On Fri, Feb 22, 2002 at 05:38:57PM +0530, Mohit Mahajan wrote:
> Hi,
>
> I'm new to PIC. I am planning to implement a control system using PIC16F877.
> I'll be using a keypad, LCD, almost all I/O pins (to input the control
> variables, operate four switches, PWM etc.).
>
> The code for all this is going to be large, no doubt. A PIC expert could
> optimise it to fit in the 8K ROM of the PIC, but as I'm a novice I'm not
> sure I'll be able to do so. In any case, since I'll either use JAL or a
> C-compiler, the code will be more than 8K. So now my question is:
>
> Can we interface an EEPROM [specifically: 24LC256, 32Kbyte Flash EEPROM
> (I2C)] to the PIC in such a way that the firmware code can be written in the
> EEPROM, from where the PIC can fetch it in packets and run it?

Not with the standard midrange PICs. The 17C (and 18C series I think) have
such a microcontroller mode.

But I think you may be making a faulty/hasty assumption by presuming that
the code won't fit. Being a novice you simply don't have enough expertise to
make that presumption. Nothing that you listed above consumes a significant
amount of code. The only variable is your application code computation.

So why not simply get the project started and see how far you get. I have
a sunrise/sunset controller with display, knob/pushbutton interface, timer
code, calandar, sunrise/sunset interpolation, and relay control interface.
It takes barely 600 bytes of assembly. Even at 3x code bloat by switching to
an High Level Language, that's less than 2K.

>
> It doesn't matter to my application if the effective operating speed
> decreases, because I'll be controlling pH, temperature, dissolved oxygen
> (all these take few minutes to adjust).
>
> If no, and the firmware is greater than 8K, then what should I do? Is there
> anything available with Microchip that has a larger (flash) ROM and as many
> I/O as the PIC 16F877? I've read about PIC18F242, but it still isn't out in
> the market.

Simple. Split the tasks between two PICs. It effectively doubles your code
space and I/O space. Use the USART for the two parts to communication to
one another.

But frankly I think you're overanalyzing the problem. 8192 instructions will
take you a lot further than you think.

BAJ

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