RX610 Group
11. Buses
R01UH0032EJ0120 Rev.1.20
Page 257 of 1006
Feb 20, 2013
11.2
Description of Buses
11.2.1
CPU Buses
The CPU buses consist of the instruction and operand buses, which are connected to internal main bus 1. As the names
suggest, the instruction bus is used to fetch instructions for the CPU, while the operand bus is used for operand access.
Connection of the instruction and operand buses to on-chip RAM and on-chip ROM provides the CPU with direct access
to these areas, i.e. access is not via internal main bus 1. However, only reading is possible in direct access to on-chip
ROM by the CPU; programming and erasure are handled via an internal peripheral bus.
Units for the arbitration of bus requests for instruction fetching and operand access are on-chip RAM, on-chip ROM, and
internal main bus 1. The order of priority is operand access then instruction fetching.
If requests for access via the instruction bus, operand bus, and internal main bus 1 are for different slave modules, the
various forms of access can proceed at the same time. For example, parallel access to on-chip ROM and on-chip RAM or
to on-chip ROM and external address space is possible.
11.2.2
Internal Main Buses
The internal main buses consist of a bus for use by the CPU (internal main bus 1) and a bus for use by the other
bus-master modules, i.e. the DMAC and DTC (internal main bus 2). Requests for bus mastership from the DMAC and
DTC are arbitrated by internal main bus 2. The order of priority is DMAC and then DTC, as shown in table 11.2.
Units for the arbitration of bus requests from the two internal main buses are slave devices on the external and peripheral
buses, and on-chip memory. If the CPU and another bus master are requesting access to different slave modules, the
respective bus-access operations can proceed simultaneously.
Internal main bus 2 (for bus masters other than the CPU) has priority over internal main bus 1 (for the CPU). However,
when the CPU executes the XCHG instruction, requests for bus access from masters other than the CPU are not accepted
until data transfer for the XCHG instruction is completed. Furthermore, requests for bus access from masters other than
the DTC are not accepted during reading and writing-back of transfer control information for the DTC.
Table 11.2 Order of Priority for Bus Masters
Priority
Bus Master
High
Low
DMA destination
DMA source
DTC
CPU
Summary of Contents for RX600 Series
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