# 56803 PWM duty cycle sweep speed

Started by June 14, 2004
 Hi We are presently trying to use the DSP56F803 PWM (channel 4) as a simple independent PWM output. We are sweeping the output duty cycle from 0% to 100% and therefore, updating and reloading the PWM timer on every timeout interrupt of the PWM. When we then observe the PWM output on a scope, the rate at which the PWM duty cycle is changing, is taking much longer then we would expect (4 to 5 sec to go from 0 to 100 % duty cycle). We have also seen that the rate at which this output duty cycle can be changed is somehow tied to the configured base frequency of the PWM. Is this normal operation as per the 56803 implementation of the PWM functionality? Has anyone gotten the duty cycle to change faster? The prescaler and modulus are set for PWM freq = 4.32khz. We expected the duty cycle to change in microseconds or milliseconds, not seconds. Thanks Mike Kertesz
 Hi We are presently trying to use the DSP56F803 PWM (channel 4) as a simple independent PWM output. We are sweeping the output duty cycle from 0% to 100% and therefore, updating and reloading the PWM timer on every timeout interrupt of the PWM. When we then observe the PWM output on a scope, the rate at which the PWM duty cycle is changing, is taking much longer then we would expect (4 to 5 sec to go from 0 to 100 % duty cycle). We have also seen that the rate at which this output duty cycle can be changed is somehow tied to the configured base frequency of the PWM. Is this normal operation as per the 56803 implementation of the PWM functionality? Has anyone gotten the duty cycle to change faster? The prescaler and modulus are set for PWM freq = 4.32khz. We expected the duty cycle to change in microseconds or milliseconds, not seconds. Thanks Mike Kertesz
 Hello We are presently trying to use the DSP56F803 PWM (channel 4) as a simple independent PWM output. We are sweeping the output duty cycle from 0% to 100% and therefore, updating and reloading the PWM timer on every timeout interrupt of the PWM. When we then observe the PWM output on a scope, the rate at which the PWM duty cycle is changing, is taking much longer then we would expect (4 to 5 sec to go from 0 to 100 % duty cycle). We have also seen that the rate at which this output duty cycle can be changed is somehow tied to the configured base frequency of the PWM. Is this normal operation as per the 56803 implementation of the PWM functionality? Has anyone gotten the duty cycle to change faster? The prescaler and modulus are set for PWM freq = 4.32khz. We expected the duty cycle to change in microseconds or milliseconds, not seconds. Thanks Mike Kertesz
 The PWM duty cycle can only be changed at PWM period intervals. You can write to the value registers any time, but they only take effect at the beginning of the next PWM period.  So at 4.32 kHz, the PWM duty cycle can only be changed every 231 usecs.  If you have 16-bits of resolution, and you change the duty cycle one count at a time, it will take (231 usecs * 65535) = 15.17 seconds to sweep from 0 to 65535.  If you have 15-bits of resolution, it will take 7.59 seconds, etc.   Doug Holub ----- Original Message ----- From: Corey, Rick To: 'm...@yahoogroups.com' Sent: Monday, June 14, 2004 3:18 PM Subject: [motoroladsp] 56803 PWM duty cycle sweep speed HelloWe are presently trying to use the DSP56F803 PWM (channel 4) as a simpleindependent PWM output. We are sweeping the output duty cycle from 0% to100% and therefore, updating and reloading the PWM timer on every timeoutinterrupt of the PWM.When we then observe the PWM output on a scope, the rate at which the PWMduty cycle is changing, is taking much longer then we would expect (4 to 5sec to go from 0 to 100 % duty cycle). We have also seen that the rate at which this output duty cycle can bechanged is somehow tied to the configured base frequency of the PWM.Is this normal operation as per the 56803 implementation of the PWMfunctionality?Has anyone gotten the duty cycle to change faster?The prescaler and modulus are set for PWM freq = 4.32khz.We expected the duty cycle to change in microseconds or milliseconds, notseconds.Thanks Mike Kertesz M...@dpconline.com_____________________________________Note: If you do a simple "reply" with your email client, only the author of this message will receive your answer.  You need to do a "reply all" if you want your answer to be distributed to the entire group._____________________________________About this discussion group:To Join:  m...@yahoogroups.comTo Post:  m...@yahoogroups.comTo Leave: m...@yahoogroups.comArchives: http://www.yahoogroups.com/group/motoroladspMore Groups: http://www.dsprelated.com/groups.php3
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