6551 vs 6850 vs 8250 vs ?

[Dajgoro]

Hello!

I am building a 6502 based system, and now i was looking to this ic, and i found them all three on ebay for a few $. So whitch is better for a 6502 based system(1mhz cpu): 6551 vs 6850 vs 8250 or something else?

[BigDumbDinosaur]

Dajgoro wrote:

Hello!

I am building a 6502 based system, and now i was looking to this ic, and i found them all three on ebay for a few $. So whitch is better for a 6502 based system(1mhz cpu): 6551 vs 6850 vs 8250 or something else?

Of the three, the 6551 is easiest to interface to a 65xx bus—simple glue logic will handle chip selects, and the rest of the control lines attach directly to their 65xx counterparts (e.g., /RESET). However, the 6551 has a quirky (ugly, in my opinion) programming model and unless you do something hinky with the baud rate clock, is limited to 19.2 Kbps. Also, there's a minor booby-trap involving the DCD signal.

The 6850 isn't too difficult to interface to the 65xx bus, as it was designed for the Motorola 6800 series MPU, which has a similar bus to that of the 65xx. The 6850 can be run at higher baud rates than the 6551, and has a much better programming model. It lacks a hardware reset function, so you'd have to cobble up something to toggle Vcc when /RESET is asserted. Otherwise, I recommend it.

I would stay away from the 8250, but that's just me not liking its general design. If you're going to go that route you might as well look at the 16550, which has an onboard RxD FIFO to better handle high data rates.

My personal choice for a UART is the NXP 2692A, which is what I used in my POC computer.

[Dajgoro]

I think go for the 6850(most expensive on ebay), because i can use external clock, i don't have to buy specific quartz (no electronic store in my town). what is the max input clock for the 6850?

[GARTHWILSON]

The 6850 does not have its own onboard oscillator and baud rate generator IIRC.  The 6551 can use an external oscillator if you want.  It's not restricted to the internal one, or even the internal baud-rate generator.

I know BDD doesn't like the 6551, so I have to stick up for it a bit.  I have used the 6551 at all standard baud rates from 50 to 19,200 (the slower rates being when I was experimenting with cheap modems for cassette tape 25 years ago), plus 31,250 for MIDI, using 500kHz generated by a 6522's PB7 output off T1 connected to the RC (pin 5) input, and never had any trouble with any of it.  I know it can go to at least 125kpbs with external clock input on pin 5 for both transmit and receive.  My 6551 hardware and software has always worked on first try except when I lacked the 22pF capacitor from pin 6 to ground in the crystal circuit.

The only thing I don't like about the 6551 is that a couple of the controls are merged into the same control bit, where it would be nice to control them separately.

Do use the CMOS version though (65c51) because the NMOS did have a quirk in that, IIRC, if its CTS\ goes false during the transmission of a byte, that byte does not get finished.  Instead, it is lost.  The CMOS version will finish the byte.

For any future designs I will probably just use the 14-pin-DIP SPI MAX3100 which has 8-byte transmit and receive buffers, and talk to it through a 6522 or Daryl's 65SPI.

[Dajgoro]

For external clock generating, can i divide the cpu clock with a 4040, and use it's outputs?

[GARTHWILSON]

That kind of thing is pretty standard with the 6850, right?  (I'd have to dig into some old info I don't access very often.)  It's not necessary with the 6551 though since it has its own onboard baudrate generator which can be used even if you choose to use an external oscillator.

[Dajgoro]

I will use the 6850, i will try using divided cpu clock for interfacing it to my pc (using max232 chip), will my pc be able to read serial data with weird(not to fast, but some frequency with many decimals) input clock? So i can use it as a i/o to test EhBasic before i finish the video module with the MC6847 CRTC chip?

[GARTHWILSON]

It is not clear what you want to do. Does the PC have an RS-232 port?  If so, it can read standard RS-232 speeds, but you will get loads of errors if the speed you send is more than a couple percent off of the speed expected by the PC.

[Dajgoro]

I just need to wire up the 6850, so that it can communicate with a normal pc COM port at some normal speed. So how do i wire up the 6850 so that i can communicate with my pc (i will have a max232 module)?

[BigDumbDinosaur]

Dajgoro wrote:

I just need to wire up the 6850, so that it can communicate with a normal pc COM port at some normal speed. So how do i wire up the 6850 so that i can communicate with my pc (i will have a max232 module)?

Take a look at this article before you get yourself too confused. Also, see here for a data sheet.

Your questions suggest to me that you need to learn more about the use of ACIAs in general before you start soldering stuff together.

[BigEd]

Dajgoro wrote:

I just need to wire up the 6850, so that it can communicate with a normal pc COM port at some normal speed. So how do i wire up the 6850 so that i can communicate with my pc (i will have a max232 module)?

Hi Dajgoro
Please have a look at Deiter's page for an example. Several people here have hooked up similar systems, so you might try a search like this for other posts mentioning max232.

Cheers
Ed

Edit: removed commentary on previous comment

[BigDumbDinosaur]

BigEd wrote:

Please have a look at Deiter's page for an example.

From Deiter's page:

• Typical values for the capacitors connected to the charge pump are 2.2 to 10 uF. (There also is a version of the MAX232 that runs with 100 nF capacitors.)

Once again, I must point out that Dallas Semi's (formerly Maxim) datasheet says the charge pump capacitors should be 1 uF, not 2.2 to 10 as stated above (please refer to page 17 of the datasheet).

I was advised by a Maxim application engineer a while back that exceeding the charge pump capacitor recommendation will not improve the device's operation, but will cause an excessive amount of inrush current to flow at power-on, overstressing the device and eventually resulting in failure.

[BillO]

To me anyway, the MAX232A IS a version of the MAX232. Just like the 6502A or 65C02 are versions of the 6502.

In fact, so many different manufacturers have made or make so many different versions of the MAX232, it boggles the mind.

...or am I totally lost in space...?

[BigDumbDinosaur]

BillO wrote:

To me anyway, the MAX232A IS a version of the MAX232. Just like the 6502A or 65C02 are versions of the 6502.

The MAX232A has a higher slew rate than the MAX232, but accomplishes it at the expense of drive strength. For general use, the MAX232 is a better (and usually less expensive) choice. The 232 will handle 115.2 Kbps data rates with ease over at least a fifty foot cable.

Quote:

In fact, so many different manufacturers have made or make so many different versions of the MAX232, it boggles the mind.

...or am I totally lost in space...?

Only Maxim made all the versions, as far as I know. This was a highly proprietary design and I don't believe Maxim licensed most of the product line for second source production. However, there were a few pin-compatible devices available from other sources. BTW, it was only in the last few years that the original patents on the MAX232 series expired.

[Dajgoro]

Well, i wanted to be able to connect my project to any computer, so i bought this from ebay:
http://www.ebay.com/itm/CP2102-Module-S ... 3a69272f3b

Usb to ttl module, but i if i got it right this module does not have any clock input or output, how do i connect it the to the 6850(or elsewhere)???