CMOS 6502 alternative - Page 1
Forum section: electronics / general

author: netdudeuk
author_rank: Frequent Contributor
timestamp: October 02, 2014, 07:43:14 am
content: HiThese (W65C02S8P-10 ) are pretty cheap and faster than the classic MOS 6502s but does anyone know if they would still work happy amongst regular 74LS support chips ?http://www.ebay.com/itm/1PCS-W65C02S8P-10-65C02-6502-8-Bit-10MHz-MPU-DIP40-NEW-D18-/300948544902?pt=LH_DefaultDomain_0&hash=item4611ee5d86I haven't used many CMOS devices. Is there a good chance that these CPUs would be damaged without proper ESD precautions ?Thanks

author: Kjelt
author_rank: Super Contributor
timestamp: October 02, 2014, 08:36:03 am
content: were you planning on running it on a higher frequency? If the clock stays the same as in the original design I don't see the problem?

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 02, 2014, 08:54:55 am
content: Hello,to my knowledge, this is only another 2nd source for the CMOS 65C02:http://en.wikipedia.org/wiki/WDC_65C02.Maybe that the C versions already have some extensions to the instruction set (bit manipulation, addressing modes, shr, shl,...). But it's fully downward compatible..The 65C02 chips also had CMOS counterparts for the peripheral chips, as VIA, PIA, UART, and so on.If you instead need to interface directly to TTL - LS logic, you may compare the original 6502 versus the 65C02 datasheet, if the CMOS version has the same fan-out capability ( I assume so).Otherwise, better replace the TTL chips by their CMOS counterparts also.Frank

author: netdudeuk
author_rank: Frequent Contributor
timestamp: October 02, 2014, 09:16:48 am
content: Thanks for the replies.It looks like I'll be better off getting an N-channel version so I can use my existing TTL LS chips.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 02, 2014, 10:25:35 am
content: Quote from: netdudeuk on October 02, 2014, 09:16:48 amNo, the CMOS version seems to have equivalent output drivers..See datasheets on:http://www.6502.org/documents/datasheetsThe output levels vs. fan-out = 1 are specified identical between e.g. Rockwell 6502 and W65C02.And I doubt, that you will still get NMOS parts today, for a reasonable price.Frank

author: tggzzz
author_rank: Super Contributor
timestamp: October 02, 2014, 11:30:05 am
content: Quote from: netdudeuk on October 02, 2014, 07:43:14 amI'll avoid giving a direct answer (since i don't know these specific devices), but I'll note three topics that you may care to examine.Firstly, and simply, check the input logic thresholds are compatible with the output levels, both for processor->logic and logic->processor.Secondly, when you are replacing a "slow" device with a "faster" equivalent device at the same clock rate, problems can be caused if the propagation delay to an output "has become" shorter than the driven input's required hold time.Similarly, if the clock is faster, you should ensure that the propagation time to an output does not violate the driven input's require setup time.

author: SeanB
author_rank: Super Contributor
timestamp: October 02, 2014, 04:45:17 pm
content: You could retrofit the original ( and all the clones) of the Apple ][ with the 65C02, and it would work. Fan out is not likely to be a worry, as you would have LS parts there not regular TTL, and they are lighter in loading. Worst case you would have to install a set of bus buffers to the address lines and a set of bidirectional buffers to the data lines.That would only be a worry in larger systems, where the CPU bus is sent to a few cards over a long backplane. If you are doing a simple system with a CPU, and a bus decoder and ROM and RAM with a few other memory mapped chips like a display driver and VIA then it will not be needed.

author: westfw
author_rank: Super Contributor
timestamp: October 02, 2014, 05:58:10 pm
content: QuoteDidn't NMOS have fanout capabilities that were at least as crappy as the later CMOS parts? Pretty much the only TTL you can drive with either sort of chip is a bus-driver :-) I mean, an NMOS output stage looks like an open-drain CMOS output stage with an internal pullup, right? (You could get systems more complex than "1 NMOS output drives 1 TTL Driver chip by building the system with other CMOS/NMOS chips.)

author: SeanB
author_rank: Super Contributor
timestamp: October 02, 2014, 06:17:40 pm
content: As I said, fan out is not a worry, the original part was barely capable of driving one TTL standard input load, or 2 LS inputs. If you used it in anything more than minimal with one ROM, a CMOS ram and a VIA you needed buffers.

author: guido
author_rank: Regular Contributor
timestamp: October 02, 2014, 08:21:47 pm
content: The CMOS version does not support some undocumented opcodes. So depending on the application, there is a chance it doesn't work. E.g. some Acorn games used those.The nice thing about the CMOS version is that you can stop the clock. I've used that in the past.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: October 03, 2014, 07:44:11 am
content: Quote from: Dr. Frank on October 02, 2014, 10:25:35 amThere are still lots of places that sell NOS parts for just a few dollars, from ebay, to places like Unicorn electronics.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: October 04, 2014, 09:09:01 am
content: Thanks for the replies. Interesting comments.I picked up a brand new N-channel one for only $6. It's the 3MHz part and that's ok by me.I've got plenty of LS244s and LS245s to buffer the buses, just in case.

author: SeanB
author_rank: Super Contributor
timestamp: October 04, 2014, 02:49:34 pm
content: Buffer all busses and control lines, the faster edges and better drive capability, along with only having to replace a cheap bus driver are worth it. For added protection place 100R resistors in the data bus between the processor and the data buffer, just in case you actually use DMA and halt the processor during a write and override the data bus direction.