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AMBA AHB vs AXI: Key Differences Explained

This article compares AMBA AHB and AXI, highlighting the key differences between these two important AMBA (Advanced Microcontroller Bus Architecture) interface bus types.

Introduction to AMBA

AMBA stands for Advanced Microcontroller Bus Architecture. It defines AMBA protocols used for connections on boards with various functional blocks and peripherals. This is especially useful when you have multiple microcontrollers, microprocessors, memory, DSPs, DMAs, USBs, PCI, I2C, and other peripherals on a single board.

AMBA standard evolution timeline

Figure 1: AMBA standard evolution timeline

AMBA standards cover various bus types, including:

AHB (Advanced High-performance Bus)
ASB (Advanced System Bus)
APB (Advanced Peripheral Bus)
ATB (Advanced Trace Bus)
AXI (AMBA Extensible Interface)

AMBA AHB (Advanced High-Performance Bus)

AMBA AHB is designed for high-performance, high clock frequency system modules. It allows efficient connection of processors, on-chip memories, and off-chip external memory interfaces.

Key features of AMBA AHB:

High performance
Pipelined operation
Multiple bus masters
Burst transfers
Split transactions
Single clock edge operation
Single cycle bus master handover
Non-tristate implementation
Wide data bus configurations (64/128 bits)

AMBA AHB Architecture

Figure 2: AMBA AHB Architecture

Figure 2 shows an AMBA-based SoC design using AHB or ASB protocols for high-bandwidth interconnects and APB protocol for low-bandwidth peripheral interconnects. An AHB to APB bridge or ASB to APB bridge connects the high-bandwidth and low-bandwidth peripherals.

AMBA AXI (AMBA Extensible Interface)

AMBA AXI is designed for high-performance, high-frequency, and high-speed submicron interconnect. It’s well-suited for high-bandwidth and low-latency designs. It’s also backward compatible with previous interfaces like AHB and APB. AXI enables high-frequency operation without the need for complex bridges.

AMBA AXI architecture

Figure 3: AMBA AXI architecture

There are different versions of AXI interfaces, including AXI3, AXI4, and AXI-Lite, each defined in various standard specifications.

As shown in Figure 3, an AXI system consists of multiple master and slave devices connected using interconnects.

AMBA AHB vs. AMBA AXI: Key Differences

The following table highlights the comparison between AMBA AHB and AMBA AXI:

Features	AMBA AHB Bus	AMBA AXI Bus
Architecture	Single channel, Shared bus	Multi-channel, read/write optimized bus
Bus width and Speed	128-bit bus running at 400 MHz	64-bit bus running at 200 MHz
Burst mechanism	Address and data are locked together (single pipeline stage), HREADY controls intervals for address and data	One address for the entire burst, simultaneous read and write transactions
Address Transmission	Requires transmitting the address of every data item transmitted on the bus	better bus utilization as burst mode requires transmitting address of only the first data item on the bus.
Throughput	Good	Excellent
Latency	Excellent	Good
Power consumption	Low	High
Burst mode	No fixed burst mode	Fixed burst mode for memory-mapped I/O peripherals
Access	No exclusive access support	Exclusive data access (semaphore operation) support
Security	Simple protection and cache hint support	Advanced security and cache hint support
Timing Isolation	No inherent support for timing isolation	Register slice support for timing isolation
Power interface	No low power interface	Native low power clock control interface
Topology	Default hierarchical bus topology support	default bus matrix topology support