The document discusses energy optimization techniques for ARM® Cortex-M processors, focusing on the role of the instruction cache (i-cache) in enhancing energy efficiency. It highlights the importance of i-cache in reducing current consumption by minimizing costly read routines from external memory, particularly for IoT and wearable devices. Results from energy profiling illustrate significant differences in power consumption when the i-cache is enabled versus disabled.

1. ARM® Cortex™ M Energy Optimization
Techniques - Using Instruction Cache
Raahul Anand Raghavan, Lead Systems Architect,
Glyton Solutions
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2. Agenda
• Before We Proceed!
• Importance of energy optimization with Cortex M profile in
context
• Instruction cache in ARM® Cortex™ M
• Energy Profiling & Results comparison
• Turn ON I-CACHE
• Turn OFF I-CACHE
• Concludes
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3. Before We Proceed!
• In this presentation, we will be investigating role of
I-CACHE when it comes to energy efficiency optimization
• Involves Cortex M0+ based EVM and a suitable IDE
• Official Documentation from ARM®, Silicon Labs will
override information provided here. Treat official TRM’s as
complete guides on Subject Matter
• ARM® Cortex® M trademark - ARM Ltd
• EFM®32, Silicon Labs SDK etc are registered trademarks
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4. ARM® Cortex™ M Importance of Energy Optimization (1 / 2)
• Typical products - Cortex M usage scenarios
• Perpetually powered sensor products for industrial control
• Energy harvesting is one key technology used to charge the battery pack when it comes to
perpetually powered low power consumption sensor products
• Products powering IOT (Internet of Things)
• Wearable products catering to
• Fitness trackers
• Elderly / Patient assistance
• ….
• Whats one critical factor judging better end user / enterprise experience across all the products
above ?
• ALWAYS ON functionality
• How do we achieve ALWAYS ON functionality ?
• Best in class energy efficiency
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5. ARM® Cortex™ M Importance of Energy Optimization (2 / 2)
• What would be one of the best methods for extreme energy efficiency ?
• Taking advantage of Architectural features
• We will be discussing how one such architectural feature in ARM® Cortex™ M micro-controllers
• I CACHE - Instruction Cache
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ARM® Cortex™
Core
Cache Look-Up
Logic
SRAM CACHE
Implementation
IDCOD MUX
CODE
Memory Space
(NV-FLASH)
ICODE!
AHB-LITE BUS
ICODE!
AHB-LITE BUS
Instruction Cache
DCODE!
AHB-LITE BUS
IDCODE!
AHB-LITE BUS

6. ICACHE Implementation - EFM32 MCU’s from Silicon Labs
• ICACHE
• Connected directly to Cortex M Core
• Acts as memory access filter between processor core and memory system
• Consists
• Access filter
• Look-up logic
• 128x32 SRAM (512 Bytes)
• How does it work ?
• Access filter checks if instruction access address falls in code space
• On match , cache look-up and SRAM is enabled and instruction fetch happens
• If no match , then ICACHE is bypassed and access is forwarded to memory
system , then to NV memory such as NAND flash
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7. How does ICACHE Impact energy efficiency ?
• In the absence of ICACHE
• Every instruction in CODE section, gets fetched from
external FLASH
• This involves read routines are always costly as it
involves external peripheral access
• The read routines to fetch instructions from external NV
memory such as NAND is always costly (in terms of
current consumption) compared to on-chip components
• Another factor is that , there could be a risk of involving
multiple read cycles for fetching instructions from
external flash
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8. Energy Profiling Results (1 / 2)
• Following data might give an idea with respect to what
would be the impact of having ICACHE turned ON and OFF
• ICACHE Turned ON / OFF in EFM32 Zero Gecko
• API used — MSC_EnableCache(bool );
• Does required configuration to turn ON or OFF ICACHE in
memory system controller
• Simple application measuring ambient temperature used
• Simplicity Studio - Energy Profiler from Silicon Labs
• On Board J-LINK debugger
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9. Energy Profiling Results (2 / 2)
Average Current Consumption — 60.45μA
Average Power — 200.78μA
Total Energy — 36.50mJ
Average Current Consumption — 73.03μA
Average Power — 242.48μA
Total Energy — 44.28mJ
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ICACHE ON
ICACHE OFF

10. Concludes!
• Lets ensure to
• Take advantage of any architectural functionality when
it comes to achieving high energy efficiency
• Take advantage of any ARM® Cortex™ M, silicon
implementer specific functionality when it comes to
achieving high energy efficiency
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