Technical Reference Manual
002-29852 Rev. *B
Core, Bus and Memory Access
The Cortex-M4 CPUSS uses AHB-Lite also for the system interconnect. Most of the IPs are compliant to the AHB-
Lite protocol.
The AHB-Lite protocol is little endian; the least significant byte of a word is mapped onto the word's lowest
byte address IPs use a bus data width of 32 bit.
This allows for byte (8-bit), halfword (16-bit) and word (32-bit) transfers. We distinguish two types of
transfer types:
- MMIO registers: These registers are typically used for IP control and status information. Unless otherwise
noted, transfers to these registers must be 32-bit transfers; any other transfer size generates an AHB-Lite bus
error. An IP's MMIO registers do not necessarily fully occupy a consecutive memory region; i.e. holes may be
present in the region
- Memory structures: This includes generic memory structures, such as system FLASH, system RAM and
system ROM, but also IP specific memory structures. Transfer to these memory structures may be 8-bit, 16-bit or
32-bit transfers; any other transfer size generates an AHB-Lite bus error. A memory structure fully occupies a
consecutive memory region; i.e. no holes are present in the memory structure region
A bus slave memory region may contain both MMIO registers and zero or more memory structures. Gaps
might exist in the bus slave's memory region, either due to gaps in the MMIO memory region or gaps between the
MMIO and memory structure region(s)
Gaps in the address space are reserved. Do not access these gaps, if accessed can result in Hard faults or
BUS ERROR depending on which bus segment or a peripheral an address space is allocated to.
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2022-04-18
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