> From: Sam Reaves <sam.rea...@gmail.com> > I have a LED with a 330 ohm current limiting resistor on pin 3 (the 74ACT240 > can sink up to 24mA and I am well below that) for the lock indicator. All of > my units lock and produce 10MHz although the 1pps never switches.
Pin 3 is not driven by the 74ACT240. It is an input to the '240, and your LED is pulling it up above the logic threshold.... see below. http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq#how_can_i_get_a_1_pps_output How can I get a 1 PPS output? A 1 microsecond wide, logic level 1 PPS signal appears on the DB-9 pin 6 whenever the unit is locked (hard to see on an analog scope, should be easy on a digital scope). The PPS signal does not appear until the unit is locked (pin 3 goes low). The current sink capability of pin 3 is weak, and if it is driving an LED + 1K resistor from +5V, that will leave pin 3 at 2.3V when locked, not enough to enable the 1 PPS signal. Advice: buffer the pin 3 lock signal before driving an indicator light. All being well, pin 1 of the 74ACT240 will drop low in step with the lock condition and the PPS signal will appear at the DB9. Alternatively, it is easy to get a 1 pps signal from the 10 MHz output with a single picDIV chip (possibly with a 0.1uF cap and two 10k resistors to bias the sine wave to 1/2 the PIC power supply voltage). picDIV parts (based on PIC12F675 cpu) from Tom Van Baak take a 10 MHz clock input and generate a 1 PPS output. See also: http://www.leapsecond.com/pic/picdiv.htm Thanks to Bob Grant for describing the pin 3 LED pin 6 PPS problem. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
