exxos's Atari, Amiga & retro forum

Hi!

I'm trying to control the JOYWE line from the GST MCU of an STE. Has any of you have experience with that?

I'm using this code (on a function called via Supexec): and I can see in the oscilloscope that the line changes, but immediately go back to the old state... So, I'm not sure what's happening.

Truth be told, this is about controlling the JOYWE line on the Combo chip on a STBook, but I'm hoping there aren't differences. ;)

Thanks!

Andres.

Seems like you are are trying to access a STACY-only register on the STE?

Well, what I'm REALLY trying to do is to control the JOYWE line in the STBook. The STBook and STacy use, I think, the same Combo chip, and I think the GST MCU in the STE is similar (I'm not versed on the STE, but the pins look very similar). Of course, there is less documentation/experience about the STBook, so that's why I'm asking about the STE.

In this post you can see that the address 0xFF827E , bit 5, is what I want to control. From the Combo schematics, that controls the JOYWE line.

I'm hoping that the same way to control the JOYWE line in the STE can extrapolate to the STBook. And I think I'm in the right track (in the oscilloscope I can see the JOYWE line changing its status), but it does not stay that way (it bounces back immediately). So I think I'm missing something about controlling that line.

Did that explain my dilemma? :-)

Andres.

mikro wrote: 01 Dec 2025 02:47 Seems like you are are trying to access a STACY-only register on the STE?

Code: Select all

-------+-----+-----------------------------------------------------+----------
$FF827E|word |STACY Display Driver                         BIT 10 9|R/W(STACY)
       |     |Backlight on/off ---------------------------------+ ||
       |     |Display on/off -------------------------------------+|
-------+-----+-----------------------------------------------------+----------

So why don't you try it on the ST Book where the register is guaranteed to be working?

Ah! Sorry that I was not clear about that: I'm trying it in the STBook, and that's where the line flips to the old state.

The reason of my question is that, I imagine, there are more people with STE experience than STBook. :-)

mikro wrote: 02 Dec 2025 22:30 So why don't you try it on the ST Book where the register is guaranteed to be working?

AndresPlazaR wrote: 01 Dec 2025 01:58 I'm trying to control the JOYWE line from the GST MCU of an STE. Has any of you have experience with that?
...
and I can see in the oscilloscope that the line changes, but immediately go back to the old state... So, I'm not sure what's happening.

I've played around with Orion_'s SD card hack on the STE joystick ports in the past and from what I remember you don't control that line directly, COMBO produces it to enact the requested behaviour.

I *think* if you perform a write to the joystick direction register that line will go low to drive the output. Conversely, if you read the joystick direction register it that line will go high to send the 373 output chip high-z. I think the line then remains in the last state needed.

So by having a rapid read/write operation there you'd probably see it go high (for the read) then low (for the write). If it were low to start with, you'd get a toggle as described.

Try only reading from it (to send it high) or only writing (anything) to it (to send it low).

BW

Aha! Thanks @Badwolf , I will test that and report back. :-)

Andres.

Badwolf wrote: 03 Dec 2025 20:30 I've played around with Orion_'s SD card hack on the STE joystick ports in the past and from what I remember you don't control that line directly, COMBO produces it to enact the requested behaviour.

I *think* if you perform a write to the joystick direction register that line will go low to drive the output. Conversely, if you read the joystick direction register it that line will go high to send the 373 output chip high-z. I think the line then remains in the last state needed.

So by having a rapid read/write operation there you'd probably see it go high (for the read) then low (for the write). If it were low to start with, you'd get a toggle as described.

Try only reading from it (to send it high) or only writing (anything) to it (to send it low).

BW

So, now I understand why @mikro was confused with my question: I was mixing two different things, and my question was completely invalid. :-)

My problem is not with the JOYWE line from the Combo chip; actually, that part I have it already working. My problem is with the GOUT2 line coming from the Shadow chip (that's the 0xFF827E address I was mentioning). And because the Shadow chip is only used in the STacy and the STBook (and GOUT2 is not even used in the STacy), my hope to find someone that used it are... basically nill.

Well, unless someone have documentation about the Shadow chip (I have never found any), this will be one of those Unsolved Mysteries.

Thanks anyways, @Badwolf !

Andres.