Cpu Timing - ZiLOG Z8018 Series User Manual

Z8018x
Family MPU User Manual
18

CPU Timing

UM005004-0918
The user must program the Operation Mode Control Register
Note:
before the first I/O instruction is executed.
This section explains the Z8X180 CPU timing for the following operations:
•
Instruction (Op Code) fetch timing
•
Operand and data read/write timing
•
I/O read/write timing
•
Basic instruction (fetch and execute) timing
•
RESET timing
•
BUSREQ/BUSACK bus exchange timing
The basic CPU operation consists of one or more Machine Cycles (MC).
A machine cycle consists of three system clocks, T1, T2, and T3 while
accessing memory or I/O, or it consists of one system clock (T1) during
CPU internal operations. The system clock is half the frequency of the
Crystal oscillator (that is, an 8-MHz crystal produces 4 MHz or 250 nsec).
For interfacing to slow memory or peripherals, optional Wait States (TW)
may be inserted between T2 and T3.
Instruction (Op Code) Fetch Timing
Figure 9 illustrates the instruction (Op Code) fetch timing with no Wait
States. An Op Code fetch cycle is externally indicated when the M1
output pin is Low.
In the first half of T1, the address bus (A0–A19) is driven from the
contents of the Program Counter (PC). This address bus is the translated
address output of the Z8X180 on-chip MMU.
In the second half of T1, the MREQ. (Memory Request) and RD (Read)
signals are asserted Low, enabling the memory.