Yet Another ZX-Spectrum AT Keyboard Interface ( YAZSAKI ? )

So what exactly is a zx-spectrum ?  Well here's almost everything you could want to know about this old 8 bit system from the 80's can be found here World of Spectrum and also here Planet Sinclair.

Plus a BBC news story about the beastie How the Spectrum began a revolution.

Theory

For anyone unfamiliar with the Spectrum the keyboard rows were read by a z80 'in' op-code with one or more of the high address lines, A8 to A15, being pulled low and the 5 column bits, D0-D4, being read from the Sinclair ULA. The pressed key connected, via a diode, the ULA's keyboard input bit, which was pulled high by a 10k resistor, to the low address line and so forcing the input low.  A very cheap, cheerful and effective way of implementing a 40 key keyboard,  8 address lines x 5 bits = 40 keys.

So to emulate how the original works, it's just a case of reading in an AT keycode, turning it into a 5 bit 'row value' and putting it on the spectrum databus when a read of the keyboard is made for the row of the corresponding high-address bit. Simple eh ? 

Design

The design has only two major components, an Amtel AVR ATtiny45 micro-controller (MCU)  and a XC9572 Xilinx complex programmable logic device (CPLD).

The Atmel micro-controller initializes and talks to the keyboard. The keyboard is put into the mode "scan code set 3", which is a simplified key mode where no extended codes are used which makes life easier. Also the key auto-repeat is disabled. Auto-repeat is handled by the spectrum so no need to use it on the keyboard end. The controller program translates the 127 possible AT key codes into spectrum keyboard column bits using a look-up table to map one to the other. See the AVR source code for details.

Finally the micro-controller writes serially the state of all 40 keyboard keys to the Xilinx programmable logic. The CPLD  which acts like a 40 bit memory device storing the key states of up or down and passing these to the spectrum when requested.

The most important thing the CPLD does, apart from storing the key information, is handling I/O reads from spectrum ! When the spectrum reads the keyboard using a z80 'IN' opcode the CPLD detects the IORGE with the Z80 /RD line going low, which is the signal for a keyboard read, and places onto D4 to D0 the state of the 'key bits' array. The bits are controlled by the A8 to A15 address lines. A8 to A15 enables a particular keyboard row and each of the D4 to D0 bits corresponds to a keyboard column. As the ULA is effectively 'isolated' from the specturm databus, the CPLD has effective priority, so it's data is read by the spectrum and there's no contention with the ULA.

The Advantage of this design

Why's this design different from others ? Well.. on the original Spectrum you could read more than one row of keys at a time by having more than one address line being low during a keyboard read. The big advantage of this CPLD approach is that it can simulate this behavior. So if both address line A8 and A10 are low, the CPLD will output the up/down key status of BOTH those rows onto the databus, just like you'd see happening with a real Spectrum.

Technical Notes on the Program & CPLD

The serial shifter for the MCU to the CPLD is very simple, the MCU pulls the 'mem reset' line high, then low to set the CPLD's internal address counter to zero.  Next it puts each bit representing the key state onto the CPLD's 'mem-data' line and pulls the 'mem clk' clock input high, at which point the CPLD stores that key state in it's internal 40 bit memory. The MCU pulls the clock low and gets the next bit ready to send. At this falling edge of the clock signal, the CPLD updates its internal address counter to point to the next bit location in the 40 bit 'key bits' memory array. After all bits have been written out the MCU pulls 'mem reset' high causing the CPLD's internal memory address counter to be reset back to zero. See the diagram for details,

The micro-controller AT keyboard conversion table allows for multiple spectrum keys in two different rows to be marked as 'pressed' simultaneously. It can also set all the keys in one row as 'pressed' at the same time too. This is really useful for the AT key 'delete', which is translated to the spectrum codes for 'shift' and '0'. Also the arrow keys are decoded to the spectrum 'shift' + direction keys.

The pinning of the CPLD was made with reference to make the board layout as easy as possible and ideally fit on a single sided PCB. In the end only one jumper was required. So that pretty much met the design goals.

Schematics & PCB

Full schematics plus the prototype PCB designed in Cadsoft's Eagle v4.1.

Software - Microcontroller + CPLD

The micro-controller program was written in AVR assembler, source code for AVR studio 4.1 is provided.

The CPLD code was written in VHDL using Xilinx webpack ISE 8.2, the source and project files are provided.