Hi All,

I am just starting out with PICs. Some of the resources I am using use the PIC16F84A for examples. As I cannot buy this chip from Altronics, they only sell the PIC16F84A-20/P, what are the differences between the two chips and what problems may I face attempting to program it?

Cheers.

Additionally - Im using this PIC because the books I have use it as an example. Some webpage resources recommend the use of the PIC16F628 instead. Please comment.

Thankyou for your time.

 

-20/P is the frequency and packaging designations. It is a 20MHz part in commercial DIL package. It is an obsolete part, but if you want to strictly follow the book, go for it. Otherwise F628 or F88 is cheaper and better alternatives.

 

I agree with n9...

After some circuits as per the book when you decide to kick your fisrt own one, buy two 16F88 (or, why not?, one from the 18F family) and go ahead.

 

dont forget microchip has a nice sample program on their website. you can a couple of i think 3 or 4 different chips. they make their money when you turn around later and buy hundreds of your favorites. or hundreds of thousands for production of that great gizmo you designed. oh and if you decide to replace that 84a with a 628 or 88 pin for pin compatible chip, remember they're newer and have more things like comparators etc that have to be turned off for your 84a program to work.

 

and to further your original question... 16f84a is the chip they come in different speeds and packages. the -4 has max speed of 4mhz, the -20 max speed of 20 mhz. you can use a -20 running at 4mhz in place of a -4 with no problems. just not the other way around. P is dip, SO is small outline surface mount, J is ceramic etc. the last part of the data sheet should have all identifications explained. data sheets in pdf form available at microchip.com

 

Following fabrication, samples of all integrated circuits are batch tested at various speeds and temperatures , and those batches of which a small percentage fail at the higher speed (and temperature range) are awarded a slower speed/temp rating, with the very best devices getting a MilSpec rating.

Thus, a 4Mhz component is guaranteed to function 100% at 4Mhz, but the device might have come from an otherwise ok 20Mhz batch of which only a few failed, so it might just be worth trying the chip at progressively higher speeds, until it 'falls over' - then reduce the clock speed by (say) 20% so that you will then be operating within a safe area.

Colin

 

I think the 4MHz and 20MHz versions are identical chips,
and perhaps the 20MHz versions are tested at the higher speed.
I've clocked many 4MHz pics at 20MHz, and there has never been any problem with them,
even running for weeks at a time.
I've done this with exactly those two pics (F84/A, and 628).
Art.

 

Many manufacturers out there just mark higher freq. chips as lower freq. ones. When the specified maximum speed is not even close to the maximum speed the chip can actually handle, it is really difficult to find lower performance chip during testing while the market demand might be quite high.

For these kind of chips, it is much easier, cheaper and faster to test just for one minimum speed and then mark them differently according to the market demand.