my Gigatron RAM and IO expansion does not work

Using, learning, programming and modding the Gigatron and anything related.
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gfernval
Posts: 38
Joined: 24 Jan 2022, 00:39

Re: my Gigatron RAM and IO expansion does not work

Post by gfernval »

Very simple, supply additional current to the board, the miniusb connector with the time (after many insertions) will not do good contact.

The socket label Y1 XTAL is only a socket with pins 4 and 5 connected, if inserted (like showed in the photo), the gigatron works with the 6.25MHz crystal, but this socket (top socket only) can be removed and replaced by a 4 pin oscillator (of the size of a 14 pin IC) of 6.144MHz, 12.288MHz or 16.257MHz to make overclocking experiments. The other board I have (is a common standard PCB gigatron board) has all the parts 74F series, including a 74F595 instead 74HC595 and fast 10ns static SRAM, such board with the 16.257MHz oscillator, the v3y ROM and a pluggy mcplugface board programmed with the standard babelfish.ino (but running such attiny85 chip at 16MHz instead 8MHz) runs fine at 16.257MHz, including games and Mandelbrot (at 18MHz does not work) I have not tested the gigatron IO board with 16MHz, problem with ROM v3y is that the screen image is small.

I have used two Atmel ATF22V10C 15ns rated GALs in a standard Gigatron (6.25MHz) When I saw that Sandisk SDcards didn´t work, I exchanged the GALs by two GALs rated 7ns (also Atmel brand), but same failure with the Sandisk SDcards. I use 15ns GALs insteaf 7ns GALS because run more cooler during Gigatron operation.
gfernval
Posts: 38
Joined: 24 Jan 2022, 00:39

Re: my Gigatron RAM and IO expansion does not work

Post by gfernval »

- Does exist any keyboard key that when pressed bypass the loading of the SDcard and goes to the Gigatron menu when the Gigatron is reset?
- Does esist a rom for the gigatron running at 12.5MHz that has the screen centered like the normal 6.25MHz gigatron? The rom v3y is not centered
bmwtcu
Posts: 111
Joined: 01 Nov 2018, 12:02

Re: my Gigatron RAM and IO expansion does not work

Post by bmwtcu »

AFAIK, nobody has worked on a 12.5MHz compatible ROM beyond v3y.
lb3361
Posts: 186
Joined: 17 Feb 2021, 23:07

Re: my Gigatron RAM and IO expansion does not work

Post by lb3361 »

gfernval wrote: 08 May 2022, 01:02 Very simple, supply additional current to the board, the miniusb connector with the time (after many insertions) will not do good contact.

The socket label Y1 XTAL is only a socket with pins 4 and 5 connected, if inserted (like showed in the photo), the gigatron works with the 6.25MHz crystal, but this socket (top socket only) can be removed and replaced by a 4 pin oscillator (of the size of a 14 pin IC) of 6.144MHz, 12.288MHz or 16.257MHz to make overclocking experiments. The other board I have (is a common standard PCB gigatron board) has all the parts 74F series, including a 74F595 instead 74HC595 and fast 10ns static SRAM, such board with the 16.257MHz oscillator, the v3y ROM and a pluggy mcplugface board programmed with the standard babelfish.ino (but running such attiny85 chip at 16MHz instead 8MHz) runs fine at 16.257MHz, including games and Mandelbrot (at 18MHz does not work) I have not tested the gigatron IO board with 16MHz, problem with ROM v3y is that the screen image is small.

I have used two Atmel ATF22V10C 15ns rated GALs in a standard Gigatron (6.25MHz) When I saw that Sandisk SDcards didn´t work, I exchanged the GALs by two GALs rated 7ns (also Atmel brand), but same failure with the Sandisk SDcards. I use 15ns GALs insteaf 7ns GALS because run more cooler during Gigatron operation.
To run at 16Mhz, did you have to change thr resistors of the diode forest and eliminate the CLK2-CLK1 delay (remove cap C3)? I am asking because clock variations have been one of the tricky points of the 512k expansion board. It needs precise timings in order to make multiple memory accesses during each clock cycle. But the CLK2-CLK1 delay varies from machine to machine in a manner that is load dependent. In addition the diode forest delay depends on both the resistor (+- 20%) and the properties of the selected diodes. Finally different chip brands in the ALU have different propagation times. Fortunately they are all better than advertised. I ended up taking huge margins around the Gigatron memory access, then doing all other accesses very tightly.
gfernval
Posts: 38
Joined: 24 Jan 2022, 00:39

Re: my Gigatron RAM and IO expansion does not work

Post by gfernval »

Yes, I did exactly: (see 10MHz, 12.5MHz and Beyond! forum entry)

- All 74F logic, including 74F04 for clock and 74F595 for input
- Pull-up resistors in control unit lowered from 2.2kΩ to 680Ω
- BAT42 signal diodes
- C3 removed, used a 16.257MHz oscillator (DIP 14 pins size) instead XTAL base Colpitts oscillator
- 10ns RAM
- 45ns OTP Atmel 27C1024 EPROM

a question: does exist any keyboard key,shortcut,.. that when pressed bypass the loading of the SDcard (if one SDcard is inserted in the gigatron IO board) and goes to the normal Gigatron menu when the Gigatron is reset?
lb3361
Posts: 186
Joined: 17 Feb 2021, 23:07

Re: my Gigatron RAM and IO expansion does not work

Post by lb3361 »

The Gigatron boots to the main menu unless it detects the presence of a sd card. Therefore, to bypass the sd card boot, the most intuitive solution is to either remove the sd card or use the push-push mechanism of the socket to disconnect it electrically (a kind of a button in fact.) Booting the Gigatron will then go directly into the main menu application.

The sd browser also has an option to return to the main menu. Just use the left arrow to go up directories until you see the sd0/sd1/rommenu dialog box. Select rommenu and you're done.

When the browser starts, it looks for a program named autoexec.gt1. If it finds one, it runs it immediately unless the B button is depressed. One would have to pick another button to prevent the boot code from running.

Updated: see also the collection of stub programs (https://github.com/lb3361/gigatron-os/tree/master/stubs) that start the corresponding rom program if it is present in the rom. They can be used to compose a customizable rom menu in fact.
gfernval
Posts: 38
Joined: 24 Jan 2022, 00:39

Re: my Gigatron RAM and IO expansion does not work

Post by gfernval »

Thanks, in testings I have done with the overclocked gigatron board I saw that a DIP14 size oscillator is more stable and less noisy than a XTAL based Colpitts oscillator like the one used in the Gigatron board
lb3361
Posts: 186
Joined: 17 Feb 2021, 23:07

Re: my Gigatron RAM and IO expansion does not work

Post by lb3361 »

gfernval wrote: 08 May 2022, 15:46 Thanks, in testings I have done with the overclocked gigatron board I saw that a DIP14 size oscillator is more stable and less noisy than a XTAL based Colpitts oscillator like the one used in the Gigatron board
Chances are that the DIP14 oscillator contains about the same circuit, but with tighter tolerances and with everything carefully optimized...
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