Hi Martin, The difference is in the "always set bits" in the FSR of the two devices. In the 12C508, bits 5, 6 and 7 are always set (ie the values for the 12C508 are always 0b0111xxxxx) whereas in the 12C509, bits 6 and 7 are always set (ie 0b011xxxxxx) and bit 5 can be programmed. The issue for me was, I was creating a sixteen byte circular buffer. In the application I was working on, I was just at 512 instructions. If I used a '508, I could use the code to "push" into the buffer: movwf INDF ; Save the byte in "w" incf FSR bsf FSR,4 ; Keeping Pointing to Address 0x010 to 0x01F If I used a '509, this code would balloon to: movwf INDF ; Save the byte in "w" incf FSR bsf FSR,4 ; Keep Pointing to Address 0x010 to 0x01F bcf FSR,5 This extra instruction meant that I couldn't keep the code within 512 instructions. Cost was a big issue and the $0.12 difference between the '508 and '509 was a customer concern (they were worried about the final cost of the product). Not being able to keep the code within 512 instructions meant that I had to modify the program to change PA0 in the '509s STATUS register for some jumps... Now, I could have macro'd the program up so that it would work in both a 12C508 and 12C509 depending on how it was assembled. But,... It was a lot easier harassing Active until they had a few windowed '508s in and held for me. Incidentally, I had the same problem with Digi-Key, everytime they'd get a hundred or so in, they'd be sold out that day. myke Funny story about this with Active regarding this; trying to get the windowed '508s from them, one of the kids on the floor told me he had just finished reading "Programming and Customizing the PIC Microcontroller" and said that there wasn't any difference in the FSR between PICs part numbers. He then suggested that I join the PICList and contact the author of the book, who could explain it to me... I got five windowed '508 parts two days later. > For the last couple of months whenever I've gone to Active they only > had the 509 device, OTP and windowed, but you got my attention - I > didn't realize there was a difference in the FSR between the 508 and > the 509. Could you elaborate? > > > TIA - Martin. > > >______________________________ Reply Separator _________________________________ >Subject: Re: PIC12C508 >Author: pic microcontroller discussion list at >Internet >Date: 5/19/98 11:21 AM > > >Martin, > >Active Components at Victoria Park and Gordon Baker do stock the windowed >12C508, but they seem to sell out of them quickly (to buy five, I had to go >three times - for my application, I couldn't use a 12C509 because of the >differences in the FSR register). > >Give them a call (498-9886) and they'll reserve some for you, or at least >tell you how many they have on hand or when they're getting more. > >They cost around $12 (CAN). > >myke > >> Here in Toronto Canada none of the local stores seem to stock the >> windowed version of the 12C508, but they do have the 12C509. Since >> the only difference is the memory space, I used one of these to >> develop a 12C508 circuit. It worked fine. >> >> >> CIAO - Martin. >> >> >>______________________________ Reply Separator >_________________________________ >>Subject: PIC12C508 >>Author: pic microcontroller discussion list at >>Internet >>Date: 5/19/98 9:26 AM >> >> >>I 've seen in Microchip's Datasheet that the PIC12C508 should be available >>in UVPROM version. Is it easy to find this part in the US as It's almost >>impossible to get it in France. >> >>Thanks, >> >>Richard. >> >> > >"If people don't know what you're doing, they don't know what you're doing >wrong." - Sir Humphrey Appleby K.C.B > > "If people don't know what you're doing, they don't know what you're doing wrong." - Sir Humphrey Appleby K.C.B