I'm switching over to PICs from Basic Stamps due speed, capacity, and price reasons. I've already purchased a semi-universal programmer, and am looking a couple good PICs to start with. I was thinking about a lower end PIC for simple projects, and a mid-range PIC for more advanced stuff. I has thinking about the 12F629 for the low-end stuff, and the 16F88 for mid-range stuff. Would these be good PICs to start with?
Also, apparently these come with their own internal oscillators. Am I correct in understanding that these will not need crystals unless I want to clock them at different speeds? Do external oscillators add any benefit for these PICs? Speed? etc? Thanks in advance for any insight...
most of the pics have the internal osc, you only need to supply Xstal or Cap (depending in which one) to get the internal clock to operate. after you have master the pic, there are also things like the ATMEL chips (AVR), they use linear addressing.
I've never used a 12F myself, but that's just because I didn't have any. They should be fine for small projects where you only want a few i/o pins. Since I've never used one personally, the rest of my response will be on the 16F88.
For the 16F's, I started on the F84 (a few moons ago ;-) and then switched to the 16F628. Now, I'm tinkering with a 16F88. I like it, but it's a bit sensitive to PS noise. Be sure to use bypass caps on the cpu or you will likely see mysterious device resets. This is the first PIC I've used that has a real ADC in it (the F628 just has comparators). I just used the ADC for the first time yesterday, worked like a champ first time out. ;-) The 16F88 is also a member of the nanowatt PIC family and really uses a miniscule amount of power.
Note: I just noticed a warning in the datasheet saying to use a pull-up resistor >1k on the MCLR line instead of tying it directly to Vdd or latch-up may occur due to spikes. Hmm....I guess I better change my methods. I've always just tied it high on the other PICs with no ill effects.
Yes, the 88 has a multiple speed internal oscillator that runs from
31KHz to 8MHz. It's reasonably accurate for most projects (+/- 1% typical at 25C). I wouldn't depend on it in production if rs-232 comms are going to be used, especially over any realistic temperature range.
I would recommend you use crystals if you need clock time accuracy or even with serial comms. If you can live with a little bit of inaccuracy, the internal oscillator is fine.
You can crank the F88 up to 20MHz if you use a crystal.
The 18F452 might be interesting to you. It will run at up to 40MHz using a 10MHz crystal giving you a 100nS instruction cycle time. It also has certain advantages over the 16F's in terms of the instruction set and the architecture.
Thanks Jeff. As a follow-up, will the internal oscillator be good enough for serial communication up to, say, 9600 bps? Under what circumstances would I need tighter tolerances for the USART?
I've used the 16f628 serial I/O at 19,200 with the internal 4 mHZ oscillator without errors. If the oscillator changes frequency by 1 part in 18, you lose the last data bit. 1 part in 16 loses bits 6 and 7, etc. But the internal oscillator seems to be pretty good at 1% or better. I think I put the circuit in the freezer, and then under a heat gun, and it survived both tests.
That depends on what you need. The 12F629 is a bit short on I/O pins, but of course very cheap. The 16F88 is the king of the 18-pins 14-bit core chips, but you might want more pins (maybe an 16F877A) and/or more CPU power (18Fxxx). Personally I think the 16F630/676 are very good value for money. There is so much to choose from. Just stay clear of the 16x84(a) unless you are building an existing project and don't want to modify the code.
The internal oscillator means the you need fewer components and (for some chips) can change speed on the fly. Be aware that the internal oscillator is not very accurate.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.