Device for filtering a signal and corresponding method

1. A device for filtering a signal representative of a phenomenon of determined instantaneous period, delivered as output by at least one sensor installed in a motor vehicle, characterized in that it comprises:

a comparator (A) offering as output a first logic signal (S A ) representative of the positive and negative transitions of the output signal (S M ) from the sensor,

a clock (G) delivering a signal (S G ) serving as time base for the whole device,

means (E, D, F) making it possible to fix a filtering time (T Filt ) of said output signal (S M ) from the sensor and comprising:

a counter (F) whose counting is triggered by said control block (C) when said second logic signal (S B ) changes state, said counting not exceeding a maximum value (t dmax ), said counter (F) being reinitialized by said control block (C) when said maximum value (t dmax ) is reached or when said second logic signal (S B ) changes state,

a register (E) storing a fraction of the value contained in the counter (F) on receipt of a storage command signal (Str) emanating from the control block (C), said storage command signal (Str) being dispatched when at least one change of state is detected on said first logic signal (S A ) during said filtering time (T Filt ), and

a down counter (D) containing the value stored in said register (E) and of which a first countdown is triggered by said control block (C) when at least one change of state is detected on said second logic signal (S b ),

a suppression block (B) receiving as input said first logic signal (S A ) and delivering as output a second logic signal (S B ) whose duration of holding (tn) in a logic state depends at least on said filtering time (T Filt ), and

a control block (C) managing at least the set of the signals of the device, and having as input signals at least said first and second logic signals (S A , S B ).

2. The filtering device as claimed in claim 1 , characterized in that said counter (F) is triggered when said second logic signal (S B ) changes state.

3. The filtering device as claimed in claim 1 , characterized in that the value stored in said register (E) is forced to a minimum value (t dmin ) when the value arising from said counter (F) is less than t dmin .

4. The filtering device as claimed in claim 1 , characterized in that said down counter (D) is reloaded with the value contained in the register (E) when said first logic signal (S A ) changes state and so long as said first countdown has not reached the value zero, a new countdown by said down counter (D) being triggered thereafter by said control block (C).

5. The filtering device as claimed in claim 4 , characterized in that the control block (C) is designed to reload the down counter (D) only a predefined number of times during said filtering time (T Filt ).

6. The filtering device as claimed in claim 1 , characterized in that said suppression block (B) operates selectively in “transparent” mode, said second logic signal (S B ) then being identical to said input signal (S A ), or in guard mode, said second logic signal (S B ) then remaining unchanged irrespective of said first logic signal (S A ),

and in that said suppression block (B) initially adopts said “transparent” mode, passes to guard mode after a first transition of said first logic signal (S A ), and thereafter adopts said “transparent” mode or said guard mode depending on whether or not said down counter (D) has reached the value zero.

7. The filtering device as claimed in claim 6 , characterized in that said counter (F) is triggered when said second logic signal (S B ) changes state.

8. The filtering device as claimed in claim 6 , characterized in that the value stored in said register (E) is forced to a minimum value (t dmin ) when the value arising from said counter (F) is less than t dmin .

9. The filtering device as claimed in claim 6 , characterized in that said down counter (D) is reloaded with the value contained in the register (E) when said first logic signal (S A ) changes state and so long as said first countdown has not reached the value zero, a new countdown by said down counter (D) being triggered thereafter by said control block (C).

10. The filtering device as claimed. in claim 9 , characterized in that the control block (C) is designed to reload the down counter (D) only a predefined number of times during said filtering time (T Filt ).

11. A method for filtering a signal (S M ) representative of a phenomenon of determined instantaneous period, delivered as output by at least one sensor installed in a motor vehicle, consisting at least in

detecting a first positive or negative transition of the output signal (S M ) from the sensor;

converting the first positive or negative transition into a first logic signal (S A );

creating a second logic signal (S B ) having the same logic state as the first logic signal (S A ) and holding the second logic signal (S B ) in this same logic state;

storing in a register (E) a fraction of a value contained in a counter (F) if the fraction of the value is not equal to zero;

storing in the register (E) a predefined minimum value (t dmin ) if the fraction of the value is less than the minimum value t dmin ;

initializing a down counter (D) with the value stored in the register (E);

triggering the down counter (D);

triggering the counter (F), the counting being unable to exceed a predefined maximum value (t dmax );

reloading the down counter (D) with the value stored in the register (E) if the countdown has not reached the value zero and if a new positive or negative transition of the signal (S M ) is detected, the number of reloadings of the down counter (D) not exceeding a predefined integer number; and

placing the second logic signal (S B ) in the same logic state as the first logic signal (S A ) if the down counter (D) has reached the value 0.