Preface: I am using an automotive alarm as a remote trigger to enable/disable another circuit
The 'armed' state is a low output; the 'disarmed' state is 'open' (high from a pull-up) - this gives me a basic enable/disable signal for my circuit.
My problem is that when it powers on, it always powers on in the disarmed state; therefor even when it has been 'armed', if the power is removed and restored, it will re-energize in the disarmed state. (I can't do anything about this, the module is what it is).
I need to be able to overcome this so that I always have a 'disable' condition at power-on.
After it has been set to armed after initial power-on, it will control normally between the two states - it is only the default power-on that is a problem.
My thought is using a simple flip-flop as a latch for the initial power-on condition:
the S would be tied to the power rail; R would have a pull-up to rail and also connected to the Alarm output; (So the R input would be 1 for Dis-Armed and 0 for Armed)
The logic works so that at initial condition with both S & R =1 it would have Q at 1;
System then Armed - Reset then goes low, Q goes to 0
System then Dis-Armed - Reset then goes high, Q remains at 0
And then continuing as only the R can change from 0 to 1 to 0 and the Q output remains latched at 0
Latch Truth Table Requirement
Power On - Q=1
Armed Q=0
Disarmed Q=0
Armed Q=0
(I can then combine this logic with the Armed/Dis-Armed signal to give me correct Enable logic)
My Overall Truth Table Requirement
Power-on - Dis-armed - Disabled
Armed - Disabled
Dis-Armed - Enabled
Armed - Disabled
etc
It will always follow this sequence.
Correct so far as in the latched state after the initial Reset going low?
The question is what happens at power on when effectively S&R = 1 - how can I ensure that Q is high?
Can I just use a capacitor on S input to have it be 0 when power is applied then becoming 1 (so it would be S=0, R=1, Q= 1 at initial power-on, then becoming S=R=1, whereby Q would remain 1?)
After the initial power connection, S would remain high unless power removed, so it's only what happens when it is initially powered that is of consequence
Am I on the right thought train or totally off?
What other considerations would I need?
Any alternative suggestions to satisfy my requirement?
My follow-on circuit in in SMD - don't have a lot of available real estate left so the fewer components the better!
The 'armed' state is a low output; the 'disarmed' state is 'open' (high from a pull-up) - this gives me a basic enable/disable signal for my circuit.
My problem is that when it powers on, it always powers on in the disarmed state; therefor even when it has been 'armed', if the power is removed and restored, it will re-energize in the disarmed state. (I can't do anything about this, the module is what it is).
I need to be able to overcome this so that I always have a 'disable' condition at power-on.
After it has been set to armed after initial power-on, it will control normally between the two states - it is only the default power-on that is a problem.
My thought is using a simple flip-flop as a latch for the initial power-on condition:
the S would be tied to the power rail; R would have a pull-up to rail and also connected to the Alarm output; (So the R input would be 1 for Dis-Armed and 0 for Armed)
The logic works so that at initial condition with both S & R =1 it would have Q at 1;
System then Armed - Reset then goes low, Q goes to 0
System then Dis-Armed - Reset then goes high, Q remains at 0
And then continuing as only the R can change from 0 to 1 to 0 and the Q output remains latched at 0
Latch Truth Table Requirement
Power On - Q=1
Armed Q=0
Disarmed Q=0
Armed Q=0
(I can then combine this logic with the Armed/Dis-Armed signal to give me correct Enable logic)
My Overall Truth Table Requirement
Power-on - Dis-armed - Disabled
Armed - Disabled
Dis-Armed - Enabled
Armed - Disabled
etc
It will always follow this sequence.
Correct so far as in the latched state after the initial Reset going low?
The question is what happens at power on when effectively S&R = 1 - how can I ensure that Q is high?
Can I just use a capacitor on S input to have it be 0 when power is applied then becoming 1 (so it would be S=0, R=1, Q= 1 at initial power-on, then becoming S=R=1, whereby Q would remain 1?)
After the initial power connection, S would remain high unless power removed, so it's only what happens when it is initially powered that is of consequence
Am I on the right thought train or totally off?
What other considerations would I need?
Any alternative suggestions to satisfy my requirement?
My follow-on circuit in in SMD - don't have a lot of available real estate left so the fewer components the better!