Controller Decision Matrix

On this page all information for possible controllers for the inverter are collected. After listing all the necessary functions a rating for the best suited controller will be done.

For a first rough estimation of the needed computing power some considerations:
If we want to achieve a control loop frequency of 16 kHz and we use a controller with 100 MHz core clock we have 6250 clock cycles to gather all needed (analog) input values, do the calculations and set the outputs. Besides that there may be some serial communication that also needs to be addressed.
The choice of controller clearly sets the limits how fast and how precise the inverter will be able to operate.
It may be possible to use a less performant controller (and save costs) when the first implementation indicates that there is plenty of time to perform all necessary operations.
So a controller family that covers a bigger performance spectrum gets a better rating than a controller that's only available in a single configuration.

Because of the preferred support from FreeRTOS (see [#1]) all listed controllers below must be covered in the list at the FreeRTOS ports list. This also excludes 8-bit controllers.

The ADC resolution must be at least 12 bit. For the AC current the resolution should be as fine as possible to prevent any DC current flowing into the grid. For 10 A max. AC current and 12 bit resolution this would provide a resolution of (20 A_pp)/(2^12)=~5 mA. To allow some headroom the scale will be set higher than 10 A max. With 12 bit this still allows a resolution better than 10 mA. More than 12 bit resolution would be overkill due to the noisy environment inside an inverter and also due to the high dynamic of most of the signals.

The STR7 and STR9 controller families from ST are no fit due to insufficient ADC features.

A list of controllers was assembled from the supplier pages and then limited with prices from digikey. All controllers more expensive than 5 EUR were regarded out of limit for this project.

The whole list can be found in the files section controller-overview.ods.

Following the list of controllers that were left with the applied limits regarding to the requirements:

Manufacturer	Device Name	CPU	Frequency (MHz)	RAM(KB)	Flash(KB)	PWM(Ch)	DMA(Ch)	ADC Channels	ADC Resolution (Bit)	I2C (TWI)	UART(SCI)	SPI	Timers 16 bit	Timers 32 bit	PWM Channels	GPIO	IO Supply(V)	Package Group	Pin Count	Digikey Price (€)
ST	STM32F030K6	ARM Cortex-M0	48	4	32	6	5	10	12	1	1	1	4	-		27	2.4 - 3.6	LQFP 32 7x7x1.4	32	1.53
ST	STM32F030C6	ARM Cortex-M0	48	4	32	6	5	10	12	1	1	1	4	-		39	2.4 - 3.6	LQFP 48 7x7x1.4	48	1.75
ST	STM32F030C8	ARM Cortex-M0	48	8	64	6	5	10	12	2	2	2	6	-		39	2.4 - 3.6	LQFP 48 7x7x1.4	48	2.10
ST	STM32F030R8	ARM Cortex-M0	48	8	64	6	5	16	12	2	2	2	6	-		55	2.4 - 3.6	LQFP 64 10x10x1.4	64	2.19
ST	STM32F100C4	ARM Cortex-M3	24	4	16	6	7	10	12	1	2	1	6	-		37	2 - 3.6	LQFP 48 7x7x1.4	48	3.14
ST	STM32F070RB	ARM Cortex-M0	48	16	128	6	5	16	12				8	-		51	2.4 - 3.6	LQFP 64 10x10x1.4	64	3.51
Freescale	MKV30F64VLH10	ARM Cortex-M4	100	16	64	10	4	29	16	1	4	1	11	4		46	1.71 to 3.6	QFP 64	64	3.81
Freescale	MKV30F128VLH10	ARM Cortex-M4	100	16	128	10	4	29	16	1	4	1	11	4		46	1.71 to 3.6	QFP 64	64	4.36
Freescale	MKV31F128VLH10	ARM Cortex-M4	100	24	128	12	4	29	16	2	4	2	13	4		46	1.71 to 3.6	QFP 64	64	4.77
NXP	LPC11U67	ARM Cortex-M0+	50	16	128	16		8	12	2	3	2			11	34	2.4-3.6	LQFP48	48	4.78

According to that list two controller families need to be investigated: from ST the STM32F030 family and from Freescale the MKV30F family.

The Freescale controllers offer more RAM and a higher core frequency which may provide more flexibility in the system design.
On the other hand the ST controllers are about half the price and offer smaller packages which may save board space and reduce the whole system costs. But due to the lower CPU performance they may limit the maximum control loop frequency, which then can lead to higher system costs.

Both controller families are supported by FreeRTOS with GCC. The Freescale controllers are according to the side also supported by Eclipse where the ST controllers are only supported by non-free tools.

So for now the design will be aimed to run on a Freescale MKV30F64 controller.