how can i get the values of a samples before and after zero crossing of a sinusiodal signal?

hello, all

if i have discrete voltage sinusoidal signal in simulink , so which blocks may i use to get samples on the negative and positive sides of a positive-going zero-crossing,to calculate this equation:

T =(v1/(v1 − v0))Δt +(vm/(vm+1 − vm))Δt + (m −1)Δt

where:

(v0) and (v1) are the voltage samples on the negative and positive sides of a positive-going zero-crossing, respectively; (vm) and (vm+1) are the voltage samples on the negative and positive sides of a next successive positive-going zero-crossing, respectively; (m) is the total number of samples in one cycle of the voltage signal and (Δt) is the sampling interval.

[Walter Roberson]

duplicates http://www.mathworks.co.uk/matlabcentral/answers/66575-how-can-i-know-the-value-of-a-sample-before-and-after-zero-crossing-of-a-sinusiodal-signal