eZ80 CPU Card Breathes New Life Into RC2014 Z80-Based Computers
This eZ80 CPU module plugs into a carrier to replace the discontinued Z80 in computer designs.
With the Zilog Z80 microcontroller's end-of-life (EOL) date looming, Z80 enthusiasts eagerly seek modern alternatives for their Z80-based computers. The eZ80, with its binary-compatible mode and ongoing production, presents a promising solution. However, it is not a simple drop-in replacement. So, Dean Netherton took up the challenge of designing an eZ80 CPU card compatible with the RC2014 computer and bus standard.
Introduced by Zilog in 2001, the eZ80 microprocessor (MPU) followed the original Z80 microcontroller, which powers many 8-bit computers. The eZ80 can operate at higher speeds, boasts a range of newer peripherals, and has a binary compatibility mode. Unlike its discontinued predecessor, the eZ80 is still an active product. Netherton chose the eZ80F92 for the eZ80 CPU card project. This processor operates up to 20 MHz with 128 KB of on-chip Flash and 8 KB of RAM.
The RC2014 is a modular computer based on the Zilog Z80. It is not a clone of a specific computer but a modern creation. One unique characteristic is that there is no motherboard. Instead, the RC2014 bus module specification calls out the design requirements for a bus that modules plug into, including the CPU card.
The eZ80F92's 100-pin LQFP package pushed Netherton's limited surface-mount soldering abilities, but improved soldering skills were one of the project's objectives! The final eZ80 CPU card took four iterations to achieve. The final four-layer card contains the MPU, passive components, a crystal for the on-chip RTC, and pins to connect to a carrier board.
Netherton needed a backplane adapter PCB for use with a computer like the RC2104. This board contains compatible pin sockets for the eZ80 CPU module, an oscillator, 74HCT245 buffers, and the right-angle header pins to connect to the RC2014 bus. There are additional interfaces broken out from the eZ80. For example, the UART header can send data from the MPU at 115200 baud.
As with most projects, blinking LEDs are often a first milestone. Netherton successfully demonstrated that the eZ80 module attached to the backplane adapter could drive LEDs on a peripheral card also plugged into the RC2014 bus. It is a small demo but an important one.
The eZ80 runs firmware to manage its start-up, configure on-chip devices, handle interrupts, and finally switch into Z80 compatibility mode. You can find the firmware in this GitHub repository. To build the firmware, you will need the Zilog ZDS IDE and a 6-pin Zilog Smart Cable adapter. This proprietary interface can flash, control, and debug the MPU.
Check out the Dinoboards eZ80 for the RC2014 backplane project site for more information. It contains project logs and schematics.