Continuing my adventures with the XGS I decided to make use of the larger breadboard bought the other week from maplins (along with a nice pre cut set of jumper leads).

My long term plan is to build a simple games console capable of loading games from an external flash rom (possibly cartridge based on simply onboard via switching to access a handful of pre-loaded games). This is a lot more work than it sounds since the SX range of chips can only execute instructions from their internal rom, 2K on the SX28 and 4K on the SX52. There are ways around this which I’ll cover at a later date.

There are a lot of hurdles between now and the simple console I’m picturing, so the first versions of the console will be limited to 2K of ROM and 144 bytes of RAM, basically the onboard rom/ram for the SX28. The SX28 (parallax micro-processor) is going to be the heart of my games console. The reason, because thats what the XGS used which means I have a boat load of reference docs, manuals and papers about it plus a local supplier and I’m already slightly familiar with SX assembly. Also, the SX28 is available in a DIL package which makes prototyping much simpler than the surface mount versions of the SX48 and SX52 (although I’ve a few of those ready for a future project

So the plan currently is to start very simple, and progressivly add more and more “features” to the console such as graphics/sounds/external flash rom & sram/joystick port etc If you’ve read the XGS ebook then you’ll probably notice many similarities to the PICO and XGS which are been used as a base reference, however I plan to expand the feature set of the PICO much further. For example with the addition of graphics and sound hardware. How far this can be taken with just the SX28 and the limited output pins, I’m not sure. At the expensive of speed I can probably serialise a number of the systems although worst case scenario will be stepping the design up to use the SX48. Thats the rough plan anyway, I’m sure it will change as I learn more.

Step one was getting a power supply up and running, actually very easy to do, a 7805 regulator did all the hard work plus theres quite a good section on power supply regulation in the XGS ebook I’m still not 100% certain on how the capacitors reduce the ripple in the DC supply, well I understand the concept but don’t quite understand the calculations enough to work out which frequencys are getting through and which are filtered. Something to work on in the future.

Anyhow, enough reading, heres a picture of a very basic SX28 project. In the top right of the picture can be seen the 7805 voltage regulator that takes a 9V DC supply and produces a 5V supply, along with the smoothing and noise filtering capacitors to clean up the 5V output.

For a breakdown of the circuit as well as the source code Read the rest of this entry »

Having only had a few hours Sunday night to work on the last part of the code I wasn’t expecting to actually reach my goal of a pixel on the screen. In the end though, I managed to get a square block drawn in the center of the screen synced well enough to show flicker free on both my TV and Digital TV card in the PC.

The actual TV picture is a nice vibrant yellow, the screen capture doesn’t do it justice looking a little washed out. Oh, that reminds me, the colour burst wasn’t that hard afterall, just a case of making sure the colour is set in the upper 4 bits and the luma in the lower 4

Adding in the VSync and overscan code wasn’t that difficult, the hard part was counting all the clock cycles to make sure they met the timing requirements. In fact the most trouble was with the comparison loop where I check whether the scanline is within the block and needs to be drawn in colour. This results in 3 possible code paths all of which had to have exactly the same number of cycles before reaching the final DEALY(174-3) instruction. A little fudging goes a long way though

The theme for this GID was “Extreme Forces” well the forces exerted on my TV by all the badly formed signals I sent throughout Saturday were pretty extreme, does that count?

For those that care the source code is rar’d up and available for download. Although it works fine on my TV that doesn’t mean the code is correct so I’d love to hear back from any of the more experienced XGS users if you can see any obvious flaws. The final code weighed in at roughly 374 bytes (don’t quote me on that ) After looking at a few other peoples delay macros I can see a saving of several bytes there alone, so I’m sure this code can be optimised significantly, maybe another day

Check out the rest of this entry if you want to see the final code

Read the rest of this entry »

Having nailed the delay code I’ve spent most of the evening reading up on the SX52 memory architecture and the PAL timing signal. To start with I’ve created the signal needed for each scanline (see previous blog with timing diagram)

The first attempt didn’t go too well

It was only a few minutes though before I realise my timing was out by a few clock cycles. The reason? Well in order to generate say the SYNC pulse 0.0V on RE port for 4.7µs we need to delay for 4.7µs*1000/12.5ns = 376 clock cycles, so I did the following

[ASM]
mov RE, VID_SYNC
DELAY(376)
[/ASM]

The problem though is that really we’ve taken up 378 clock cycles and thus the code is executing for 25ns longer than we need it to. Couple this with all the other mov/delay statements for the other parts of the signal and we end up many cycles out of sync. The solution is simple, a mov takes 1 cycle or in the case of a compound mov (which this is) 2 cycles, so we just reduce the delay by 2 cycles to give

[ASM]
mov RE, VID_SYNC ; 2
DELAY(376-2) ; 376-2
[/ASM]

The result is a lot more promising, using VID_WHITE (1.0v) for the output port RE gives us a nice white screen, however we’re not finished yet, since there is not handling of the vertical sync yet.

So thats as far as I managed to get by the end of Saturday. I expected to have a good few hours sunday to work on getting the vsync sorted and plotting a specific pixel, but plans change. If I’m lucky I’ll squeeze in a few more hours tonight and hopefully finish things off.

GID j posts - first entry

As mentioned in the last blog entry, timing is cricical down to the cycle. In order to make sure all branches of code execute in the exact same amount of time I’ve created a delay macro. This isn’t an optimal method, I’m sure there are ways to reduce the number of bytes this uses however it does the job.. at least it does after a little debugging.

After coding up the above I programmed it into the SX52 and put the SXkey into debug mode which allows you to single step the processor one instruction at a time and view the programs memory/registers and flags. Using various delays of 1, 7, 10, 14 etc I noticed the NOP repeat was only generating a single nop rather than the expected number, this turned out to be because missed the ending ENDR directive

[ASM]
;
; DELAY for a specified number of clock cycles
; EXPECTS:
; “counter” 1 byte variable defined
DELAY MACRO clocks
; eat up clock cycles 10 at a time
IF ( (clocks / 10) > 0)
mov counter, #(clocks / 10) ; 2
:loop
jmp $ + 1 ; 3 jmp to next instruction
jmp $ + 1 ; 3 jmp to next instruction
djnz counter, :loop ; 2/4 (4 if <> zero)
; 10 cycles per loop, 8 for last loop
ENDIF

; handle remaining delay 1-9 using nops
IF ( (clocks // 10) > 0)
REPT (clocks // 10)
NOP ; 1
ENDR
ENDIF
ENDM
[/ASM]

The following screenshot (click for a larger view) shows the debugger running the DELAY(14) code and circled in red the lone NOP instruction. A simple mistake putting an extra slash but one which would if not caught render the video output useless. Glad I caught it early, otherwise I might have wasted hours debugging my video code.

With the delay loop out of the way, its time to final make a start on generating accurate h/v signals. Hopefully by the end of tonight

GID j posts - first entry

Satuday 2pm and I’m a little closer to understanding how to plot a pixel, although only a little. The first attempt to generate a video signal resulted in, well see for yourself, detune a tv channel and look at the white snow, thats pretty close to what I’m looking at right now

The reason been, I’ve not setup any kind of timing for generating the video signal yet but I know that at least I’m outputting to the correct port to generate “a” signal. After a little digging through the XGS docs I’ve got the information on the PAL signal specs, the main bits of interest are:

• Front Porch 1.5µs
• Active Video 53.1µs
• Sync 0V
• Black 0.3V
• White 1.0V

Based on this we can calculate the number of pixels a PAL tv can cope with (@5.5Mhz), we can change the signal at a rate of 1/5.5Mhz = 182ns which gives us 53.1µs / 182ns = 292 pixels per scanline (best case; although I think my tv will do 720 due to extra bandwidth, but we’ll go for a conservative value)

So in order to get a pixel (assuming no colour) on the screen I need to generate a video signal to display 312 scanlines each comprised of at most 292 pixels. Heres the full scanline signal needed for a PAL system. (I’m ignoring colour burst for now; image obtained from Haagans website)

Once the horizontal signal is up and running the vertical signal should be a snap, its nothing more than another timing signal, so we’ll be sending a H Sync signal for each line followed by 4-10 lines of vsync. I’m keeping this simple and going for 274 lines at 50Hz (50 frames per second) with 16 blank lines top/bottom to account for tv variations and the 6 lines of vsync giving a total of 312 lines.

The key to this is the timing, with the SX52 processor that the XGS uses running at 80Mhz, 1 clock cycle = 12.5ns we need to ensure that the number of cycles used to generate the signal matches the above timing diagram.

There is a whole lot more to the signal generation than the overview above, if you’re really interested in the details either get the XGS and read the book (which in addition covers designing and building the actual video hardware) or google it.

So the plan for the larger part of today is to get a suitable delay macro up and running and outputting the appropriate voltages to generate a stable H/V signal. Initially using a fixed value for each pixel (we’ll get to plotting a single specific pixel later.. for now its a screen fill) Fingers crossed that is.

Its that time of the month again, we’re now on GID j, for a change I thought I’d code something for the XGS. I sense this GID is going to be rather painful just setting things up ready for tomorrows start brought problems.

Lesson1: When routing the video/audio output through your digital tv card and you only get black and white check the Crystal. I’d forgot that the last time I used the XGS I’d hooked it up to my TV which supports PAL or NTSC and had been playing around with some of the NTSC only demos. I’d left the 3.57Mhz crystal in the XGS rather than the 4.43Mhz crystal needed for PAL output.

Lesson2: Prepare for things to only get worse from here on in. This little voice keeps telling me this is a bad idea. Trying to learn the details of programming a chip such as the SX52 and assosiated hardware is bad enough, but considering how rusty my asm skills are we’re in for a bumpy ride.

The chances of a successful GID appear to be slim to say the least, so I’m lowering my expectation (and I mean reeeealy lowering) and instead doing a PID; pixel in a day (if you hadn’t already guessed ). In other words, if I manage to get a pixel up on the screen within the time constraints of this GID I’ll be happy, anything beyond that is a bonus. On a good note I’ve already written code to handle interfacing with the joystick port (see previous blog) so if we can get a pixel up, maybe we can get it moving, perhaps even a several pixels

Hey with 4k of direct memory who knows we may even get as far as displaying a sprite, maybe two but lets not get too carried away.

Anyhow the XGS is rigged up with video/audio routed through my pc to make for easier screen grabs (erm that is assuming we get as far as displaying anything, otherwise you’re in for some code dumps instead ) so, for what remains of Friday night and a few hours of Saturday morning I’m going to hit the SX Programming manuals and read over the XGS hardware specs.

I think this first calls for a fresh cuppa.