Chadwick said:
Newb questions:
A 555 timer has an internal SR flipflop according to the data sheets,
and it appears that I can feed the SR FF any 0/1 pattern I choose by
picking the appropriate voltages for the pins of the 555.
Does the "don't assert S and R at the same time" rule still
apply in the case of the 555? Even when reset is asserted?
There is no such rule for an SR flip flop. Just don't
expect the two outputs of an SR flip flop to be opposites of
each other if you assert S and R at the same time.
Depending on the construction of the flip flop, either both
outputs will be high or both will be low. And stopping the
dual assertion at the same time will produce an
indeterminate result with the two outputs returning to their
normal, opposite states in one of two possible pairs.
Since you are dealing with only one of the two flip flop
outputs in the 555, you have to get into the structure of
the internal flip flop to determine what happens when you
assert S and R at the same time.
This web page and tutorial shows an approximate schematic of
what is inside the original, bipolar transistor version of
the 555:
http://www.uoguelph.ca/~antoon/gadgets/555/555.html
The flip flop is made up of transistors Q16 and 17, two
inverters connected in a loop. Q17 drives both the push
pull output stage (Q20-24) and the discharge switch (Q14) so
it is the output you can see from the outside.
The reset input drives Q17, directly, overriding whatever
Q16 is doing, so it also overrides and set and reset inputs
from the two voltage input comparators.
The trigger comparator's output is Q15, a switch to ground
that turns off Q16, regardless of whether it is being turned
on by the positive feedback from Q17, or by the current
sourced from the threshold's comparator output, Q6. So
trigger overrides threshold.
Unless I have gotten tangled up.
