Author:
Graeme Clark, Principal Engineer, Renesas Electronics
Date
10/28/2021
PDF
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Figure 1: Power consumption model
The spectacular growth of the IoT is driving the need to develop new, battery powered sensing devices, that are smarter, faster and can last for longer on smaller, cheaper batteries. To satisfy the demand for higher performance, we must steal time from many of the other system components, reallocating the time - and the energy required - to the key system components.
Renesas has developed an ultra-efficient microcontroller power management architecture, based on its unique Silicon on Thin Buried Oxide (SOTB) process technology.
There are two main ways a microcontroller consumes power.
· Dynamic power consumption
· Static power consumption
Reducing dynamic power is key to reducing power, we must reduce the clock frequency wherever possible, and balance power consumption against higher performance.
Static power consumption is specific to the device and environmental conditions and cannot be reduced by the application.
Dynamic power consumption depends on the internal switching rate in the device. Current flows when both PMOS and NMOS turn “ON” at the same time, current flows to charge or discharge the parasitic capacitance of the wiring and the next level gate capacitance. These, along with the power supply voltage are constant, so dynamic power consumption is proportional to frequency. Reducing the frequency reduces power consumption.
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Figure 2: Dynamic power consumption current path
Static power consumption is caused by analogue features such as the internal regulator, which consumes power independently of frequency. Stopping operation or changing the mode can reduce this power consumption. Static power consumption is also created by leakage current caused by the manufacturing process itself, so we need to keep to appropriate voltage and temperatures to prevent this increasing.
SOTB process technology allows the reduction of both active and static power consumption. The digital functions of the device, such as the SRAM, CPU, and peripheral functions, are supplied by a regulated internal voltage and are implemented on SOTB transistors, which have a dopant less channel structure. This can suppress the variance of the transistor’s threshold voltage and operates with lower voltage, reducing dynamic current. A planar double gate structure enables the control of bias voltage of the back-side gate reducing leakage current in low-speed operation and standby modes.
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Figure 3: SOTB structure
The RE has a range of power supply modes to optimise the power consumption for any specific operating condition. Using these, it is possible to suppress dynamic current while operating at up to 64MHz operation and during 32.768kHz operation and standby.
The microcontroller is divided into 4 power domains, each can be isolated from the power supply. The power supply for these domains can be selected from:
· ALLPWON mode
In this mode, all functions in this MCU are available.
· EXFPWON mode
The flash power domain is shut off so current is reduced.
· MINPWON mode
Power is only supplied to functions that are required so the current of the unused functions is reduced.
Power consumption can be reduced in Operating mode (OPE), Sleep mode (SLEEP), Software Standby mode (SSTBY) and Snooze mode (SNOOZE) by optimising the power supply depending on frequency.
· BOOST mode ... Max 64 MHz
High speed operation with low power
· NORMAL mode…Max 32 MHz
Power consumption is reduced the internal regulated voltage is lowered.
Selecting the modes below enables the optimisation of the internal regulator.
· Low-leakage current mode (VBB mode) Max 32.768 kHz
Leakage current is reduced as a back-bias voltage is applied to each transistor.
Stopping the clock to the CPU or peripheral functions can further reduce the power consumption.
· SLEEP mode
Stops the CPU operation
· SNOOZE mode
Some peripheral functions can operate, CPU is stopped.
· Software standby mode (SSTBY mode)
Only the 32 kHz clock or peripheral functions using this clock can operate.
In this mode, MCU operates with minimum power and leakage current can be reduced using VBB.
· Deep software standby mode (DSTBY mode)
Power supplied to limited functions.
After reset, the MCU wakes up in NORMAL high-speed mode and can switch to BOOST mode or VBB mode under software control. Power supply modes can further reduce current consumption.
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Figure 4: Relative power consumption
The user can switch between different supply modes and shut down different domains when not required. When power is cut, that domain doesn’t consume dynamic power and the static power consumption is reduced. Figure 5 shows the functions available.
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Figure 5: Available functions in each domain
In ALLPWON mode, power is supplied to all domains and all functions can be used. The MCU can operate at 32MHz, by switching to BOOST from NORMAL, the MCU can increase speed to 64MHz. In EXFPWON mode, the flash is powered off, so code is executed from SRAM. By combining power supply control mode and frequency control settings, the MCU can operate with both high-performance and low power. In MINIPWON mode, the power is only supplied to a few key low power functions on the device. As in EXFPWON mode, code is executed from SRAM.
In Normal mode, the MCU operates at up to 32 MHz, this can be optimised by selecting the mode for the required operating frequency.
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Figure 6: Power control mode operating frequencies
VBB mode is the lowest power mode in the RE Microcontroller, supporting operation at 32.768 kHz. In this mode, we control the application of the back-bias voltage, reducing leakage current. This is useful for applications that stay in standby mode for long periods and the CPU is only occasionally active. Peripherals such as the A/D converter are still available so analogue data can be acquired while still consuming very little current.
The Low Power modes are selectable according to the functions required, selecting the optimal mode means that the current is minimised. These modes include Sleep, Snooze, Software Standby and Deep standby, each provides specific benefits in terms of low power design. Sleep mode halts CPU operation, temporarily stopping the CPU’s clock, ideal for operation when reaction time and processing speed is needed. Software Standby Mode is lower power as the CPU, Flash and some peripheral functions are disabled. Deep standby has the lowest power consumption, taking as little as 100 µA.
SNOOZE mode allows some peripherals, such as the A/D or USART, to operate without waking up the CPU. Figure 7 shows an analogue input measurement using the timer and the A/D converter in SNOOZE mode.
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Figure 7: Using SNOOZE mode to measure an analogue voltage
The RE microcontroller allows for applications to steal time and increase product life. The devices in this family can achieve power consumption as low as 12 µA/MHz in operation, and power as low as 100 µA in standby mode. These levels of power consumption combined with the high-speed performance of the RE family allow us to achieve the highest benchmark score for any current general purpose microcontroller in the EEMBC's ULPMark-CP benchmark, with a value of 705. This is achieved by using the ability of the RE to be optimised for each specific application and function, thus reducing the power consumed while maximising performance for every part of the application.
