I want a condition to buy CE ATM contract when the 5EMA crosses above 20EMA on the Nifty spot chart. And the opposite for PE.
Why opening two topics …bro? 
Wait…You have asked your question mid-night
Probably, you will get answers tomorrow 
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I have created a strategy as per your requirement. Click on the below link to view the strategy conditions.
The above strategy is for CE, you can create the conditions for your PE strategy by following a similar approach used in the above strategy.
Hope this helps.