The Cortex-M0 is an ARM processor core designed for microcontroller applications. It is part of the Cortex-M series of cores, which are designed for embedded and IoT applications requiring low power consumption. The Cortex-M0 is ARM’s smallest and simplest Cortex-M core, making it suitable for cost-sensitive and size-constrained applications.
AHB stands for Advanced High-performance Bus. It is the main system bus used in Cortex-M0 systems to connect the processor core to on-chip peripherals and memory. The AHB bus provides high-bandwidth and low-latency connectivity within the Cortex-M0 system.
Cortex-M0 Core
The Cortex-M0 is a 32-bit RISC processor core built on the ARMv6-M architecture. It is optimized for low-power operation while still providing good performance for embedded applications. Some key features of the Cortex-M0 core include:
- 32-bit RISC architecture with Thumb-2 instruction set
- Up to 48 MHz maximum frequency
- 3-stage pipeline to achieve good performance at low MHz operation
- Single-cycle 32-bit multiplier
- Low-latency interrupt handling
- ARMv6-M architecture with bit banding and single cycle I/O
The streamlined 3-stage pipeline and single-cycle multiplier allow the Cortex-M0 to achieve 1.25 DMIPS/MHz. This provides good performance even at lower clock speeds. The Cortex-M0 implements the Thumb-2 instruction set, which provides improved code density compared to older Thumb instruction sets. Thumb-2 provides a balance of performance, code size, and simplicity.
AHB Overview
The AHB bus provides the main system interconnect in Cortex-M0-based microcontrollers. It connects the processor core to memories, peripherals, and external interfaces. The AHB uses a centralized bus architecture with multiple bus masters. All bus masters can access the shared bus using an arbiter.
Some key features of AHB include:
- High bandwidth – up to 1 data transfer per clock cycle
- Burst transfers for improved efficiency
- Split transactions for non-blocking transfers
- Single-cycle bus master handover
- Centralized arbitration for multiple bus masters
- Configurable bus width (32/64/128 bits)
The AHB supports multiple bus masters, such as the Cortex-M0 core, DMA controllers, and other direct memory access (DMA) peripherals. An arbiter handles arbitration between bus masters to determine which one gets access to the bus. This allows concurrent operations from different bus masters.
Cortex-M0 AHB Interface
The Cortex-M0 core connects to the AHB bus through its AHB-lite interface. This provides low-latency access to memories and peripherals in the system. The interface has a 32-bit data bus and operates at the same frequency as the core clock.
The Cortex-M0 AHB interface supports:
- 32-bit data bus configuration
- Up to 48 MHz AHB clock speed
- Single-cycle access latency for most AHB transfers
- Burst transfers for sequential accesses
- Unaligned accesses supported by interface
- Multi-layer AHB topology connections
The single-cycle latency allows the Cortex-M0 to perform zero-wait state accesses to memories and peripherals on the AHB bus. Burst transfers improve efficiency for sequential accesses like instruction fetches or buffer loads/stores. The AHB interface also handles details like unaligned accesses and AHB protocol compliance.
AHB Memory Map
The Cortex-M0 AHB interface connects to various memories and peripherals based on the system’s memory map. A typical memory map includes:
- Internal SRAM – On-chip memory for data and instructions
- Boot ROM – Read-only memory for boot code
- External memories – Off-chip memories accessed via interface
- Peripheral registers – On-chip peripheral control registers
- External peripherals – Off-chip peripherals accessed via interface
The processor fetches instructions from boot ROM or internal SRAM. Data accesses go to SRAM regions for stack, heap, and globals. Peripheral registers allow control of on-chip peripherals like timers, GPIO, etc. External memories and peripherals are accessed via bridges/interfaces connected to the AHB.
The memory map is configured to allow efficient access to different regions. Instruction and data accesses require low latency, so SRAM is placed close to the processor. Peripheral registers have defined addresses for easy access. External memories can be placed in address ranges that don’t conflict.
AHB Transactions
The AHB uses different types of bus transactions to read/write data and transfer control between bus masters.
Some common AHB transactions include:
- Single access – Simple transfer of 1-4 bytes
- Burst transfer – Sequential transfer of 4-256 bytes
- Split transfer – Large transfer broken into smaller bursts
- Interrupt transfer – Alerts processor to pending interrupt
- Lock transfer – Atomic transaction for shared resource
The Cortex-M0 uses single accesses for most control registers and data structure access. Burst transfers speed up instruction fetching and buffer/stack operations. Interrupt transfers quickly signal the processor to handle new events. Locked transfers prevent interference when accessing hardware semaphores.
The AHB protocol uses handshake signals for transfer arbitration, timing, and control. This allows it to handle multiple transaction types efficiently.
AHB Bus Arbitration
Since multiple masters can access the AHB bus, an arbiter is needed to control access. The arbiter uses a priority scheme and handshaking signals to manage bus arbitration.
Common AHB arbitration policies include:
- Fixed priority – Masters have predefined priority levels
- Rotating priority – Masters take turns in round-robin fashion
- Fairness scheme – Equal share of bandwidth to masters
The Cortex-M0 AHB interface uses HBUSREQ/HGRANT signals to request and grant bus access from the arbiter. The arbiter usespolicies like fixed priority to ensure critical masters like the core get frequent access while also allowing DMA and peripherals adequate access.
Priority arbitration prevents bandwidth hogging and enforces determinism in the system. Latency-critical masters can be prioritized by the arbiter.
AHB Flow Control
Flow control signals like HREADY allow slaves to throttle data transfers. A slave can hold HREADY low to force wait states until it is ready for the transfer.
For example, an SPI peripheral may use wait states while receiving incoming serial data before signaling the transfer to complete. Slow memories may insert wait states to meet access timing requirements.
The Cortex-M0 bus interface handles flow control automatically. Software just needs to check for errors if transactions take longer than expected. Proper peripheral and interface configuration prevents excessive wait states.
AHB Multi-layer Bus
Complex Cortex-M0 designs may use multiple AHB buses in different clock/power domains. Bridges allow transactions to cross between these buses while handling the clock/power domain changes.
For example, many chips have:
- Core bus – for core, memory, critical peripherals
- Peripheral bus – for lower bandwidth peripherals
- External bus – for off-chip memory and devices
Multi-layer buses improve performance isolation and power management. The Cortex-M0 interface uses standard AHB signals/protocol across the buses. Interconnect components handle protocol conversion and clock crossing.
Cortex-M0 AHB Example
Here is an example Cortex-M0 system using the AHB to connect the core to peripherals and memories:
- Cortex-M0 core – Master
- System control block – AHB decoder, bus control
- SRAM – Internal memory
- Timer, GPIO – On-chip peripherals
- APB Bridge – To APB peripherals
- AHB-Lite Bridge – To external SDRAM
The Cortex-M0 core uses the AHB bus as the main interface to the system. The core fetches instructions from SRAM and accesses peripherals like timer and GPIO using single/burst AHB transactions.
Bridges allow connecting to secondary buses. An APB bridge attaches lower bandwidth peripherals. AHB-Lite bridge connects to external SDRAM. Bridges handle protocol conversion and interconnect details.
The AHB interfaces, arbiters, and bridges allow the Cortex-M0 to access the diverse peripherals and memories in the system efficiently.
