3.7.2 Access to On-Chip Register Field and External Devices
The on-chip register field (I/O ports, on-chip supporting module registers, etc.) and external devices
are accessed in a cycle consisting of three states: T
1
, T
2
, and T
3
. Only one byte of data can be
accessed per cycle, via an 8-bit data bus. Access to word data or instruction codes requires two
consecutive cycles (six states).
Figure 3-15 shows the access cycle for the on-chip register field. Figure 3-16 shows the associated
pin states. Figures 3-17 (a) and (b) show the read and write access timing for external devices.
Figure 3-15. On-Chip Register Field Access Cycle
Write data
Bus cycle
T1 state
T2 state
T3 state
Internal address bus
Ø
Address
Internal Read signal
Internal data bus (read)
Read data
Internal Write signal
Internal data bus (write)
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