Original Publication Date: 1993-Feb-01
Included in the Prior Art Database: 2005-Mar-18
Emeigh, RD: AUTHOR [+3]
A circuit for detecting when a clock signal has stopped switching is disclosed below. This circuit is designed such that the clock signal has to stop for several cycles in time before an error is indicated.
A circuit for
detecting when a clock signal has stopped
switching is disclosed below. This circuit is designed such that the
clock signal has to stop for several cycles in time before an error
transition detector circuit schematic is shown in the
figure. This detector circuit operates on differential signals only.
The clock signal being monitored enters at the + IN and - IN pins
which are connected to the differential pair Q0 and Q1. This
differential pair amplifies the voltage swing of the differential
clock signal. The amplified signal is emitter-followed by Q2 and Q3
which drive capacitors C0 and C1, respectively. Q2 and Q3 charge the
capacitors very quickly but allow them to discharge at a slow rate
that is dependent on the current in the current sources ID. The ID
current sources have very small currents in them so that the
capacitors only discharge a few millivolts while the clock switches.
Thus, the capacitors stay almost fully charged while the clock is
switching because the emitter-followers Q2 and Q3 keep recharging
them. The voltage on each capacitor is monitored by its own Schmidt
trigger circuit formed by Q4,Q5,Q6 and Q7,Q8,Q9 and related
resistors. Thus, if the voltage on a capacitor drops below a certain
threshold, the differential output of the corresponding Schmidt
trigger (POUT/NOUT) will change states. Schmidt triggers are used to
eliminate spurious switching when the capacitor v...