bcm2835 pwm 데이터시트

balanced 모드(기본값)에서는 N/M (date / range)

When MSEN=0, which is the default mode, data to be sent is interpreted as the value N of the algorithm explained above. Number of clock cycles (range) used to send data is the value M of the algorithm. Pulses are sent within this range so that the resulting duty cycle is N/M. Channel sends its output continuously as long as data register is used, or buffer is used and it is not empty.  

mark-space 모드에서는 M/S (data / range)

When MSEN=1, PWM block does not use the algorithm explained above, instead it sends serial data with the M/S ratio as in the picture below. M is the data to be sent, and S is the range. This mode may be preferred if high frequency modulation is not required or has negative effects. Channel sends its output continuously as long as data register is used, or buffer is used and it is not empty. 

[링크 : http://www.farnell.com/datasheets/1521578.pdf] 139page