[sdiy] PIC help

[Eric Brombaugh]

Scott Juskiw wrote:
> 1. What programmer/debugger do you use or recommend? I don't want to 
> build one, or get some third party box. I've narrowed the choices down 
> to either the PICkit 3 ($70) or MPLAB ICD3 ($220) and am wondering if 
> there's any significant advantage to the ICD3 that makes it worth being 
> 3X as much as the PICkit 3.

I've had trouble with 3rd-party programmers, so I use a true-blue MCHP 
ICD2. Never have had any trouble with it. You could start out on the low 
end and migrate up if you have problems though. I know some folks are 
perfectly happy with other programmers. YMMV.

> 2. How do you decide which PIC chip to use? I know some of you are using 
> the dsPIC33FJ128GPxxx series (Tom, Seb, Eric) and I'm more than happy to 
> follow your lead, but it might be overkill for my needs. But they aren't 
> too expensive, should I just go with the flow and use the latest and 
> greatest chips?

I used to use the old 8-bit PICs. Never again. dsPICs are cheap cheap 
cheap - you can get low-end ones for less than $4 in small qty from 
Microchip's store. I'm not looking back because the 16-bit parts (dsPIC 
and PIC24) are so easy to use.

> 3. Some of the dsPIC chips have a built-in 16-bit DAC but it appears to 
> be best suited for two channels of audio, not for 24 channels of control 
> voltages. Would I be better off using an external DAC to get 24 analogue 
> outs and avoid the internal DAC?

I agree - the '80x dsPICs with stereo audio DACs are really aimed at 
audio use because the voltage specs are pretty sloppy. Go with an 
external SPI DAC if you need tight control. Also, the on-chip DACs are 
sigma-delta, so there is some latency and built-in filtering that would 
be a nuisance in CV applications.

> 4. If using an external DAC, is there any advantage to using one DAC 
> with lots of multiplexing to get 24 outputs vs. using several 8-channel 
> DACs (e.g. TLV5610)?

One DAC with muxing will be a lot cheaper. Takes more code of course. 
Your call.

> 5. It seems that dsPIC chips don't have enough built-in EEPROM to store 
> all the info I want for the presets. Is it best to avoid trying to max 
> out the internal EEPROM and use an external EEPROM? (I see that Seb has 
> hooked up an EEPROM to his Digi-Mod.)

You can use the internal program flash of the dsPIC for storage of 
long-term data. There are some concerns about the number of write cycles 
you can use on the 33F series parts, but I haven't run into that myself. 
Use an external I2S or SPI EEPROM if your worried - they're cheap.

> 6. What do you recommend for an LCD display (2x40 character or less) 
> that would be simple to hook up to a PIC? Do these things have some kind 
> of common interface (SPI)?

Most cheap LCDs (<$10) these days use a parallel interface first 
designed by Hitachi more than 20 years ago. It takes at least 7 bits of 
GPIO to talk to one of these parts. Some enterprising folks make little 
daughter cards that will convert UART/SPI/I2C to the parallel format, 
but you pay extra for that. Your call - pay more for a larger PIC with 
sufficient GPIO, or pay more for the interface cards.

Eric