Zilog Z80 CPU NOP operation tester to test Zilog Z80 CPU and compatible clones.

Fully and assembled tester only. CPU and USB cable are not supplied.


This is a simple Z80 CPU tester which exclusively executes NOP (No Operation) instructions to just check for basic functionality tests that the CPU can execute instructions. In addition it gives an indication if the clock input responds correctly.

Tester will not test all address lines and every function on the Z80, but gives you a quick indication if your treasure is good, bad or ugly. This little widget can also be used as easy to understand learging tool as it's minimal computer in itself. You might learn basics about how CPU works by observing system control lines linked to LED’s.


The CPU tester works with all NMOS and CMOS types of Z80 CPUs and compatible clones.


Design overview


What is NOP instruction?


NOP does nothing for 4 clock cycles. It is Useful for a short time waster (for example, it's common to put 8 clock cycles between output and input from the Key Port).


A NOP is most commonly used for timing purposes, to force memory alignment (may be used for pipeline synchronization), to prevent hazards, to occupy a branch delay slot, to render void an existing instruction such as a jump, as a target of an execute instruction, or as a place-holder to be replaced by active instructions later on in program development (or to replace removed instructions when reorganizing would be problematic or time-consuming).


Instruction mnemonic: NOP, Bytes: 1, Opcode 0x00, Notes: There are some other instructions without any effect (and the same timing): LD A, A, LD B, B etc.


How does it all work?


All data bus pins (A0-A7) are pulled to ground by pull-down resistors and the CPU executes NOP instructions (opcode 0x00) when the memory is read in an endless loop. The CPU starts executing these commands from address 0 after reset or data jump.


Addresses A0 to A7 are connected to LED's for visual display and will pulse on and off when the memory locations are checked. If the CPU executes the NOPs, the addresses A0 to A7 should be continuously incremented and CPU acts as a crude BCD counter, where A0 = 1, A1 = 2, A3 = 4, A4 = 8 and so on. You might be spooked by LED A7 flashing. Please don’t be, this is a standard artifact. Z80 executes a periodical memory refresh cycle, where memory access and memory refresh run synchronously.


The clock signal is generated by a 555 timer with variable frequency from 1 - 1200 Hz, so you can determine if the Z80 is executing instructions at different clock speeds. Address locations A0 - A7 are tested in about 3 seconds (1 operation per 4 cycles).


Please be aware, that since the CPU is under-clocked below specification, it is quite possible that in extreme cases the NOP tester might give you incorrect indication.


But what are all the LED’s for anyways?


M1 - Machine Cycle One (output, active Low). M1, together with MREQ, indicates that the current machine cycle is the op code fetch cycle of an instruction execution. M1, when operating together with IORQ, indicates an interrupt acknowledge cycle.

MREQ - Memory Request (output, active Low, tristate). MREQ indicates that the address bus holds a valid address for a memory read or a memory write operation.

IORQ - Input/Output Request (output, active Low, tristate). IORQ indicates that the lower half of the address bus holds a valid I/O address for an I/O read or write operation. IORQ is also generated concurrently with M1 during an interrupt acknowledge cycle to indicate that an interrupt response vector can be placed on the data bus.

RD - Read (output, active Low, tristate). RD indicates that the CPU wants to read data from memory or an I/O device. The addressed I/O device or memory should use this signal to gate data onto the CPU data bus.

WR - Write (output, active Low, tristate). WR indicates that the CPU data bus contains valid data to be stored at the addressed memory or I/O location.

CLK - Clock (input). Single-phase MOS-level clock. Four clock cycles per one machine cycle operation.


Specifications:

Board Type: Tester

Board Material: FR4-Standard Tg 130-140C Epoxy Glass Composite

Layers: Double sided

PCB Colour: Green colour solder mask, white silk screen lettering

Surface Finish: Lead free HASL-RoHS

External size (assembled): 76.2 x 76.2 x 15.0 mm

Board Thickness: standard 1.6mm

Copper Thickness: 35um

PCB weight: 10g

Assembled weight: 47g


How to test the CPU:




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