A bad design example in almost every way. - Electronic Circuits Projects Diagrams Free

blueroomelectronics · 9th January 2008, 04:58 AM

The following project (actually a neat idea a Mastermind calculator) is flawed in many ways but is a good example how NOT to design a small microcontroller.
http://www.mystrobl.de/ws/pic/mm47/index.htm

Anyone care to guess what's wrong?

Pommie · 9th January 2008, 05:13 AM

Switches straight to ground isn't a good idea. They could easily be pulled low via RA4.

Mike.

simrantogether · 9th January 2008, 05:20 AM

What i can understand from this figure is that - it is combination of lines and boxes with 8 8 8 8 written in them...

I prefer colorful figures...

Regards,

Simran..

Stellarcore · 9th January 2008, 05:33 AM

1. Driving lots of LED displays directly off of the PIC too much current.

2. The buttons are connected to the display. I take it the intention was to also multiplex the buttons with the display by reading it periodically, but if you hold the buttons down it will connect the pins directly to ground either turning on or off the corresponding segments of the display and also possibly sinking lots of current from the output pins. Should have used the unused pin RA4 (see #6).

3. As mentioned earlier, no pull up resistors on the switches.

4. MCLR connected directly to Vcc, I usually at least use a resistor between them.

5. No load caps on the crystal, probably wouldnt even start oscillating.

6. RA4 tied directly to ground, could sink too much current if the pin is incorrectly set in software.

7. No caps on Vdd near the PIC, possible dirty power.

EDIT: Strike number 5, didnt notice ceramic resonator. I've never used those personally, so I dont know if its use was correct or not.

kchriste · 9th January 2008, 05:34 AM

At first I thought it was RA4 to ground, but it's OK because it is an open drain IO. RB6 & RB7 are multiplexed between input and output modes so there really should be some 470

hm: resistors in series with the two switches to limit current if the switches initially close during output mode. I guess the worse offenses are that there is no decoupling cap on the supply rails and RA0-3 really should have transistors driving the displays common anodes.

blueroomelectronics · 9th January 2008, 05:59 AM

Quote:

Originally Posted by Stellarcore

1. Driving lots of LED displays directly off of the PIC too much current.

2. The buttons are connected to the display. I take it the intention was to also multiplex the buttons with the display by reading it periodically, but if you hold the buttons down it will connect the pins directly to ground either turning on or off the corresponding segments of the display and also possibly sinking lots of current from the output pins. Should have used the unused pin RA4 (see #6).

3. As mentioned earlier, no pull up resistors on the switches.

4. MCLR connected directly to Vcc, I usually at least use a resistor between them.

5. No load caps on the crystal, probably wouldnt even start oscillating.

6. RA4 tied directly to ground, could sink too much current if the pin is incorrectly set in software.

7. No caps on Vdd near the PIC, possible dirty power.

EDIT: Strike number 5, didnt notice ceramic resonator. I've never used those personally, so I dont know if its use was correct or not.

Correct, no current limiting resistors on the segments and no transistors to drive the anodes. Displays may work with fancy software tricks but is very poor design practice.
Right, a couple of diodes and a single resistor could fix this, RA4 was availabe. Note: I would have put them on the Anode drivers, you get a free time delay for debouncing while switching digits
ditto
Tell that to Nigel
Correct in the edit
RA4 is the notorious open collector pin on many 16 & some 18 series PICs, no need to tie it to ground, simply make it an output.
Ah yes, decoupling caps seem to always be ignored, same with filter caps.

I think you got them all, go to the head of the class

Gayan Soyza · 9th January 2008, 06:57 AM

Quote:

Originally Posted by Stellarcore

3. As mentioned earlier, no pull up resistors on the switches.

Can use internal weak pullups.

Here in my area PIC 16F84As price is expensive than PIC 16F88

blueroomelectronics · 9th January 2008, 07:34 AM

Quote:

Originally Posted by Gayan Soyza

Can use internal weak pullups.

Here in my area PIC 16F84As price is expensive than PIC 16F88

Yes but since the switches are tied to gnd the segments will light up when the buttons are pressed, bad design

16F88 are the best of the 18pin 16F series, they've got the works. The 18F84A is expensive and pretty old.

Edit one more error in the schematic, the busses are not labeled. How do you know which segment goes to which I/O pin?

simrantogether · 9th January 2008, 07:37 AM

Quote:

Originally Posted by blueroomelectronics

The 18F84A is expensive and pretty old.

You mean 16f84...

Regards,

Simran..

Gayan Soyza · 9th January 2008, 07:37 AM

Quote:

Originally Posted by blueroomelectronics

Yes but since the switches are tied to gnd the segments will light up when the buttons are pressed, bad design

Of course it needs the two resistors.

blueroomelectronics · 9th January 2008, 07:41 AM

Both the 16F84 and 84A are relics. Use the 16F628A instead, cheaper too.

And yes it would need the pullups normally to read those switches.

geko · 9th January 2008, 09:10 AM

Quote:

5. No load caps on the crystal, probably wouldnt even start oscillating.

The schematic doesn't show a crystal it shows a three terminal ceramic resonator which will have internal load capacitors.

Quote:

Edit one more error in the schematic, the busses are not labeled. How do you know which segment goes to which I/O pin?

Not shown on the schematic, but on the original web site the schematic was ripped from there is more detail on which I/O pins go to which segments.

I guess he's not a hardware guy since the switches are a very bad implementation. He even recognised the problem and kludged the software. I quote from the original web site (http://www.mystrobl.de/ws/pic/mm47/index.htm)

Quote:

For example, during the multiplexing, those two port B pins which are connected to the switches too, are made input frequently, in order to enable the pull-ups and to check the switch state. As soon and as long as a closed switch is detected, all pins are held in input state, in order to avoid short-circuiting a driving pin for more than a few ms. This method worked remarkably well. There is one drawback - the display is dark as long as a button is pressed.

One final point, the web page is dated 1998, that's ten years ago. The 16F84 might not be the PIC of choice today but back then it was so there is nothing wrong with the choice of PIC when taken in context.

Pommie · 9th January 2008, 09:40 AM

The tyeing of MCLR to Vdd was recommended for the 16F84/A. It is only on later chips this is inadvisable.

Mike.

**Nigel Goodwin** · 9th January 2008, 10:06 AM

Quote:

Originally Posted by geko

One final point, the web page is dated 1998, that's ten years ago. The 16F84 might not be the PIC of choice today but back then it was so there is nothing wrong with the choice of PIC when taken in context.

Even back then the 84 was probably already obselete?, and replaced by the 16F628 - and back then there was even a migration document telling you of the few tiny changes required to use it!.

Pommie · 9th January 2008, 10:44 AM

I don't think the 628 was released until 99. I have the 1996 Microchip Microcontroller databook and even the 16F84 isn't mentioned, just the 16c84. I still have my UV eraser that I used to erase them.

No ICSP then.

Quote:

Originally Posted by Nigel Goodwin

and back then there was even a migration document telling you of the few tiny changes required to use it!.

Mike.

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9th January 2008, 04:58 AM	(permalink)
blueroomelectronics	A bad design example in almost every way. The following project (actually a neat idea a Mastermind calculator) is flawed in many ways but is a good example how NOT to design a small microcontroller. http://www.mystrobl.de/ws/pic/mm47/index.htm Anyone care to guess what's wrong? __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

9th January 2008, 05:13 AM	(permalink)
Pommie	Switches straight to ground isn't a good idea. They could easily be pulled low via RA4. Mike.

9th January 2008, 05:20 AM	(permalink)
simrantogether	Hmmm... What i can understand from this figure is that - it is combination of lines and boxes with 8 8 8 8 written in them... I prefer colorful figures... Regards, Simran.. __________________ Simran.. 8051 Specialist..

9th January 2008, 05:33 AM	(permalink)
Stellarcore	1. Driving lots of LED displays directly off of the PIC too much current. 2. The buttons are connected to the display. I take it the intention was to also multiplex the buttons with the display by reading it periodically, but if you hold the buttons down it will connect the pins directly to ground either turning on or off the corresponding segments of the display and also possibly sinking lots of current from the output pins. Should have used the unused pin RA4 (see #6). 3. As mentioned earlier, no pull up resistors on the switches. 4. MCLR connected directly to Vcc, I usually at least use a resistor between them. 5. No load caps on the crystal, probably wouldnt even start oscillating. 6. RA4 tied directly to ground, could sink too much current if the pin is incorrectly set in software. 7. No caps on Vdd near the PIC, possible dirty power. EDIT: Strike number 5, didnt notice ceramic resonator. I've never used those personally, so I dont know if its use was correct or not. Last edited by Stellarcore; 9th January 2008 at 05:45 AM.

9th January 2008, 05:34 AM	(permalink)
kchriste	At first I thought it was RA4 to ground, but it's OK because it is an open drain IO. RB6 & RB7 are multiplexed between input and output modes so there really should be some 470hm: resistors in series with the two switches to limit current if the switches initially close during output mode. I guess the worse offenses are that there is no decoupling cap on the supply rails and RA0-3 really should have transistors driving the displays common anodes. __________________ --- The days of the digital watch are numbered. ---

9th January 2008, 07:41 AM	(permalink)
blueroomelectronics	Both the 16F84 and 84A are relics. Use the 16F628A instead, cheaper too. And yes it would need the pullups normally to read those switches. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

9th January 2008, 09:40 AM	(permalink)
Pommie	The tyeing of MCLR to Vdd was recommended for the 16F84/A. It is only on later chips this is inadvisable. Mike.