CZ80CPU Megafunction Datasheet
Pin Description
Name
Type Polarity/
Bus size
Description
clk
I
Rise
Clock Feeds internal clock counters and all synchronous circuits.
reset
I
High
Hardware reset input A high on this pin for two clock cycles while the oscillator is running
resets the device.
wait_n
I
Low
Wait A low on this pin indicates to the CPU that the addressed memory or I/O devices are not
ready for a data transfer. The CPU continues to enter a wait state as long as this signal is active.
int_n
I
Low
Interrupt Request This signal is generated by an I/O device. The CPU honors a request at the
end of the current instruction, if the internal software-controlled interrupt enable flip-flop is
enabled.
nmi_n
I
Low
Non-maskable Interrupt This pin has a higher priority then int_n and is always recognized at
the end of the current instruction independent of the status of the interrupt enable flip-flop and
forced the CPU to restart at address 0066h.
busreq_n I
Low
Bus Request It has higher priority than nmi_n and is always recognized at the end of the
current machine cycles. Active state on this pin forces the CPU address bus, data bus, and
control signal to go to a high-impedance state, so that other devices can control these lines.
busak_n O
Low
Bus Request Acknowledgment Low on this pin indicates to the requesting device that the
CPU address bus, data bus, and control signal have entered their high-impedance state and it
can now control these lines.
m1
O
High
Machine Cycles One This pin together with mreq_n indicates that the current machine cycle is
the opcode fetch cycle of an instruction execution. It together with iorq_n indicates an interrupt
acknowledge cycle.
addr_o
addr_tri
O
O
8
High
Address Bus Addr_o forms a 16-bit address bus. The address bus provides the address for
memory data bus exchanges (up to 64K bytes) and for I/O device exchanges.
data_i
data_o
data_tri
I
O
O
8
8
High
Data Bus (input/output, 3-state) 8-bit bidirectional data bus, used for data exchanges with
memory and I/O.
mreq_n
mreq_tri
O
O
Low
High
Memory Request Indicates that the address bus holds a valid address for memory read or
memory write operation.
ioreq_n
ioreq_tri
O
O
Low
High
I/O Request Indicates that the lower half of the address bus holds a valid I/O address for an
I/O read or write operation.
rd_n
rd_tri
O
O
Low
High
Read rd_n indicates that the CPU wants to read data from memory, or that an I/O device or
memory should use this signal to gate data onto the CPU data bus.
wr_n
wr_tri
O
O
Low
High
Write Indicates that the CPU data bus holds valid data to be stored at the addressed memory
or I/O device.
rfsh_n
O
Low
Refresh Timing This signal together with mreq_n, indicates that the lower seven bits of the
system’s address bus can be used as a refresh address to the system’s dynamic memories.
halt_n
O
Low
Halt State low on this pin indicates that the CPU has executed a Halt instruction and is awaiting
either a nonmaskable or a maskable interrupt before operation can resume.
Functional Description
The CZ80CPU core is partitioned into modules as shown in the Block Diagram and described below.
Cycle Control
The main control machine, which synchronizes all the others. It has an instruction register and all registers
controlled interrupts, bus request cycle, wait states etc. This unit controls bus control signals too.
Bus Control
Registers are triggered on the falling edge and or gates. These are used to form the bus control timing,
changed on both clock edges. This is the only unit that has registers synchronized on the falling clock edge.
CAST, Inc.
Page 3
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