Hi guys

I’m planning the next revision of my 65C02GPD design (v2).

As part of that, I’m looking into changes that I want to make. Here’s my list so far:

- Change the CPU and VIA’s from DIP packages to PLCC. Affords more room on the PCB.
- Change the DIP socketed 55ns RAM to 10 or 12ns soldered SOJ RAM
- Change the single row pin headers used by the VIAs to IDC sockets
- Add two additional VIAs for a total of three.
- One VIA to be a system VIA and deal with onboard devices (see below)
- Add an I2C BIOS settings EEPROM - attached to system VIA
- Add an I2C user settings EEPROM – attached to system VIA
- Add an I2C real time clock (RTC) module – attached to system VIA
- Add an external set timer – attached to system VIA
- Add a buzzer/sound header – attached to system VIA
- Add VGA or HDMI display
- Replace the WDC 65C51 with a different ACIA

Questions

ACIA: Can anyone suggest a suitable replacement for the WDC 65C51? It needs to tolerate 14MHz at least (hopefully higher). I’ve looked around in the past for such a device but could only find low speed ones.

VIDEO: This is something that I would dearly like to add to the design, but have no experience with.
I know in the past that the RAM was shared between the video circuitry and the main buses (the BBC micro uses a ‘245 buffer for this). The master clock is split down into two clocks which are 180 degrees out of phase with one another. I.e. when one is high the other is low and vice versa.
One clock is used as PHI2 and the other is the video clock. When PHI2 is low, the video clock is high and the ‘245 buffer is disabled thus allowing the video circuitry access to the RAM without risk of contention. When PHI2 is high, the ‘245 is enabled,, thus allowing the CPU access to the RAM.
Is it still done this way? Also, does anyone have access to a details (but simple) guide?

Depending on how many channels you want, I suggest the Philips/NXP SC28L91 (one channel), SC28L92 (two channels) or SC28L194 (four channels). All three have a common programming model, 16-deep FIFOs on both RxD and TxD, and excellent performance, with a maximum standard data rate of 230.4 Kbps and 960 Kbps using a non-standard setup. I'm currently running the 28L92 in POC at 115.2 Kbps on both channels, with Ø2 at 12.5 MHz. I have tested it at 15 MHz.

The SC28L series can be configured to work with either an Intel or Motorola 68K bus interface. In POC, I used the Intel setup, as it turned out to be easier to interface to the 65C816. Also, I used a 3.6864 MHz oscillator instead of a crystal. The link to the TIA-232 lines is through a MAX-238. POC V2 will use the MAX-248, since it will have four TIA-232 ports instead of two.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

What are the requirements of the video in respect of colors and resolution? For simple text displays I use dual port RAM, large DP RAM are normally only available in packages with very low pitch, 0.5mm.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Some other things to consider, though I don't really know what the "theme" of your computer is...:

- A to D converters?
- D to A converters?
- A header for I2C "expansions", though of course any one of your VIA expansion headers could be used
- Mass storage? CF/IDE, SD card, etc?
- Keyboard?

From the looks of it you have everything, except a keyboard, to make the computer "stand alone". So I'd definitely give some thought as to what kind of keyboard you might want to connect up, even if you don't see a need for it right away.

Video is going to be interesting. There are many choices. AVRs seem a popular one, or a TMS9918, or the beeb style MC6845 if you want to go more "retro". HDMI is probably going to be extremely difficult.

What are you using for glue?

_________________
8 bit fun and games: https://www.aslak.net/

Keyboard and SD card are a good idea, although I still need to learn how to bit bang SPI for the latter. I think there's a PS2 to VIA web page article somewhere online where I saw how to do the PS2 keyboard.
With regard to timers: As I might be swapping different speed oscillators in and out it means that timing will also vary. If I have an external timer than that shouldn't be a problem.
Not sure that I need A/D, D/A converters at the moment, but that might be down to me just not having experimented with them in the past: I'll look into implementiong at leats one of each.

One larger issue I have is board space. The max I can have is 180mm x 100mm (I think that's the maximum) due to my licensed version of Eagle. Means things might get a bit crowded. I was thinking that if I minimise the through-hole components then I can possibly have some SMDs on the underside.

My main issue with video with regard to the older devices such as TMS9918 and MC6845 is that they are designed for slower speeds and I fear that mu board will be too fast. This is is a pity as I'd planned to put in an SA5050 IC which would give me the nice fonts that teletext uses (also only uses 1 byte per screen character). Unfortunately, the max speed seems to be 6MHz if I'm reading the data sheet correctly. Might go with an AVR as these might be able to cope with the speed, although even these might be a bit slow as things are running in software on the uP.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I'm not too fussed about resolution and even getting black and white would be great.
Ideally, though, I'd like 320x200 with 8 colours. Or the teletext type SA5050 if I can manage it.

If you use one of the NXP dual UARTs you get a high resolution counter/timer (C/T) as part of the bargain. The C/T runs at a selectable submultiple of the BRG clock frequency (3.6864 MHz), making it completely independent of the Ø2 frequency. NXP's SC28C94 quad UART has two C/Ts, each of which is independent of the other.

I use the C/T in the DUART in POC to generate a 100 Hz jiffy IRQ that is the timekeeping source for the system. It's very stable and, of course, unaffected by changes in Ø2.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Yes, a great suggestion. I was somewhat annoyed that the SC16C654, used on my board, lacks a timer. Luckily I have plenty of other timers to choose from.

There's also the RTC. The DS1307, a common or garden I2C RTC, has an output pin that can be configured to generate a 1Hz, 4Khz, 8Khz or 32Khz square wave. Other RTCs might have more frequencies available.

_________________
8 bit fun and games: https://www.aslak.net/

Before relying on that periodic output as an accurate time base you might want to verify that those frequencies are what they seem to be. You may be in for a surprise.

In any case, a 1 KHz jiffy IRQ rate is kind of high for most applications. The more frequent the interrupts, the slower foreground programs will run. UNIX has traditionally used a 100 Hz jiffy IRQ and current versions of Linux are using 250 Hz, if memory correctly serves me.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Linux used 100Hz in the past, now you can choose between 100Hz, 1000Hz, or 250Hz when you build your kernel. I don't remember what's standard for pre-built distro kernels, but I think the recommendation is 1000Hz for desktops and 250Hz for servers. In any case, 100Hz served the Unix world well for a long time and is still an ok choice.

Very useful, thanks. I imagine those 32KHz, ... , 1Khz square waves are actually 32.768Khz, ... , 1024Hz, etc. It's a shame the datasheet isn't accurate.



Yes, hence I mentioned that other RTCs might have different clock rates. I doubt you'll find one with a nice "decimal fraction" though, since all RTCs I've come across use a 32.768KHz crystal. Another option, which the OP could use depending on the rest of his system, would be to use the frame interrupt from the video generator to create a jiffy interrupt.

_________________
8 bit fun and games: https://www.aslak.net/

Correct. Maxim uses a 32,768 Hz crystal in their timekeepers. Hence the 1 KHz square wave will be 1024 Hz and a 100 Hz interrupt from the watchdog will be a bit more frequent than 100 times per second.


One thing I quickly got used to years ago when I started using Dallas Semiconductor devices was reading between the data sheet lines. Dallas' parts were high quality, but their data sheets were as notable for what they left out as what was written. Lots of experimenting proved to be necessary in some cases.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Even if the actual frequency of the interrupt is 128Hz and not 100Hz it still can be used, you just need to take that into account, or am I wrong on this?