Rev. 1.0, 02/00, page 59 of 1141
2.9
Basic Timing
2.9.1
Overview
The CPU is driven by a system clock, denoted by the symbol
φ
. The period from one rising edge
of
φ
to the next is referred to as a “state.” The memory cycle or bus cycle consists of one or two
states. Different methods are used to access on-chip memory and on-chip supporting modules.
2.9.2
On-Chip Memory (ROM, RAM)
On-chip memory is accessed in one state. The data bus is 16 bits wide, permitting both byte and
word transfer instruction. Figure 2.18 shows the on-chip memory access cycle.
Internal address bus
Internal read signal
Internal data bus
Internal write signal
Internal data bus
φ
Bus cycle
T1
Address
Read data
Write data
Read access
Write access
Figure 2.18 On-Chip Memory Access Cycle