ADC does not work with external reference

[Ttelmah]

This gets interesting.

I am using an old data sheet, and quite an early chip (may even be an engineering sample - it's what I had on the shelf...). On my data sheet the table is given as:

[PCM programmer]

MPLAB reports this silicon revision for my 16F917:

[POPE19]

I am really confused. Ttelmah & PCM Programmer could you please explain.

[Ttelmah]

I have an extremely early (pre release) engineering sample chip (B0), and a paper data sheet that came with this.
PCM_Programmer has a B3 revision chip (the current chip).

His is behaving like yours. Mine is not. On mine the code has to change but it then works. On his (like yours), the chip behaves as if it is using the supply as the reference all the time.

The listed errata, do not mention the ADC anywhere.

Other people have used the ADC with an external Vref, but also with oddities (though the other way round):
<http://www.microchip.com/forums/m220537.aspx>

Another user has it working OK:
<http://www.electro-tech-online.com/threads/cricket-thermostat-kit-concept-16f917-lcd.31994/>

Now these are both using setups that match what CCS is currently doing.

So it should work. The question is what is different?.

One thing is possibly the LCD setup?. Given that the Vref+ pin is segment 15, it is possible that this is interfering somehow.

You are selecting MUX_14, and 0x1E000F for the segments. So segments 0 to 3, then 17 to 20, which looks right.
On two of the other chips, there is the following comment:

[POPE19]

Ttelmah, I am checking using LCD. As I said I am newbie I really don't know any other way to check the output of PIC in any other way. But I will really appreciate if you please guide something so that I can use serial port PCM. As it will be beneficial all newbie like me in future. I will try that too and let you know the results without using LCD.

I really appreciate you guys explaining such deeply making a newbie like me easy to understand and learn at same time. Keep it up.

Waiting for your reply.

[PCM programmer]

It's this one:
https://www.ccsinfo.com/forum/viewtopic.php?p=75407
Look at the 2nd post from the bottom. It's a bug in set_adc_channel().
Not fixed in 10 years. Probably never reported.

[Ttelmah]

That is interesting. On my chip, that makes it stop working!. It permanently gives 0. It is resulting in the Vref- floating. If I ground the Vref- pin it starts working, but is using Vdd, unless I set VCFG1.
MPLAB reports my revision as 00000001, which according to the current data sheets doesn't exist (the earliest they list is 00000002). Would have to trace my correspondence (physical letters in those days), but I suspect it was an engineering sample.

So a silly error in CCS, that may date from them writing the code for a really early chip, and never revising it to the working chips....

[PCM programmer]

I sent off a bug report to CCS support.

[POPE19]

Thanks a lot PCM Programmer & Ttelmah. Can you please suggest something so that i can watch the output of a pic on computer ?

[PCM programmer]

[POPE19]

I don't have db9 connector in my computer but I only have USB ports. I am making my circuit on breadboard. Please suggest. I read I have to get rs232 ic for serial connection so I got the hardware circuit but I don't know which software would I use and how to proceed or whether I am going in right direction or not.

[temtronic]

What you need is a 'TTL to USB ' adapter module.

I get them for about $2 CDN with free shipping from China. Usually I buy x5 to get a bit of a price break and, well, you always need a spare !!
I've never had a bad one over the years and it's cheaper than buying a PIC with USB and adding parts !

Connections are easy

PIC USB module
Ground Ground
Txd Rxd
Rxd Txd

That's it....

It's best to use the PIC's HW UART, if it has one, otherwise any I/O pin can be used to send data to the PC, CCS makes that EASY !

I cannot buy the PL2303 'USB chip', connector, resistor, PCB for $2 ! NOBODY can, so buy the module. One source is Banggood and they offer free shipping. Any local electronics supplier may have them so check around.

You can use a free 'terminal' program like RealTerm to send/recieve data. There are others but I started using it years ago and,well, it works, so why should I change?

edit: forgot to mention most(all ?) of these TTL<>USB adapter modules have LEDs for both power and data. I unsolder/remove the power LED more so because it's TOO BRIGHT...and it saves power.

edit2: I also now buy RTC/EEPROM modules instead of discrete parts. Again CHEAPER than making it AND the last ones came with RECHARGABLE coin cell batteries !

Hope this helps
Jay

[POPE19]

thanks a lot to all of you . PCM Programmer I am getting output with 3.3 V reference as per your suggestion. I am still kind of confused how to display output of a pic on my computer .

Temtronic is there a link where some detailed instructions of what you said is available especially software part . Please suggest .

[temtronic]

First, check the FAQ section of the CCS manual! They show code(easy...) as well as how to connect a MAX232 for an 'old' PC with RS232 ports.
While in the manual, lookup 'putc', 'getc',printf, and then other 'blue links' they say are 'related'.
Also look in the 'examples' folder. Most programs will use the PC as a 'display device', and check out code in the 'code library' forum here as well.

Jay

[POPE19]

thanks a lot Temtronic. will give a shot once i receive the cable.