, PWM frequency is 732 Hz, PWM period is 1.4 ms, and the start delay will vary from 0 to 1.4 ms.

If a duty cycle of 0.0% (totally off) is required, consider using a simple inverter IC such as the CD74ACT540E from TI. Or you can switch the mode of the timer to some input mode, and add an external pull-down to hold the line low when set to input.

2 comments

I am looking for a sample vi for pwm on 14 output lines on a u3... Do you have one available?

The U3 has 2 timers (see Section 2.9), so you can make 2 PWM outputs as described above and there are various examples in the LabVIEW_LJUD archive.  If your PWM frequency and resolution requirements are low enough, you could use software timing to set the various lines high and low ... see the Waveform Generation App Note.