42
2.6
Basic Operational Timing
CPU operation is synchronized by a system clock (ø) or a subclock (ø
SUB
). For details on these
clock signals see section 4, Clock Pulse Generators. The period from a rising edge of ø or ø
SUB
to
the next rising edge is called one state. A bus cycle consists of two states or three states. The
cycle differs depending on whether access is to on-chip memory or to on-chip peripheral modules.
2.6.1
Access to On-Chip Memory (RAM, ROM)
Access to on-chip memory takes place in two states. The data bus width is 16 bits, allowing
access in byte or word size. Figure 2.11 shows the on-chip memory access cycle.
T
1
state
Bus cycle
T
2
state
Internal address bus
Internal read signal
Internal data bus
(read access)
Internal write signal
Read data
Address
Write data
Internal data bus
(write access)
SUB
ø or ø
Figure 2.11 On-Chip Memory Access Cycle
