Rabbit RCM3000 User Manual

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Rabbit RCM3000 User Manual Manual pdf 118 page image

110 RabbitCore RCM3000E.4 PWM OutputsThe Pulse-Width Modulator consists of a 10-bit free-running counter and four width regis-ters. Each PWM output is high for n + 1 counts out of the 1024-clock count cycle, where nis the value held in the width register. The PWM output high time can optionally be spreadthroughout the cycle to reduce ripple on the externally filtered PWM output. The PWM isclocked by the output of Timer A9. The spreading function is implemented by dividingeach 1024-clock cycle into four quadrants of 256 clocks each. Within each quadrant, thePulse-Width Modulator uses the eight MSBs of each pulse-width register to select the basewidth in each of the quadrants. This is the equivalent to dividing the contents of the pulse-width register by four and using this value in each quadrant. To get the exact high time, thePulse-Width Modulator uses the two LSBs of the pulse-width register to modify the hightime in each quadrant according to Table E-3 below. The “n/4” term is the base count, andis formed from the eight MSBs of the pulse-width register.The diagram below shows a PWM output for several different width values for bothmodes of operation. Operation in the spread mode reduces the filtering requirements onthe PWM output in most cases.Figure E-1. PWM Outputs for Various Normal and Spread ModesTable E-3. PWM OutputsPulse Width LSBs 1st 2nd 3rd 4th00 n/4 + 1 n/4 n/4 n/401 n/4 + 1 n/4 n/4 + 1 n/410 n/4 + 1 n/4 + 1 n/4 + 1 n/411 n/4 + 1 n/4 + 1 n/4 + 1 n/4 + 1n=255, normaln=256, spreadn=255, spread(256 counts)(64 counts) (64 counts) (64 counts) (64 counts)(65 counts) (64 counts) (64 counts ) (64 counts)n=257, spread (65 counts) (64 counts ) (65 counts) (64 counts)n=258, spread (65 counts) (65 counts) (65 counts) (64 counts)n=259, spread (65 counts) (65 counts) (65 counts) (65 counts)n=259, normal (260 counts)Downloaded from Elcodis.com electronic components distributor