Driving 74LS from 3.3V uP

Haha, around here you need a state license for that level of passive-aggressiveness.

I do appreciate the advice, though. ; )

Assuming the switcher has the usual 1.2V reference, then a single open-drain MCU pin and a resistor can switch it between high and low output voltage.

To simulate open drain, you write 0 to the bit in the register and toggle the output enable.

Vout

0------R1R1R1---*--------*----0 FB | | R R 3 2 R R 3 3 | | | GND 0---------------* MCU pin (OD)

Output off -> resistor R3 not connected -> low output voltage Output on -> resistor R3 connected -> high output voltage.

The feedback pin doesn't go above 1.2V in normal operation. It'll jump up when R3 is disconnected, but should never go over 3.3V unless the voltage ratio is more than 2.5:1. If it is, a Schottky diode to the supply will fix it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

But would he get invited to the Whitehouse to show it off?

OK, so everything is working good now! I found that the boost's enable pin responds pretty quickly, so just ramping the ~5V supply voltage up via a PWM output to that pin, for about a half second, allows everything to start up well without any extra supply filtering.

OK. Now what happens if your display goes from . to 8888?

NT

It can't, there's only one tube. It counts up from 0 to 9 and back down again in 1000 mS, 900, 800, etc. intervals well...

meausre the R of the cold incandescent segments and you will your answer

"soft starting" the "enable" pin of the converter with PWM to bring up the voltage on the filament slowly got it sorted