Z80 Computer CPU Card (USB Code Loader)

Documents: Schematic 

    Are you ready for some retro computer fun?  This Z80 based computer allows for super fast experimentation without the delay or complexity that PROM/EPROM/EEPROM/NVRAM based systems introduce; no EPROM programmer (Or eraser for that matter!) required, just assemble your code, upload it immediately to the Z80 SRAM and run your program!

     The 1730 uses the Z84C0008VEC processor connected to 64kB of SRAM that fills the functions of both ROM and RAM.  This computer uses a USB interface to upload code from a hex file into the SRAM and after upload, the SRAM can be executed immediately and/or copied into the USB controller's 64kB of Flash.  If the code was previously copied to FLASH, after each power cycle the Flash is copied back into the SRAM and when the copy process is complete the bus is released, the Reset# signal is de-asserted and the Z80 is allowed to operate independently of the USB controller. Note: There is no need to clear the flash when uploading a new program to the computer, i.e. the FLASH only comes into play after a power cycle.

     The Z80 clock can be supplied from a 8MHz crystal oscillator or by the USB controller in a frequency range of DC to 16MHz including single stepping.

     With the USB controller the user also has the ability to do in-circuit-debug (ICD) of Z80 code: One way is to use the "Slow Clock" described below to stop execution at a particular address and then single step through the code to observe the program flow. Another method is to use the SLOW clocking from DC to 1kHz; the instructions are disassembled in real time and the UI has a 16 register capture window that shows the values of user selected memory addresses whenever they are written. Alternatively the BUSREQ# signal to the Z80 can be asserted at which time any number of bytes from the SRAM can be read directly and when done, BUSREQ# is deasserted and the code execution can proceed normally...This can be done while operating with any clock source.

     For added entertainment an LED signal watcher board can be plugged into the Z80 backplane or in-line with the CPU to examine every Z80 signal in real time.  Because the CMOS Z80 is fully static, it can be single stepped from the UI or push button on the CPU card or it can be clocked at any frequency from DC to 16MHz (Way overclocked!).

Hardware Features:

• 3.0 inch wide by 2.5 inch high 4 layer PWB with a 40 pin (2 x 20 x 0.1) edge mounted female connector to mate with the Z80 Backplane or user circuit.
• 8MHz Z84C0008VEC CMOS Z80 CPU: Static operation allows for single stepping.
• 64K of 45nS SRAM for Z80 code execution.
• 64K of FLASH memory in the USB controller for SRAM image storage.
• USB connection from the PC/UI to the USB supervisory processor.
• 8MHz crystal oscillator or Programmable clock from the USB controller.
• All signals, representing every pin on the Z80 are available at the 40 pin connector and are clearly labeled.
• During Z80 code execution, all signals operate transparent to the USB controller unless a WAIT#, RESET# or BUSREQ# is commanded via the UI:  This means the Z80 operates the same as if the USB controller was not there.
• LCD interface mapped to I/O addresses 0xFE (Control) and 0xFF (Data).
• Power is supplied from the USB Mini-B connection (With or without USB data interface) or from the 5V/2A regulator on the backplane (7 to 35V input).

Computer controls:

• A button at the top of the card directly controls the Z80 Reset# signal that operates in parallel with the USB controller (Wired-AND).
• A button is provided on the side of the computer to Start, Stop or single step the computer.
• A button is provided on the side of the computer for future expansion.
• A button is provided labeled "FBL" that forces the USB controller into bootload mode (Generally this is not needed, mainly for USB controller firmware development).

Memory:

• 64kB of FLASH in the USB controller (10k write endurance) is provided for non-volatile Z80 code storage.
• 64kB of SRAM that serves as both the ROM and RAM for the Z80 CPU.
• Upload from the PC to the Z80 computer SRAM can be any number of bytes from 1 to 65536, starting at any address.
• The UI reads in a standard IntelHex file which is formatted for display and prepared for upload to the Z80 computer.  Once loaded, the formatted SRAM image can be saved to a text file.
• Note: The IntelHex file must have the CR+LF line enders to load properly; otherwise an error message will be displayed. The UI help has information on how to easily correct this.
• Uploading 64kB to SRAM takes about 30 seconds (~2kB/S).
• Unused SRAM can be filled with any byte value or left in its uninitialized state to simulate a fully stand alone computer.
• SRAM can be executed by the Z80 immediately after upload without first copying to flash:  A typical, moderately sized program can go from assembler to operation in under 30 seconds.
• If desired, SRAM can be copied to FLASH after upload is complete, typically in about 30 seconds.
• Code stored in FLASH will be copied to SRAM at computer power up in under 0.5 seconds and begin execution.
• SRAM can be downloaded from the Z80 computer to the PC and saved as a text file: Download size is any number of bytes from 1 to 65536 bytes starting at any address.
• Downloading 64kB from SRAM takes about 30 seconds (~2kB/S).
• Z80 code execution can be paused, SRAM downloaded and then execution resumed.
• Up and downloading SRAM generates a 32bit CRC:  This allows detection of as little as a single bit change, useful inspect for ROM write violations, etc.

Debugging:

• When the Z80 is paused in the BUSREQ#/BUSACK# state, any portion of the SRAM can be downloaded over the serial link.
• When using the "Slow clock" described below at 0 to 1kHz or 250kHz, a break-at-address-match can be set which will halt the clock at that location.
• When using the slow clock at 0 to 1kHz the Z80 code is disassembled in real time (Windows throughput permitting).  If the windows environment is slower than the Z80 computer bus state report speed, the UI will still capture and disassemble the code, e.g. if the code operates for 10 seconds but windows disassembles at 1/2 the report rate, the disassembly will complete in 20 seconds.
• Part of the disassembly process enables the UI to watch for and report writes to 16 user specified memory locations while code is running.
• The bus state is only reported when either RD# or WR# are asserted which reduces the RS232 traffic during code execution vs having to report the bus state at EVERY clock cycle.
• Disassembled code can be saved as a test file.
• If the user copies the Z80 registers to SRAM prior to the BUSREQ# assertion the UI can decode that space into a special set of UI registers when that SRAM is read.

Clock sources:

• Fixed 8MHz crystal oscillator.
• Programmable clock generated by the USB controller from DC to 16MHz.
• "Slow clock" from the USB controller under UI control operates from DC to 1kHz or 250kHz (About 1/32th of the 8MHz clock speed) with single step and break-on-address-match capability. Slow clock count can be anywhere from 1 to (2^24-1) cycles and can be canceled from the UI or button press on the Z80 computer.
• This example is a Real time disassembly with breakpoint of  the LCD driver program (Which also shows the entire upload sequence from scratch) running on the slow clock.

Accessories:

• A label board is included that fits over the male header pins with signals clearly labeled on both sides.
• The Z80 computer can be run stand alone or can be plugged into the 5 slot Z80 Backplane for hardware expansion.
• The Z80 backplane provides 0 to 2A at 5VDC from a 5.5 x 2.5mm DC jack, reverse polarity protected with an input range of 7 to 35VDC.
• Hardware prototyping is easy with the Backplane Proto Card that has pads on a 0.1 inch grid and space for up to four 40 pin DIPs or many more smaller packages.
• Signals can be examined visually with the Signal Watcher that plugs into the backplane.
• The Piggy Back Signal Watcher has a male header on top that can be plugged directly into the Z80 computer or into the backplane.  A computer or expansion card can be plugged onto the watcher so it does not waste a backplane slot.

Firmware/Debug support:

• A free windows Z80 computer control UI is provided to upload code to the Z80 computer, download the computer memory and control the computer hardware.  This UI is also used to update the firmware in the USB controller.
• Sample code:  Z80 Library / zDevStudio Projects. The driver for the LCD interface is included in the Z80 library (LCD Display code timing).
• zDevStudio (SourceForge) is a free Z80 assembler that uses the Pasmo compiler (v0.6).
• The Z80 control UI can reformat the zDevStudio (Pasmo) debug output into a more readable list file with symbol map that can be saved as a text file.
• Loaded IntelHex file data can be saved to an RTF file in an easy to read 16 bytes per line format with ASCII translation.
  
• Downloaded SRAM data (Or any subset of it) can be saved to an RTF file in an easy to read 16 bytes per line format with ASCII translation.
• During debug, the user can step through and examine the code execution with the UI using the built in real time Z80 disassembler.