Monday, February 05, 2007

Scrolly message impass...

Damn....My new 4028 chips arrived at the weekend (the BCD to Decimal ones), and I got all excited and rewired my breadboard prototype to try it out. Once it was all wired up, I switched it on and....bugger....didnt work.... I spent the next day or so trying to figure out why!

Here's which I think is going wrong... The old chips had a LOW TTL level output of around .6volts, where as these actually output 0volts. The problem being, that since I use pull up resisters to increase the brightness I need some sort of voltage there so that the power from the pullup doesn't power the LCD matrix when I dont want it to. But with 0volts, the pull up supplies power to both. Damn.

So, next thing to try is a few transistors, although Im almost at the point where I'm thinking sod the extra expense... get the slightly larger PIC (which will probably cost about 50-60pence more per unit, but save power, complexity and wiring - it almost has to be worth it.

6 comments:

Anonymous said...

Have you tried connecting LED anode to VCC thru a resistor and connecting decoder output to LED cathode? This means (il)logical 0 in the output pin lights LED, 1 turns it off. Inverted logic means you can use your old 3-to-8 demux without inverters :)

--
TNT

Mike said...

I was going to do that initially, but this led to scanning vertically (8 times with 5 bits) rather than horizontally (5 times with 8 buts). So by using the latter, the LED stays on longer, and is a little bit brighter - which I prefer.

Currently, Im thinking that taking the hit and paying a little more will just be easier, even if it is a little more expensive. Back to less chips, less power, less soldering....less things to go wrong, and everything can be done form the PIC - all this for a modest (ish) cost increase.

Still....bummer....

Anonymous said...

It is simpler and better at the end, you'll see :)

Popmilo said...

It is simpler and better at the end, you'll see :)

forgot to sign post... :)


ps. I would love to sign in as google-blogger but dont you just hate it when someone registered your nick before you? And its ok if that one uses it, but when he didnt touch it for a year .... arrghh... :)

Levente said...

Hi Mike!,

Actually, those chips from the 40xx series are generally "weak" (ie. they can't sink or source more than a few milliampers), they're hardly capable of driving LEDs by themself. Some TTL compatible ones (74HCT, 74ACT) probably are, but I'm not sure. 74LS and co. would probably be a safe bet, but they're not really manufactured anymore. It's usually simpler to take an MCU with more I/O pins... though, you should probably check the datasheet if it will be legal for the particular chip to source or sink the required current (there's usually a rating for the individual I/O pins and the whole package's current consumption).

Mike said...

Yep...I've found that out the hard way! I got a new stack of 28 pin PIC's, only to discover that they DON'T have build in clocks! Which means yet more expense. So currently Im thinking of using some PNP's instead of pull-up's to power the LED's and stick to the cheaper PIC's.

*sigh*...its never easy.... (my new moto)