With the newest microcontrollers, engineers could face some interesting tradeoffs. In Freescale Semiconductor's Flexis products, the same package, same peripherals and same development tools are used for either an 8- or 32-bit core. The tradeoffs for the right choice involve cost, performance and power consumption.
Running at 50 MHz on the CPU with the bus and all the peripherals operating at 25 MHz, the 32-bit unit, a ColdFire V1, dissipates 27 mA and the 8-bit S08 dissipates 11 mA. The values drop to a dissipation of 520 nA for the ColdFire and 450 nA for the S08 in a sleep mode called Stop3, where internal circuits are loosely regulated and the clocks are running at low frequency. In the lowest power dissipation mode, Stop2, the external clocks are off, the real-time clock (RTC) operates from an internal reference clock and the power dissipation is identical for both units, 670 nA.
"Intuitively, you would think that there isn't any way that a 32-bit core could dissipate less power than an 8-bit," says Joe Circello, chief ColdFire architect at Freescale Semiconductor. However, there are situations, especially in sensor monitoring, where the MCU is in a sleep mode and wakes up periodically to make a measurement that can impact the power consumption. "For the cost of 2X the dissipation I have got 10X performance, then it fundamentally alters what the power over time graph looks like," says Circello.
Three different design approaches to minimize power consumption in a periodic measurement situation provide interesting results. In Approach 1, the MCU runs at a 16 kHz frequency with the RTC providing 1-sec intervals to take the analog to digital converter (ADC) readings. In Approach 2, the crystal on the MCU provides an accurate clock during Stop3. The RTC provides a wake up every second for the ADC measurement. On every fifth measurement, the MCU ramps to an 8-MHz bus frequency and processes the data. The third approach is similar to the second except the low-power oscillator (LPO) on the MCU provides the clock during Stop2.
The calculations take into account the different conversion times for each situation and use a processing time of 2,000 cycles for the S08 and 500 cycles for the V1. Using Approach 3, the 32-bit unit consumes 0.88 µA, the lowest power level and 6 µA less than the 8-bit's consumption of 0.82 µA.
Low Power Play
The approach for making periodic measurements using either an 8- (S08) or 32-bit (V1) MCU can drastically reduce the current consumption. The 32-bit MCU actually draws less current than the 8-bit unit in the lowest power consumption mode.
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