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Re: Same FTM module used for Input capture and PWM output

Question asked by Nitin verma on Dec 2, 2016
Latest reply on Jan 23, 2017 by Nitin verma
Branched from an earlier discussion
Dear Petr,
If I do not set CnSC[ICRST] bit on channel configured for Input Capture mode then Input Capture is not working as expected.
 
Following are my observations:
 
* I'm using FTM0 module Channel 5 PWM Output and Channel 6 for Input Capture.
 
* Scenario 1: FTM_C6SC[ICRST] = 0 (RESET)
 
-> Signal of 100Hz is given to Channel 6 for Input Capture.
 
CASE A:
-> PWM Output is generating 50Hz signal which is lesser than the frequency received for Input Capture.
-> Captured FTM counter values (10 Samples) for Channel 6 through Input Capture mode are:
   {2429, 8679, 4513, 347, 6598, 2432, 8682, 4516, 350, 6595}
-> For 100Hz signal captured, FTM counter value should be 6250. 
    It's capturing variable counts however I can see that the difference between first two samples (8679-2429 = 6250) is exact as desired.
    But differences between other consecutive samples are not as desired (6250 to be precise).
 
CASE B:
-> Now, PWM Output is generating 200Hz signal which is greater than the frequency received for Input Capture.
-> Captured FTM counter values (10 Samples) for Channel 6 through Input Capture mode are:
   {518, 518, 518, 518, 518, 518, 518, 518, 518, 518}
-> For 100Hz signal captured, FTM counter value should be 6250. After some time it's giving following values:
   {637, 637, 637, 637, 637, 637, 637, 637, 637, 637}
-> It's capturing constant values however not at all desired (6250 to be precise). Even counter values are increasing gradually (from 518 to 637).
 
* Scenario 2: FTM_C6SC[ICRST] = 1 (SET)
 
-> Signal of 100Hz is given to Channel 6 for Input Capture.
 
CASE A:
-> PWM Output is generating 50Hz signal which is lesser than the frequency received for Input Capture.
-> Captured FTM counter values (10 Samples) for Channel 6 through Input Capture mode are:
   {6250, 6250, 6250, 6250, 6250, 6250, 6250, 6250, 6250, 6250}
-> Captured FTM counter values are as desired but PWM Output is not as desired.
 
CASE B:
-> Now, PWM Output is generating 200Hz signal which is greater than the frequency received for Input Capture.
-> PWM Output is as expected but Input Capture is not as desired.
 
* Are these all observations are because of common counter available for entire FTM module ? Please review above findings and let me know whether it's a hardware limitation that one FTM module can not be used for PWM Output and Input Capture simultaneously. Looking forward for your response
BR,
Nitin verma

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