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• More documentation online

  roelh • 10/05/2025 at 07:57 • 0 comments

  
  In the Hackaday file section, I now placed the almost-latest version of the schematic, and 3 assembly drawings that will make it easy to find the components on the pcb when you build it. For convenience, the typenumbers of the IC's are printed on the pcb.

  
  The new pcb's (ISA2532) have arrived from my sponsor PcbWay. As always, the quality is very good and they deliver quite fast.
  Builders of the KIT will now receive this pcb, so there is no need any more for the modifications with all the wires. The pcb was tested by building an Isetta with it, and it works perfectly.

  The SymbOS version for Isetta is now practically bug-free.

  SymbOS has a shutdown command, that switches VGA screen and the power LED off. After pressing the button at the front, it will start-up again.

  The real-time clock/calendar is now also working. It keeps running during shutdown, and during ZX-Spectrum or other applications.

  On october 11th/12th, Isetta will be at the SymbOS stand in the Nixdorf Museum in Paderborn (Germany):
  https://www.hnf.de/veranstaltungen/events/date/2025/10/11/cal/event/tx_cal_phpicalendar/retro-computer-festival-1.html

  For all of you who want to build one, I sell pcb's on my new Envionic website (not a HTTPS connection possible yet).

• Third PCB design finished

  roelh • 07/30/2025 at 08:40 • 0 comments

  The new pcb design was finished. I did not send it yet for production, because experience says, that within ten minutes of the final submit for production, I think of something that I forgot in the design. So I just wait a few days before ordering.

  
  What changed (w.r.t. ISA2442 version):

  - The wire modifications, as on prev version, are no longer needed
  - slightly changed form factor, to better fit in the housing
  - Have a 1.5 W sound amplifier on board, can now directly connect a speaker. The 
      output for an amplifier is also still on board.
  - Due to speaker dimensions, a bigger housing is used, RM2055M (instead of RM2055S)
  - Have a 'wakeup' button at the front. (A SymbOs command can put Isetta asleep). The power
       LED will be off while Isetta is asleep.
  - Several changes to improve EMC behaviour
  - Amount of Microcode flash ROM can be doubled, now 32 instead of 16 microcode pages.
      (The option to use 8-cycle instructions was removed for this.)
  - Max number of consecutive microcode cycles is now 512 (was 256). Now it is easier
      to display 640 pixels per line (two pixels per cycle, so 320 consecutive cycles)
  - The selection between 640 or 320 pixels per line is simpler now (uses ctl_upd_n)
  - The 4 blinkenlights were removed
  - Added a LED to indicate file read/write
  - Changed the optional WiFi module to XIAO ESP32S3 (WiFi + Bluetooth BLE)
  - The WiFi module has an option to add LoRaWan
  - One extra IC used. And one for the sound amplifier.

  New Option connectors:
  - I/O A: 20 pin connector. Bus for parallel input/output. Has 8 datalines, 3 address lines.
  - I/O B: 20 pin connector. Supports 4 input and 4 output bits. Also useable for alternative power
                              supply (and battery backup). This option can also select a lower CPU speed
                              for less power consumption. 
  - I/O C: 40 pin connector. A board to give more video modes can be connected here.

  - I/O D: 20 pin connector. Connects to RPi. This was "I/O C" on prev version. Removed some unused signals.

  This is the 3D impression of KiCad:

• We have Sound !!

  roelh • 06/10/2025 at 09:24 • 2 comments

  Finally, the PSG (Programmable Sound Generator AY-3-8910) is coming to life. It was harder than I thought. But it now has the 3 channels for tone or noise, with independent frequency and volume control. Only the the envelope generator is not present yet [edit: a month later, envelope was also working]. The sound is generated by microcode, no actual AY-3-8910 is used, but the sound is controlled in the same way as on a real sound chip.

  It generates the typical sound of the eighties computer games.

  It can now provide sound for the SymbOs operating system !  And here you see the first Youtube video of Isetta running SymbOs (made by Prodatron):

  
  

  (The start-up screen of Isetta will have to be modified, it's quite a mess at the moment.)

  A new version of the Isetta programming manual is now in the file section. It includes the commands to control the sound generator.

  OPERATION

  There are four 1 kB sound buffers, one for each of the three channels, and a fourth one for noise.

  The squarewave sounds are generated in an unrolled loop. So if a wave has 5 samples 'high' and 5 samples 'low', to generate one period there will be two sequences that each put samples in the buffer, with each having five "(pc++) <- T" micro-instructions in a row. (Note the program counter is used as auto-incrementing pointer to the buffer).

  The value in T is the required amplitude for this sound (positive value for high samples, negative value for low samples). So there is no separate volume-control stage, like the real PSG has.

  At each scanline interrupt, a byte-sized sample is taken from each of the four buffers, the four samples are added and sent to the D/A converter (resistor array RN1). So the sample frequency is the same as the VGA line frequency, 31.25 kHz.

  To control this, there are two Z80-output ports. One to select a AY-3-8910 register, and another one to write a byte to that register. The Z80 output ports can also be used by the 6502 processor.

  EFFICIENCY

  For each channel, producing samples takes around 3 cycles per sample. (The unrolled loop produces 8 samples on average, single cycle per sample, with also 16 cycles overhead).
  For a full 525-line frame, this is 1575 cycles per channel. Noise has it's own channel, so there are 4 channels, together taking 6300 cycles each frame.

  
  In each frame, 120 lines are for the CPU, minus interrupt it's around 350 usable cycles per line. Per frame, this is 120 * 350 = 42000 available cycles. So the sound takes 15% of the CPU time. For frequencies higher than 1kHz, there are just a few samples produced in each loop, so there is more overhead.

  LINKS

  There are several very nice articles about sound generation.

  Want to know everything about synthesizers:

  https://www.soundonsound.com/series/synth-secrets-sound-sound

  Building the AY-3-8910 fully from TTL chips:

  https://github.com/mengstr/Discrete-AY-3-8910
  

  Very good description of the AY-3-8910:

  https://www.cpcwiki.eu/index.php/PSG

  Article about Linear-feedback shift registers, used to generate noise:

  https://en.wikipedia.org/wiki/Linear-feedback_shift_register
  

  The Isetta noise generator comes from this article. It's a 16-bit Galois type, quite simple:

         unsigned msb = (int16_t) lfsr < 0;   /* Get MSB (i.e., the output bit). */
          lfsr <<= 1;                          /* Shift register */
          if (msb)                             /* If the output bit is 1, */
              lfsr ^= 0x002Du;                 /*  apply toggle mask. */
  
  

  Sound generation in the Novasaur TTL computer:

  https://hackaday.io/project/164212-novasaur-cpm-ttl-retrocomputer/log/184708-roll-your-own-sid-chip

  Interactive demo, shows harmonics...

  Read more »

• Do-It-Yourself KIT available

  roelh • 05/21/2025 at 17:45 • 0 comments

  KIT Available

  The first Isetta kit was successfully built by Vadim (see picture):

  And the second kit was built by Hans-Jürgen in Germany.

  In [this log] you can see all ins and outs of the kit. Of course, with the kit, you will receive the latest schematics and software versions. [ edit: You can order your kit at the new Envionic website. ]

  On the first pcb, a few modifications had to be done (the wires that you see on the pictures). But there is now a new pcb design where these modifications are not needed.

  SymbOS

  Good progress was made on the SymbOS implementation. The most serious bugs are fixed now. Currently I am working on the microcode for a sound generator, that will work similar to the AY-3-8910, and that can also be controlled by SymbOS. 

• Come and see Isetta running Symbos

  roelh • 04/27/2025 at 20:23 • 0 comments

  On may, 10th you can see Isetta running SymbOs !

  Isetta will be at the "MSX Vriendenclub Mariënberg", together with Prodatron (designer of SymbOs), and myself.

  https://raymondmsx.nl/home_mvm_eng.html   English
  https://m-v-m.lookpages.nl/home/  Dutch

  Place:
  Buurtgebouw de Grendel
  Wicher Hentostraat 17
  7692 AL Mariënberg

  Time: may, 10th 2025, from 10 a.m. till 4 p.m.

  As a first time visitor, the meeting is free. 

  It would be nice if you send me a reply or PM if you want to come.

• Programming manual, microcode and Blitter

  roelh • 03/12/2025 at 12:10 • 0 comments

  PROGRAMMING MANUAL

  An Isetta Programming Manual was made, that describes all features that are available to the assembly program.
  You can find it in the File Section.

  It describes most of the I/O ports. 
  It also describes the video system, bankswitching, ports used for the Blitter, and describes a debugger that I made.

  MICROCODE INTRO

  One of my plans was, to have some microcode support for writing to the screen. 

  Writing to the screen, in Z80 (or 6502) assembly language, can be very tedious. The screen is pixel based. There are several complications:

    - in 640x400 mode, there are two pixels per byte
    - we might have graphics encoded as 2 bits per pixel (4 colors), this must be converted to 4 bits per pixel
    - We might have transparent pixels (for sprites). So sometimes, in a byte, one nibble must be written but the other stays the same.
    - the upper and lower half of the screen are in different memory banks
    - perhaps the 4-bit color pixels must be converted to another 4-bit color code

  A Z80 assembly program to do this will be difficult to write, especially because it should be quite fast (to get the screen written in an acceptable time).

  MICROCODE FEATURES

  To get a good performance, it would be better to write screen handling in microcode. On Isetta, programming in microcode has the following features:

    - A microcode instruction is 24 bits, and is totally different from a Z80 or 6502 instruction.
    - While the main purpose of the microcode is, to implement Z80 or 6502 instructions, it can also be used to do other functions.
    - A 24-bit micro-instruction can be fetched from microcode flash memory while a data byte is transferred to/from data memory (Harvard style). A micro-instruction always executes in a single cycle (80 nS).
    - The 24-bit instructions are addressed by the instruction register (8 bit), a step counter (4 bit), a page register (4 bit), and the flag F (1 bit).
    - The micro-instructions can read and write flags, and have conditional execution, that lets the F flag choose between two micro-instructions.
    - During execution of a Z80/6502 instruction, when the required steps are done, a micro-instruction will load the instruction register (IR) from an address that is in the program counter (PC++), reset the step counter to 0, and might set the page register to a new value.
    - If, at the end of the 16 steps, the instruction register (IR) is not loaded again, the IR will automatically increment, because the lower 4 bits of the instruction register is also a counter. So a sequence of micro-instructions can be 256 cycles long. If IR is still not loaded, it will go back to the first of these 256 steps. 
    - If the instruction register is loaded, it does not have to be loaded from the PC address. It can also be loaded with a constant (to jump to a fixed next instruction), or can be loaded from a register in memory. It is possible to write simple programs without using the program counter.

  MICROCODE DETAILS

  What do the micro-instructions do, more in detail ?

    1) Read the next 24-bit micro-instruction (pipelined), from one of two 'tracks' chosen by the F flag.
    2) Determine memory address and memory bank. Memory address can come from:
        - the data pointer (DPH/DPL), or
        - the program counter (PC) with optional auto-increment, or
        - a small fixed value (included in the 24-bit instruction). This is divided in two ranges (selected by microcode):
             - A) A fixed range of 64 bytes
             - B) A selection between two other ranges of 64 bytes. Selection is done by the output bit EXX. The BC, DE and HL registers of the Z80 are stored in this range.

       There is an option to set the lowest address bit to 1. This is useful to read the MSB of a word, without having to increment...

  Read more »

• Announcing kits on the last day of 2024 !

  roelh • 12/31/2024 at 12:44 • 0 comments

  Both prototypes that I have built worked without any HW problem (of course there were SW problems now and then). On the first one, I ran The Great Escape (in its self-running mode) continuous for almost a week (and then I stopped it). This gives confidence.

  SELLING KITS

  I could now soon start with selling kits.  If you are interested, let me know in the comments or in a private message.

  [edit may 2025: Kits are available NOW. The first kit was successfully built by Vadim. ]
  

  The price of the kit will be around 100 Euro (USD 110) (Shipping costs not included). You will need to have some experience in building electronic kits.

  - You buy the bare pcb from me, for 29 Euro. I will make instructions and a detailed list of parts.
  - You buy all parts from an electronic parts supplier, like Mouser or Digikey. Cost around 65 Euro, free shipping at several suppliers.
  - You get the PDF of the actual schematic
  - An enclosure is not yet included. But the pcb is designed to fit in a Hammond RM2055S enclosure.

  Selling complete Isetta's might come later.

  What more do you need

  You need several other things, that you might have lying around:
  1. A universal programmer to program the three flash chips with microcode. Or the Isetta programmer pcb.
  2. A Raspberry Pi (RPi), for transferring files to the 32MB on-board storage of Isetta
  3. A 5 volt power supply with USB-B plug
  4. A monitor (CRT or LCD) with VGA connection, with cable and 15-pin connector
  5. A PS/2 keyboard
  6. A PS/2 mouse (not needed for all applications)
  7. A micro-SD memory card (might be needed by some applications)
  8. An adapter to connect the micro-SD card to your Windows/Linux/Mac computer
  9. A speaker with amplifier, with 3.5mm (stereo) plug. For sound.

  Programming the microcode

  You can use a regular, universal programmer to program the microcode in the three flash chips (PLCC44 case). (I have not done that myself yet, I use the RPi). But, it is very likely that new microcode versions will appear, so you have to use sockets on the pcb to be able to reprogram them. Sockets can be unreliable, however.

  You can also use the special Isetta programmer pcb (that I will sell you for 9 Euro). It needs 10 TTL chips, and connects the Isetta to the Raspberry Pi. The Raspberry Pi can now program the microcode flash chips while they are soldered on the board. The programmer pcb also allows the RPi to send files to the on-board storage, and allows low-level testing and single-stepping the Isetta. The low-level testing is VERY useful if you soldered your own Isetta.

  The RPi also needs a monitor (HDMI connection), USB keyboard, USB mouse, and power supply.

  Note that, as soon as the microcode is programmed and files are transferred, Isetta can work fully stand-alone without needing the RPi or the programmer pcb. In the future, Isetta should be able to fetch files from the micro-SD card, or through its WiFi connection. [edit: SymbOS uses the micro-SD card]

  If you only need file transfer, you can connect the RPi with 4 wires to Isetta, without needing the programmer pcb.

  Future plans

  - Make a good description of the available I/O instructions
  - Provide more microcode support for writing to the screen
  - Implement a nice sound generator
  - Support micro-SD card and WiFi
  - Have an editor/assembler on Isetta
  - Add applications
  - Have a nice operating system

  I could use help ! But you'll first need the hardware...

  What did I do since the last log ?

  - Write text to the new 640x400 screen
  - Implement the RST #38 interrupt (interrupt mode 1) at each video frame. Can be enabled/disabled with EI, DI instructions.
  - Made a mouse driver in microcode. An I/O instruction can now read messages from the mouse.
  - Completed several changes that were needed due to HW changes on the new pcb
  - Created an output instruction to do bankswitching

  END OF THE YEAR

  Best wishes for the new year ! That you and your loved ones all be happy and healthy !

• New pcb, new video mode

  roelh • 11/17/2024 at 13:07 • 0 comments

  A few weeks ago I received the new pcb (ISA2442). Several things have changed with respect to the first pcb, so several software things had to change.

  One of the main changes was the 640 x 400 pixel mode. On the first pcb, a byte from memory represents 8 pixels, and the byte tells for each pixel if it has foreground color or background color (these colors being programmable).

  On this new pcb, a fetched byte has information for only two pixels (pixels last 40nS). So we can simply have 4 bits (16 colors) per pixel. The color of each of the 256000 pixels can be set independent from the others.

  Today I got this new mode to work. 
  On the picture you see mainly uninitialized memory, that gives a random, noisy pattern of colored pixels.
  In the first row I programmed the 8 basic colors, and the second row shows the same colors with low intensity. So the upper two rows show all 16 available colors in this mode.

  On the last row I made horizontal and vertical stripes, and a few checkerboard-patterns of two colors, that give the 
  impression of another color (orange, and variants of green and blue).

  You might ask why this is only 400 lines instead of the standard 480. This is because the processor can not run the user program during the active part of the video frame, because it is busy rendering the screen. By using only 400 lines, the processor has more time to run the user program.

  The 320 x 200 pixel mode (used for the ZX Spectrum screen) was essentially unchanged. 

• Second PCB version ordered

  roelh • 10/19/2024 at 08:49 • 1 comment

  Yesterday I finally ordered the new PCB. This PCB is sponsored by the kind people of PCBWay, the well-known high-quality PCB fabrication company from China. During production you can always check in which production state your pcb is, and I watched an impressive series of videos that show all steps in the production process.

  This is the KiCad 3D-impression of the new PCB:

  With respect to the first PCB, the main changes are:

   - fix mistakes (VGA connector)
   - add Mouse connector
   - add HW for sending info to keyboard (and mouse)
   - add memory bank select
   - change video pixel/color generation for 640x400 mode
   - change WiFi module type
   - simple 4-wire connect to Raspberry Pi (for file transfer)
   - instead of a SPI header, a micro-SD card connector is mounted
   - changed SPI reading, it can now be done in 2 cycles/bit (instead of 5)

• Changed bankswitching system

  roelh • 10/14/2024 at 12:59 • 0 comments

  In this previous log I described the new bankswitching system. But I wanted to add another mode (Linear mode). It would be nice if a program could access the whole 512kB memory without bankswitching. The address would be in a 3-byte register, with 19 significant address bits. A simple processor with 19-bit addresses could then be implemented.

  For programming the Z80 or 6502, it is not needed to know all this. You only need this if you implement a bankswitching system or a new processor microcode that accesses the whole memory. Probably it is only for myself.

  I came up with the following: (All images can be enlarged by clicking them)

  LINEAR MODE

  The functions are essentially the same as in the mentioned previous log, but I added the Linear mode. For program code, the upper bits A18 and A17 come from a bank select register, and A16 comes from microcode. For data, upper bits A18, A17 and A16 come from another bank select register. So, the upper three address bits can be different for program and data. Also new is, that this bank select register can be loaded from a memory-resident register in a single cycle.

  APPLICATION and KERNEL

  It is intended that, for Application and Kernel modes, BS_2 and PC18 are always written with the same value. That makes our table slightly less complex:

  This diagram shows all modes. The modes are controlled by signals in the microcode and the bank select registers, as shown.
  

  HARDWARE IMPLEMENTATION

  The result is a bit complex because I didn't want to add more chips. Another complication is, that the address must be available quite fast, so there can not be long propagation delays. The following diagrams (there are seven) explain all modes, and also the way it is implemented. The used colors are the same as in the table above.

  You find the bankswitching control registers at the top of the diagram. The microcode control signals are at the left side. At the right side, you find the five highest address bits, their value indicated by '0' or a color.
  

  REGISTER MODE

  This is the simplest. The (memory resident) registers are in the very first 256 bytes of memory. This mode is selected by CTL_M1 = 1. The CTL_M1 signal switches all upper address outputs OFF, so all upper address bits are zero. The register in question is selected by several bits in the microcode.
  

  The value in the bank select registers (U13 and U50) is irrelevant.

  TABLE / ZPAGE

  In this mode only the lower 8 bits of a data or code address are used. The other bits are zero, but A16 and A17 come from microcode. It is used for 6502 Z-page, for an identity table and a shift-right table. If A16=0 and A17=0, this mode can not be used (that is the domain of the memory-resident registers).
  

  This mode is selected by microcode signals only. The value in the bank select registers is irrelevant.
  

  NATIVE

  This is the main addressing mode of the first prototype. The upper address bits are 'hardwired' in the microcode. Bit A18 can no longer be used, that should not be a big problem because this will no longer be the main addressing mode.
  

  This mode is selected by microcode signals only. The value in the bank select registers is irrelevant.
  

  APPLICATION MODE

  Here, the upper three address bits are freely programmable in the bank select register U50. It is advisable to set BS_2 equal to PC18, otherwise the topmost address bit is different for code and data (also for the following two modes kernel slot1/slot3).
  

  This mode is selected by microcode signals in combination with APPL = 1. Yes, the APPL signal got it's name from this mode.
  

  KERNEL SLOT1

  The 64k addressing range is split up in four slots:
  

  • Slot 0 ( 0000 - 3FFF )
  • Slot 1  (4000 - 7FFF)
  • Slot 2 (8000 - BFFF)
  • Slot 3 (C000 - FFFF)

  In this kernel-slot1 mode, normally the microcode selects which 64k bank is selected (orange signals). But if the address is in slot 1, the bank number comes from the bank select...

  Read more »

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