Interface To Bus Master - Hitachi H8S/2199 Hardware Manual

24.3

Interface to Bus Master

ADR and AHR are 16-bit registers, but the data bus to the bus master is only 8 bits wide.
Therefore, in accesses by the bus master, the upper byte is accessed directly, but the lower byte is
accessed via a temporary register (TEMP).
A data reading from ADR and AHR is performed as follows. When the upper byte is read, the
upper byte value is transferred to the CPU and the lower byte value is transferred to TEMP. Next,
when the lower byte is read, the TEMP contents are transferred to the CPU.
When reading ADR and AHR, always read the upper byte before the lower byte. It is possible to
read only the upper byte, but if only the lower byte is read, incorrect data may be obtained.
Figure 24.3 shows the data flow for ADR access. The data flow for AHR access is the same.
Upper byte read
Bus master
(H'AA)
Lower byte read
Bus master
(H'40)
Rev. 1.0, 02/00, page 526 of 1141
Bus
interface
Bus
interface
Figure 24.3 ADR Access Operation (Reading H'AA40)
Module data bus
ADRH
(H'AA)
Module data bus
ADRH
(H'AA)
TEMP
(H'40)
ADRL
(H'40)
TEMP
(H'40)
ADRL
(H'40)