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• SystemBus

  Gilhad • 2 days ago • 0 comments

  There are two copies of SystemBus, both paralely conected. Either one, or both may be used to stack with other PCBs like retrocomputer or another cards together. I will use female/male connectors (socket with long legs) in the inner position, but it is also possible to use pins downwards and socket upwards, alternation positions, if the F/M connectors are ont available or practical.

  The SystemBus contains the usual common power, 8bits Data and 16bits Address, then communication with GLUE divided to A and B parts and rest of pins is reserved for signals (like MasterReset).

  MasterReset is used for reseting all connected PCBs, while normal Reset resets only this PCB without affecting others.

  |SystemBus-schema.png|

  |SystemBus-side.png| view from side with sockets in outer and pins in inner holes

• Configuration

  Gilhad • 12/28/2025 at 15:01 • 0 comments

  This PCB can be used in different ways and different configurations. There are a lot of solder-jumpers (some open, some bridged by default) to customize it for your needs.

  Serial connection

   RX/TX are on pins PE0/PE1, CTS/RTS are on pins PB4/PB5, all 4 are at their respective ports pins. DTR is connected to Reset block directly.

  Together they are on Serial connector on top edge for connecting any external Serial source (default). 

  |USB-Serial_pinhead.png|

  Solder-jumpers there enable connection to CH340G USB-Serial convertor.

  |USB-Serial_CH340G_top.png|

  Another set of Solder-jumpers is under CP2102 module and allowe use of that module.

  |USB-Serial_CP2102_bottom.png|

  CH340G and CP2102 cannot be used together, or with external Serial (signals will conflict). 

  SBC config

  To use Shared RAM in SBC configuration is needed access to its signals (otherwise managed by GLUE) and maybe to its gates to SystemBus too - simply solder all jumpers in this part and use the new scheme

  |Shared_RAM_top.png|

  |config_SBC.png|

  AxB SystemBus connection - Graphic Card

  Graphic Card configuration is the final goal - this PCB would serve as Input/output for retrocomputer - SystemBus is connected to other PCB, where is retrocomputer and its GLUE is managing Shared RAM and some more signals.

  There may be 2 such cards in system (one for VGA/RCA where screen takes 90% of time, other for SD card, PS/2 and Serial, where some transmission needs be uninterrupted by screen). One card will be named A and other will be named B and GLUE will talk to each separately. To became Card A solder left part of each of 10 jumpers, to became card B solder right part. (And in both cases unsolder SBC configuration.)

  |config_GLUE_top.png|

  |config_GLUE_top_A.png|

  |config_GLUE_top_B.png|

  |config_GLUE.png| This is communication from MCU to GLUE.

  |config_gates.png| Here is communication from GLUE to gates

  |config_shared.png| and here communication from GLUE to Shared RAM 

  Notice, that in order to read/write shared RAM the MCU need ask GLUE to set related signals (and GLUE will do it only if the Shared RAM is owned by MCU)

  HALT + LED

  For proper boot of retrocomputer may be needed to fill its RAM with the "ROM" part before it is started. To do so the HALT should be pulled DOWN (it is active low) and hold there until is all ready.

   If this PCB is supposed to do that, it would be good to solder pulldown resistor (and not pullup), after boot actively hold down the line and after setting the RAM (open gates via GLUE, manipulate A0..A16 and D0..D7 and set write signal via GLUE) hold it UP. 

  If this PCB is suppose to do that, but does not want to doo it, solder pullup resistor (and not pulldown), cut the XHalt jumper and use HALT LED for anything else (like SYSTEM_LED, but beware, it is also active low)

  If this PCB is not suppose to do that (the other (AxB) card should do it), do not solder any pull* resistor, cut the XHalt and use LED as you want.

  On SBC configuration this line is used to to write to Shared RAM, so do not solder pull* resistors, do not cut XHalt, but you may cut out the HaltLed or leave it to indicate writes to Shared RAM.

  Also you may cut the HaltLed and solder any output to this inverted LED or use the invertor for anything else.

  |config_HALT.png|

  |config_HALT_bottom.png|

  PS/2 Direct access + Inside

  If you want read the PS/2 direct and process it, solder the 2 jumpers near SPI/SD card (and change the two marked resistors to 1 kOhm ) Also you may want to cut the Inside jumper, as it is probabelly of no use in such scenario. It also interfere with SPI, which needs address in SW.

  |PS2_direct_top.png|

  SD Card

  SD Card may be cut off SPI and managed separatelly via its header, but it is probabelly of no use

  RCA source 40 x 80

  This pinhead selects source of graphic data for RCA - the pins are 40,signal,80 and the center (signal) should be connected to one of the sides  - 40 selects the UART1 output (and classical driver...

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• Debug LEDs

  Gilhad • 11/18/2025 at 19:01 • 0 comments

  As I need feedback while testing things, I usually use LEDs on Arduino, but LEDs needs IO pins and forgotten code may affect function later, so I decided to use one pin (PD4) for 6 Neopixel WS2812B which are serial driven. Setting different colors may indicate different states and should I accidentally forget somewhere some testing code, it will reveal soon and in non harmfull way.

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• RAM (External + Shared)

  Gilhad • 11/15/2025 at 06:18 • 0 comments

  External RAM may be used to have visible 64kB RAM inside MCU (but those 56kB extra is 1 tick slower to access).

  Shared RAM may be shared with retrocomputer or used as just storage with slower time then RAM but faster then anything else.

  Both are 128kB chips CY7C109D, as I found them to be cheap, accessible and 5V friendly. The truth is, that their output is  lower then 3.4V, but Atmega should be able use it anyway. 74HC245 gate may need power little lower than +5V, so I added resistor there to slightly decrease it (or maybe Schottky diode). I will see, if it is really needed.

  Note that near the RAM datalines may be permutated any way, as well as adderss lines, because it does not matter, the permutation will negate when the data are readed back. So it may be used to have better physical traces there (as I did).

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• SD Card + SPI + ISCP

  Gilhad • 11/15/2025 at 05:27 • 0 comments

  SD card formated with FAT system serve for storing and reading files. 

  Currently it uses SPI and Arduino library, but as I use timers, I update millis() just each screen refresh to make illusion of time for its timeouts.

  SPI have its LED and pinheader placed near SD card and may be separated by cutting solder jumpers (default connected). MISO/MOSI may serve also for PS/2 serial input when its (default open) solderjumpers are solrdered.

  ISCP connector is also there with pin 1 marked by white rectangle under SD reader. 

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• Serial + USB

  Gilhad • 11/14/2025 at 19:36 • 0 comments

  Common

  All marks are from MCU signal side of view. (RX is input, TX output, CTS is input, RTS is output, DTR is input).

  Default Serial uses USART0 (RX0/TX0 - PE0/PE1 pins) and RTS/CTS (PB5/PB4 pins) and DTR connected to Reset.

  There are 3 possible sources for Serial Communication

  • Serial pinhead - always conected to MCU, need all signals to be attached
  • CH340G with micro USB from Arduino Mega Pro - need to solder connect (RTS/CTS does not work with usual drivers)
  • CP2102 module with USB A - need to solder connect

  There can be two modes of communication

  • full with RTS/CTS flow controll - recomended - protect agains buffer overflow, when PC transmits more data (like a file, or Copy-Paste larger block)
  • simply RX/TX like on Arduino, buffer may overflow, cannot paste longer lines

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• PS/2

  Gilhad • 11/14/2025 at 08:02 • 0 comments

  Errata

  • all 10 kΩ resistors in PS/2 (8+2) should be rather 1 kΩ (otherwise it take too long for reading values, like 10 clock cycles)

  PS/2

  PS/2 have 2 lines - Clock and Data. See https://wiki.osdev.org/PS/2_Keyboard

  Clock runs at 10-17 kHz, 11 pulses (normal High, active LOW), data came as 

  • 1 start bit (always 0)
  • 8 data bits (least significant bit first)
  • 1 parity bit (odd parity)
  • 1 stop bit (always 1)

  The clock/data can be connected directly to PB2/PB3 MOSI/MISO via solder jumpers (default OPEN, solder to USE) and decoded as serial communication, but I will use two shift registers and R-C to detect, if we are Inside the packet or not, so I could read the data as one byte on port PF, when I do not use it as VGA data output. 

  Inside signal (connected via closed solder jumper, cut if not wanted) is HIGH when clock is changing, LOW, when clock stops and PSDat is valid. 

  PS-OE is active LOW and let the captured PS data go to port PF

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• Composite output

  Gilhad • 11/13/2025 at 10:43 • 0 comments

  The composite output is currently done via USART running on half system clock.

  One line is 64 uS long.

  Sync pulse should be 0 V, black should be 0.3 V and white should be 1 V.

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• VGA

  Gilhad • 11/07/2025 at 12:32 • 0 comments

  VGA need Vertical Sync (at start of each screen), Horizontal Sync (at start of each line) and Signal (inside the line, it is the pixels).

  Vertical Sync is generated by timer on pin PE4 (every 16.64 mS ~ 60 Hz)

  Horizontal Sync is generated by timer on pin PB6 (every 32 uS)
  

  VGA signal is generated in interrupt on timer overflow (every 32 uS = each 512  clock ticks)

  It needs pin PE7, where 16MHz (system clock) can be configured. As Arduino Mega does not have this pin connected, this was the main reason for making this PCB.

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• How it started

  Gilhad • 11/05/2025 at 12:46 • 0 comments

  Many years ago I bought  Omen Kilo (SBC with HD6309) and start to play with it. It worked, but did not anything visible (does it even run?), just communicated over Serial. Which was not so much satisfying.

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