LX-328 - 8 Bit Digital Logic Processor with two ATmega328P MCU's

I am curious, What is it your trying to do with this, you don't say

All the logic your replicating in the 74 series TTL chips is available in the ATMEGA328's already so I'm not sure of your goal

Thanks

Peter

Not sure of it intended function overall, but if I had a guess, I would say a homemade 8-bit computer of his own design. It looks to be a more complex TTL CPU complete with his own upgrades, which wonderfully demonstrates the use of simple logic gates, buffers, a binary adder, registers, multiplexors, decoders, flip-flops, and more. 7400 IC's are very common in TTL CPU projects, so it is interesting seeing this approach to say the very least.

Jason, could you share a little more about it and it intended function(s)?

Thanks,

Cory

Great guess Cory. The board is an 8-bit computer. Granted modern MCU's and CPU's handle logic internally, but I wanted to pose a challenge to myself to see if I could get something like this working.

This is my first hardware project but I've added on a bunch of SPI components, each on their own dedicated line. The 64k x 1 SRAM is unique, and nicely, I am reading and writing to it using basic gpio functions while using adc pins for reading. In all data is handled by pin toggling.

The board has one 8-bit input and an 8 bit output. Depending on which custom opcode comes in through the input 74hc164, one MCU handles which register receives the next incoming byte, and then it sends data on it's way to the appropriate logic gate. I have it sending the bytes out to all gates at the same time, but parallel loading one of nine 74hc165's at a time. When one is loaded, all the other output registers are 0x00 and have no effect on the one output register that has actual data.

The second atmega328p handles peripheral IO, such as saving data to the four SPI flash roms, writing flash or sending data out through 8 channels or reading data into any of the 8 inputs.

Even though I am more of a VB.Net programmer, it helped to understand what the C language was for. All of the components are working, even the LCD display. The hardest thing for me to do was program the application and bootloader code into the MCU flash manually by decyphering the Intel hex files to get the data and then start writing data to the appropriate locations in flash.

Trouble is, I still don't have a connection to the PC for updating firmware on the MCU's so I can use the ISP interface at any time to update the program code. I do have a USB CDC connection with a UC3A3 Xplained evaluation board connected to the PC to run programming in real time in a sort of debug mode, but the opcode interrut language is a bit complicating because the unit listens for hex chars, then takes an action per letter it receives.

For this board, the commands are 4 bit meaning 1 byte = 2 commands except when it requests data entry which is fed into the unit by keyboard on a host PC. You can think of the input as an SPI connection with clock and data line. implementing SPI with GPIO is a matter of toggling the data and clock pins in the right order. In my first attempt, all the values coming from SPI devices were multiplying the bytes by 0x02, even the shift registers all because I toggled the clock line first before checking if the corresponding adc pin was high before issuing clock.

It was a matter of gpio_set_gpio_pin() and  gpio_clr_gpio_pin() functions where one pin is assigned SCK, MISO and MOSI along with a chip select line Per device. 8 data out lines, 8 data input lines, 4 chip selects, 8 clock lines and a 4 bit address bus that handles writing data to the SRAM just by setting an address.

I gave the board 16 priliminary opcode instructions 0-F

They set a uint8, uint16 and uint32 for SDRAM and SRAM data and addresses, select chips, write data, and read data.

I'm definitely working on putting a user guide together, but in debug mode, what could be easier than just typing in hex chars to make the board do something.

For example, if I wanted to read a byte from SPI Flash, I would select one of four. Selecting one deselects the others. I would enter:

C10D190000900009000090B03

C1 selects chip at location C1

0D1 sets uint8 D1

90 sends the uint byte D1 to the spi chip on C1 (SPI0)

followed by 3 0x00 bytes sent to the chip on C1

B0 reads a byte from SPI0 that is on C1

3 prints the hex value of the byte read.

C0 unselects all SPI chips setting the CS lines high.

I could then store it in a temp 74hc165 by sending it with the 96 command which is Output Data Channel 7.

9 means output while the following hex char tells it which channel to export the bye at 0-F although I was currently only using outputs 0-7.

97 is write byte to LCD display. It's pretty unique. 0EE97 = set uint8 EE and send it to the LCD display.

I'll definitely make a list of opcode commands. At the rate of developement, the list can continue of acceptable commands. In debug mode, the board listens for chars with scanf() then converts them into numbers.

Outside debug mode, the whole process is automated rather than taking manual typed data entry. I can load data into ram from flash and even from MCU EEPROM by placing one MCU in ISP mode to get data with the other Atmega. We have Twice the power with two MCU's.

I'll certainly be giving updates, maybe give this thing micro sdcard. Maybe making a pluging card for it as I have the SPI flash. Maybe create universal memory where interchangeable SRAM, Flash and FRAM cards could be used if the cards had the same ponout. Maybe use 8soic components. It's all very exciting.

I Might even give this thing an R2R DAC for playing 8-bit audio. It is coming along nicely. I would consider things like IDE or compact flash but spi seems to be very stable with the unit.

Glad you like the project. Feel free to check out my videos from the links I have posted in the topic above.

I'll be giving updates the more features I can add on. Maybe I can even create a sort of command prompt for it. I gave it an OS name, Lilac and model number LX-328 to correspond with 32 MB of SPI flash and 8 bit operation.

The first sixteen opcodes are 4-bit so one byte has two instructions in it. There is also an extended opcode list accessed by 0xF and then a set of another sixteen. It can keep going. The 16 and 32 bit variables are used to set the SDRAM address while the uint16 = uint8 to fill up the 16 bit SDRAM data with MSByte set to 0x00 per address location.

Thank you for all the provided information Jason!

When you gave the example of making it read bytes from the SPI Flash using Hex it honestly reminded me of the Apple 1 computer developed by Steve Wozniak back in 1976.

I love your take on the 8-bit computer however and I like seeing these kinds of projects because there is always something new someone can add that just makes it that much more special while still keeping it what it is. I am defiantly interested in checking out updates you make on this project and I will certainly check out the videos you provided.

What are your plans for connecting it via USB for updating the firmware rather than using an evaluation board?

You mentioned possibly adding Micro SD and 8-bit Audio, are there any other additions you might be making, such as external display options, basic camera/ printer functions similar to that of the Gameboy camera/ printer for example, possibly analog support or another option for controller support for 8-bit games for example something similar to that of the Atari 2600 joystick?

Might not be too hard expanding IO. I started making a couple games for the onboard lcd display which is pretty fun. Reading button presses might be fairly easy to implement. Perhaps I could give it an SPI color display, might have to search for the shift registers with latch to do some parallel output but still stick to the SPI standard. Audio might not be a problem, I made an 8 bit DAC once before but there just might be a 7400 series DAC, and even a VGA or composite video chip.

USB-wise, I am using VB.Net to open the com port and can read/write data having full control over the board in real time sending hex chars to the board.

I'll definitely keep everyone updated. Maybe even make a mini model of this board later on, possibly on shortrun PCB.

Thanks!

It could even be fun giving the board internet access through USB where the VB.Net app could send data across the internet talking with another like-unit if there were two boards. Could be useful for real-time messaging or data transfer and could be very secure with the range of logic gates the data could go through.

I think it would be pretty awesome to add basic button inputs for 8-bit gaming purposes simply to add another twist to 8-bit computer projects such as this.

As for Audio I have seen an example of i2s DAC with Class D Output which was an entry in a 7400 contest. As it so happens I have the link handy, maybe it could be of some use.

http://willhb.com/?p=155

As for Video, you are in some luck. I know of the ADV7125. It has three separate high speed 8-bit video DACs with a standard TTL input interface. It has additional video signal options such as composite SYNC and BLANK as well. Might be a little more than what you need, albeit could be something to check into. Here is a spreadsheet of the ADV7125; http://www.analog.com/static/imported-files/data_sheets/ADV7125.pdf

A mini DIY kit of this would be an awesome project for anyone looking to play with and/or learn digital logic not to mention the opportunity for someone to build their own little 8-bit computer. I could see something like this being on kickstarter or even as a kit on sites such as E14 or Adafruit.

Looking forward to seeing updates.

I think it would be pretty awesome to add basic button inputs for 8-bit gaming purposes simply to add another twist to 8-bit computer projects such as this.

As for Audio I have seen an example of i2s DAC with Class D Output which was an entry in a 7400 contest. As it so happens I have the link handy, maybe it could be of some use.

http://willhb.com/?p=155

As for Video, you are in some luck. I know of the ADV7125. It has three separate high speed 8-bit video DACs with a standard TTL input interface. It has additional video signal options such as composite SYNC and BLANK as well. Might be a little more than what you need, albeit could be something to check into. Here is a spreadsheet of the ADV7125; http://www.analog.com/static/imported-files/data_sheets/ADV7125.pdf

A mini DIY kit of this would be an awesome project for anyone looking to play with and/or learn digital logic not to mention the opportunity for someone to build their own little 8-bit computer. I could see something like this being on kickstarter or even as a kit on sites such as E14 or Adafruit.

Looking forward to seeing updates.