Microchip PIC programming question

Wow, thanks for the quick suggestions!

Okay, now I know that signal A and B need to be different.

I can replace signal A with one with a different sensor that would read from 1.1v - 2.0v and give 180 steps which should be enough.

Or as John suggested just amplify the signal by 50. Ol' Duffer mentioned a buffer amp. Is there one that could be used as a buffer amp and multiply by 50 (two birds - one stone)?

Sensor B can also be run with a different sensor that would read from

0v- 3.5v so there should be no problem there. Since I am just learning, I didn't realize that Vref had to be less than the supply. How do I know if I need a buffer amp?

I want to learn how to do this with the PIC assembler language. I learned C in college, so if this becomes difficult, perhaps I will go to C, but for now time to learn something new. Just out of curiosity, how much easier would it be to do this routine in C instead of assembler? If it is that much easier, perhaps I would do it in a language I know first then try assembler after I learn some more basics like vref shouldn't go too high and the like.

Thanks again for your help, Andy

Use a single-supply or rail-to-rail opamp, in which A goes to the non-inverting input, and the feedback to the inverting input is through a precision voltage divider that creates 1/50th of the voltage of the output. Then, with your original sensor, you have a range of from 1.5V to 3.75V. Noise and output errors will also be multiplied accordingly. An LM324 would work with 6V Vcc.

For the 2.5 to 6V input, just subtract 1.5V from it, using the traditional opamp circuit, and it'll go from 1V to 4.5V.

Both of these ranges will be reasonably accurate on the PIC, and you'll get some buffering from the opamps.

If your reference is 6V, simply use a couple of precision resistors to divide it down to something the PIC likes. If you use the scheme above, a 4.5V reference would be ok, if you use 5V as Vdd.

C is about 20x easier to design and debug. You'll be using division and multiplication. The PIC18 series has multiplication in the instruction set, but it's 8 bit multiplication. There is no division, so that will be done in software. You can get both floating point and fixed point libraries from microchip that you can call from assembler.

Note that you can download a free C compiler for the 12 and 16 series from

(hitech-lite.)

If it is that much easier, perhaps I would do it in a

--
Regards,
  Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
                                  Thomas Henry Huxley, 1877

The PIC processors are speced to handle a supply voltage between about

2.2 and 5.5 volts. Analog inputs are allowed to be between the supply rails. The analog to digital converter has a 10 bit resolution (one part in 1024) but only if its full scale reference is most of the supply voltage. So if the chip is powered by 5 volts, and the A/D converter uses that supply as its full scale reference, then each discrete level is 5 V / 1024 = 4.9 mV That means, if you try to digitize your 30 millivolt signal, the A/D converter won't be able to distinguish it from 34.9 mV.

However, if you multiply this voltage by a fixed gain with an opamp, (say, 50 times) so that the maximum value is closer to the PIC supply voltage, the resolution will more closely approach the full .1% the converter is capable of.

The 6 volt peak value of the second input is a problem for a different reason. It will exceed the absolute maximum voltage for the chip, and once it gets higher than the A/D converter full scale reference voltage, all higher voltages will give the same full scale output. So, this signal will have to be scaled down a bit to make it compatible with the PIC. If the signal can stand a bit of current load, you can accomplish this with a two resistor divider.

The Microchip site has math libraries you can use to do the division, but it is a fun exercise to go through this development just to get familiar with how this works.

And the PIC A/D converter is a leaky piece of crap. When they say input impedance should be < 2.5K, they aren't kidding. You often need a buffer amp even if the input signal is at a reasonable level. And don't even think about driving an input above the supply rail, because this makes all the other inputs leak even worse than spec.

A buffer amp just copies the voltage (voltage gain of 1) while providing any current the load requires, without that current having to come from the signal. Any opamp suitable for the input and output voltage range can be used as a buffer or amplifier with a voltage gain higher than 1. (snip)

I like the MCP6022 dual rail to rail opamp when having to work with a

5 volt supply (but there are lots of others).

You are going to have to divide your second input voltage to bring it below the supply voltage of the chip. The same for the Vref. The first input will need amplification to give acceptable resolution.

This should fit into a single 676 _easily_. I have a board I did some while ago, that takes readings from a Ph sensor, temperature sensor, and ammonia sensor, linearises the ammonia reading based on two calibration points, and a logarithmic curve fit (a lot of arithmetic), outputing a new reading every second. At the same time it performs a servo loop controlling a heater with one of the PWM outputs, and regulating this based on the temperature sensor reading. Finally it converts th Ph sensor voltage into a Ph value. This only uses 85% of the code space in an 676. Realistically, the code would be much quicker to write, using a language like 'C', where in the versions for the PIC, most of the work to perform basic I/O, and the code needed to retrieve tables etc., is already written for you. It is down to whether you really want to learn the PIC assembler or not. There are at least three quite well known C compilers available for the PIC.

Best Wishes

For a lot of ideas on PIC's - See website below.

--
Luhan Monat: luhanis(at)yahoo(dot)com
http://members.cox.net/berniekm
"Any sufficiently advanced magick is
indistinguishable from technology."

I went to

(hitech-lite.) and couldn't find a compiler. I did a search and found Hitech PICC-Lite at htsoft.com.

Based on the PIC covered I figured I will upgrade from PIC16F676 to PIC16F684 which is covered in the software. After comparing the two for the extra couple sense it has a few nice extras.

Andy

One piece of advice that I still haven't taken to heart seriously. Get familiar with how the C flag is used during subtraction. It's a bit tricky, being the reverse of addition. You can easily get your btfsc/s instructions the wrong way round and it all gets confusing. Good luck.

Or you could just make some macros and save yourself the confusion.

ifbit macro file, bit ; easier way to think about the test/skip instructions btfsc file, bit endm

ifnbit macro file, bit ; easier way to think about the test/skip instructions btfss file,bit endm

jz macro JumpLabel ; Jump to the Specified ifbit STATUS, Z ; Address if the Zero goto JumpLabel ; Flag is Set endm

jnz macro JumpLabel ; Jump to the Specified ifnbit STATUS, Z ; Address if the Zero goto JumpLabel ; Flag is Clear endm

jc macro JumpLabel ifbit STATUS, C goto JumpLabel endm

jb macro JumpLabel ifnbit STATUS, C goto JumpLabel endm

jnb macro JumpLabel ifbit STATUS, C goto JumpLabel endm

cheers,

Al