On Fri, Jun 14, 2002 at 12:15:49PM -0700, Brendan Moran wrote:
> I'm trying to decode 2 16-bit hex numbers to BCD then display them on
> 7-segment displays from a '877.
Alright.
>
> I only need 3 digits of each, so I can easily drop rest of the data
> internally, but they do need to be full range i.e. -999 to +999. So, that
>means 3 BCD digits and a sign bit externally. (I already have a 16-bit binary
>- BCD output routine)
Well the sign bit is somewhat irrelavent. Simply save it and complement if
negative and run the absolute value through the routine.
Are you planning on capping/limiting values above 999 and below -999 to those
max values? Just curious.
> I have only PORTB used, and the rest of the ports are free, but the device
> is in an application where it only might be possible to multiplex the
> display.
That's business as usual on seven segment displays. Multiplexing is almost
always helpful. It saves both I/O and board real estate.
> Basically, I could use the tmr0 interrupt which is running at 9600
> interrups/second, but it couldn't opperate when one specific other operation
> (also interrupt driven) was running, but it would be easy to set up a flag
> for that.
As long as the other op doesn't take too much time it should be fine. However
if it delays the timer interrupt too long then you'll get uneven brightness on
the digits. In that case the higher interrupt would be responsible for updating
the display when TMR0 rolls over.
>
> What I'm thinking right now is setting up a few 22v10's to act as
> combination BCD to 7-segment decoders and latches, and using them to handle
> everything.
??????. weird head scratching, mumbled muttering....
What in the HELL are you talking about, Willis!?!?
Brendan, the whole point of having a high speed microcontroller is so that
you don't have to hang a bunch of hardware to do a task like this. Everything
you need done can be easily handled in software. All it costs is I/O pins.
>
> So, what I'm asking is which of those is better, given that I have enough
> 22v10's handy to do it, and the code written for them to handle the
> decoding/latching. Or is there a better method entirely?
Ah I see. The 22V10s are like a nice shiny new multipart power tool set that
you've gotten for Father's Day. Just itching to find a project to use on.
Only problem is that all you really need to do is hammer nails.
Leave the 22V10s for another project. You don't need them.
You have enough I/O real estate to essentially do it for free. Here's the
I/O cost:
8: Seven sement driver + decimal point*
4: Display drivers for your 3 digits + sign bit.
---
12
* 11 if the decimal point isn't required.
So it's a full port and a half. But as you stated you have that kind of real
estate available. So use it.
Attach PORTD to the display anodes (I'm presuming common cathode displays) via
180 ohm resistors (18 ma through 1.7V forward voltage LED's with 5V Vdd. Adjust
to meet the requirements of you LEDs). Take 4 pins of PORTC or PORTA (whichever
isn't tasked for anything else) and drive 4 NPN drive transistors: collector
connected to the cathode of each display, base connected to the port pin
through a 1K resistor, emitters grounded.
That's it. All the hardware you need. To operate simply drop the display value
on PORTD and raise the required digit drive pin high. Use standard multiplexing
techniques such as blanking the display between digit changes (all 4 drive
lines low). Cycle as fast as you can afford.
No hardware necessary. The PIC can do all the decoding and multiplexing.
There's no need to add any additional hardware unless you want to conserve
I/O pins.
With external hardware 4 pin interfaces are trivial, and with quite a bit of
work, 1 pin interfaces are possible. However the only purpose of using the
external hardware is to save PIC pins.
However if the PIC has enough pins, just use them. That's what they are there
for.
Just my 2 cents.
BAJ
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