Date: Thu, 22 May 2014 11:19:17 -0300 From: Luiz Otavio O Souza <lists.br@gmail.com> To: fabiodive <fabiodive@gmail.com> Cc: "freebsd-arm@freebsd.org" <freebsd-arm@freebsd.org> Subject: Re: Beaglebone Black. PWM minimum frequency of 1.52KHz Message-ID: <CAB=2f8zy7=S_Xh0REsv29_1%2BHk_=h7Ut1SDURjP-otaE9hsxBg@mail.gmail.com> In-Reply-To: <F5705D29-1EFE-4A20-89CC-CB0E52EB57F3@gmail.com> References: <F5705D29-1EFE-4A20-89CC-CB0E52EB57F3@gmail.com>
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--001a11c3669263125b04f9fdcc1b Content-Type: text/plain; charset=UTF-8 On 10 May 2014 08:56, fabiodive wrote: > Hello, > > PWM on Beaglebone Black > > during my laboratory tests I was not able to setup a frequency suitable for servo motors. > I tried several times with many values but the results was always the same. > The minimal frequency I was able to achieve was 1.52 KHz. > Also, I noticed odd results with the most of configuration values, in this case > I lost the PWM output signal. > > I tried some combinations, measuring the results with an oscilloscope, the period > configuration key appears to loose its nanoseconds meaning beyond a value: Hi Fabio! The PWM frequency is determined as SYSCLKOUT (100Mhz) / prescaler settings / period. As the current code uses 1 for the prescaler and the period is a 16bit value the lower frequency you can have is actually 1.529Khz. > # sysctl dev.am335x_pwm.1.period=1500 > # sysctl dev.am335x_pwm.1.dutyA=300 > result frequency: 66 KHz ->should be 666 KHz > length of period: 15 us ->should be 1.5 us > length of pulse: 2 us As above, this is correct: 100Mhz / 1 / 1500 = 66Khz. > > # sysctl dev.am335x_pwm.1.period=1500000 > # sysctl dev.am335x_pwm.1.dutyA=10000 > result frequency: 1.71 KHz -> should be 666Hz > length of period: 585 us > length of pulse: 100 us > > # sysctl dev.am335x_pwm.1.period=1800000 > # sysctl dev.am335x_pwm.1.dutyA=10000 > result frequency: 3.24 KHz -> should be 555Hz > length of period: 308 us > length of pulse: 100 us These two examples are overflowing the period variable. I've the attached patch which enforces the maximum values where needed (to avoid such overflows) and two new sysctls, one allows you to set the prescaler for the PWM module and the other (which is probably more interesting) allows you to check the actual PWM frequency. The sysctl.am335x_pwm.X.freq can also be use to set an desired frequency directly, it will try to get you the better setting it can find for the desired frequency, which usually means get the bigger period it can find to give the better PWM resolution that is possible for a given frequency. I've been able to drive R/C servos directly with this patch and the following set: sysctl.am335x_pwm.X.freq = 50 (digital servos may operate at higher frequencies) And then, for the cheap chinese servo i have here, i can get it to travel almost 180o using duty values from 2000 to 6000 (YMMV). I've checked the frequencies with a scope and they look fine for me. 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