Hi Satya,

As my university professor of Logic points out, in engineering (and
logic) you have to be very precise in making statements and in asking
questions. I beg your pardon, but your questions are a bit fuzzy. But
since I started PICs from zero, I can understand where the wooly-ness
comes in.

PICs are 8 bit MCUs, meaning the data that they work on is 8 bits wide.
For example, if they are programmed to add something then they add 8
bits to another 8 bits and give an 8 bit answer (with a carry). So all
data that they deal with is 8 bits (each data memory location is 8 bit
wide). You can of course extend this to 16 bit, 24 bit, etc. by simply
using 2 or 3 or more registers to do the arithmetic. The 12, 16, 17 or
18 series is a way of Microchip to identify the series [they also call
it base-line (12bit), mid-range (16bit) and high end (18bit)]. Each PIC
series is capable of addressing up to so many _program_ memory
locations. For example: "The PIC16F870/871 devices have a 13-bit program
counter (PC) capable of addressing an 8K x 14 program memory space". The
program counter is 13bit wide so it can address  2^13=8K program memory
locations. Now PIC instructions are 14bit wide, and since programs are
composed of instructions, the program needs 14bit wide memory locations
to get stored in. So PC x InstructionWidth = 8K x 14. But the actual
program space physically implemented may be lower in quite a few devices
(PIC16F870 has 2K). When you write a code for this, make sure the
compile comes to 2K or less.

>And what exactly we mean when we say PIC can address 16 bit at a time?
I don't think it can address 16 bits at a time, at least not in
PIC16F870. Using a 13bit program counter, it can address 2^13 locations.

>Is the memory addressed in RAM or ROM are they talking about?
ROM of course. You don't even need to address RAM locations per se (only
address the banks, RP<1:0>). You simply give "names" to GPRs (Temp0,
Temp1, NumHi, etc.) while SFRs already have them. And then use these in
the instructions of your program. The RAM consists of 8bit wide
registers (that's why the PIC is an 8bit MCU), that you use to add,
subtract, OR, AND, move, shift, input, output... Cut a long story short,
RAM is used durng runtime to do what a PIC is supposed to do.

Btw, can somebody tell me why when the program counter is 13bit wide,
instruction 14bit wide, data registers 8bit wide; does Microchip call it
a 16 series?

Mohit.

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Prasad, Satyaprakash
(Consultant)
Sent: Wednesday, November 26, 2003 11:26 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]:PIC Types


Hi All,

When we say that it is a PIC of 12,16,17 or 18 series, is it that we
actually mean to differentiate it with the memory bit it can address at
a time. Example PIC 18 series can fetch and process 16bit instruction at
a time, similarly PIC 12 series executes 12 bit. In this case how many
bits a PIC instruction would be(8,12 or 16)bits? Is it possible that an
application developed for PIC 12 could execute on PIC 16 series. And
what exactly we mean when we say PIC can address 16 bit at a time? Is
the memory addressed in RAM or ROM are they talking about? Regards,
Satya Prakash Prasad

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