ST AN2326 Application note

AN2326

Application note

Calibrating the RC oscillator of the

ST7ULTRALITE MCU using the mains

Introduction

The ST7ULTRALITE microcontroller contains an internal RC oscillator which can be trimmed to a specific frequency with the required accuracy. The oscillator frequency has to be calibrated by software using the RCCR register (RC Control Register) and the SICSR register (System Integrity Control/Status Register). The value entered in the RCCR/SICSR registers will switch ON a corresponding number of resistors that will modify the oscillator frequency. Whenever the ST7ULTRALITE microcontroller is reset, the 10-bit value

contained in the RCCR/SICSR registers is restored to its default value (3FFh) i.e. the lower

volatile memory and load it in the RCCR/SICSR registers afterProduct(s)each reset. However, if any of the external conditions (temperature or voltage, for instance) changes too drastically, the

possible frequency, so each time the device is reset, you have to load the calibration value in

the RCCR/SICSR registers. There are predefined calibration values stored in memory (refer

to the” Internal RC Oscillator Adjustment” section in the ST7ULTRALITE datasheet). You

can load one of these values in the RCCR/SICSR registers if one of the operating conditions

matches that in your application. Otherwise, you can define your own value, store it in non

stored value may no longer produce the required accuracy. One solution is to recalculate the RCCR/SICSR register values after each reset, bas d on an external reference.

The purpose of this application note is to pre ent a software solution using the frequency of the European standard mains (220V/50Hz) as a timebase to adjust the internal RC oscillator

of the ST7ULTRALITE to 8 MHz. The same approach can also be used for the US mains
standard (110V/60Hz).	Obsolete
	-

The basicProduct(s)software takes le than 200 ms to calibrate the oscillator and uses less than128 bytes of program memory and five bytes of RAM for its simplest version. These RAM bytes

can be freed for other purposes when the calibration is done. Another example using averages is given in this application note. This can be useful with noisy mains.

This applicati n note also contains the diagram of a low cost circuit which converts the

mains into a 5 volt power supply and protects the microcontroller from overcurrent on the Obsoleteinput connected to the mains.

April 2006

Rev 1

1/20

www.st.com

Contents	AN2326

Contents

1	Calibration software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		3
	1.1	Software principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.2	Basic version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.3	Average version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5

	2	Power supply and timebase delivery circuit . .			. . . . . . . . . . . . . . . . . . .	. 7
		2.1	Basic circuit . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . . . . . . . . . . . . . . . . . .	7
		2.2	Hardware protection . . . . . . . . . . . . . . . . . . . . .		. . . . . . . . . . . . . . . . . . . . .	8
	3	Conclusion .		. . . . . . . . . . . . . . . . . . . . . . . . . . . . .	Product(s). . . . . . . . . . . . . . . . . . .	9
	4	Software examples				10
					. . . . . . . . . . . . . . . . . .
		4.1	Single alternance . . . . . . . . . . . . . . . . . . . . . . .		. . . . . . . . . . . . . . . . . . . .	10
			4.1.1	Main program . . . . . . . . . . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . . . . . .	10
			4.1.2	Input capture interrupt . . . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . . . . . .	11
			4.1.3	Obsolete		12
				Timebase interrupt . . . . . . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . . . . . .
			4.1.4 Writing in non volatile memory for products without Data EEPROM . . .			12
			4.1.5	Writing in non volatile memory for products with Data EEPROM . . . . . 13
			4.1.6 Detailed basic version software flowchart .		. . . . . . . . . . . . . . . . . . . . . .	14
		4.2	Average version . . . . -. . . . . . . . . . . . . . . . . . . .		. . . . . . . . . . . . . . . . . . . .	15
			Product(s)			15
			4.2.1	Main program . . . . . . . . . . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . . . . . .
			4.2.2	Input capture interrupt . . . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . . . . . .	16
			4.2.3	Timebase interrupt . . . . . . . . . . . . . . . . . . .	. . . . . . . . . . . . . . . . . . . . . .	17
	5	Revision history . . . . . . . . . . . . . . . . . . . . . . . . .			. . . . . . . . . . . . . . . . . .	19
	Obsolete

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AN2326	Calibration software

1 Calibration software

1.1Software principle

The software algorithm, described in the following flowchart (see Figure 3.), uses the mains frequency as a timebase. This timebase allows the microcontroller to test if the RC oscillator frequency is above or below the targeted frequency and repeatedly transforms it by dichotomous analysis so that in 10 iterations the RCCR/SICSR registers are set to the optimum value. In order to simplify the calculation, the calibration is done at 1MHz, the 8MHz internal RC is divided by 8 thanks to the AVTCHR register.

As the timer speed depends on the RC oscillator frequency, it is easy to determine if the oscillator is too fast or too slow. The counted value can be obtained by the following equation:

	fcpu
countedvalue = ----------------------------
32	× fmains

Since the frequency of the counter is the frequency of the oscillator divided by 32 (only the 8 MSbits are saved on the 13 bits of the counter), if the oscillator is at 1 MHz, the result of the count between two edges (which have a 10 ms interval), is 138h for the European standard (220V/50Hz). For the US standard (110V/60Hz) the right value is 104h. Since the goal of the

software is to set the RC oscillator frequency to 1 MHz it means obtaining 138h as the result
of the count. So if the result of the count is greater than 138h, it means that the frequency is
too high so the program increases the value of RCCR/SICSR registers in order to decrease
							Product(s)
the RC oscillator frequency. And if the result is less than 138h, the RCCR/SICSR register
value is decreased in order to increase the RC oscillator frequency.
Figure 1. Dichotomous analysis of RCCR value
		increase oscillator				decrease oscillator
			frequency				frequency
RCCR Register		Obsolete
		-
0h					80h				FFh
			Start Value

The RCCRProduct(s)register is set to 80h initially by the program, then the dichotomization starts by adding or subtracting 40h and after each iteration the result is divided by two, so that after

10 iterations the value of RCCR is set with an accuracy of one bit. Obsolete

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Calibration software	AN2326

Figure 2. Using the timer input capture to measure the mains frequency

	Mains
Overflow
F9h
	Free-running
	Counter
0h
Capture 1	Capture 2

To measure the frequency, the software uses the Lite Timer input capture (LTIC) so that on each edge of the mains the value of the free running counter is stored as shown in Figure 2.. Then the microcontroller calculates the elapsed time between the two edges of the mains. This time is given by the following equation:

	time	Product(s)
		= nbover × F9h + capture2 – capture1
	where nbover represents the number of counter overflows during the measurement, capture
	1 and capture 2 are the values captured on the free running counter when an edge occurs
	on the mains and F9h is the overflow value of the free running counter.
	If the RC oscillator frequency is equal to 1 MHz, the result time will be 138h for European
		Obsolete
	standard (220V/50Hz) or 104h for US standard mains (110V/60Hz), so these are the
	reference values.
	This measurement result is compared to the reference value and, depending on the result of
	the comparison, the microcontroller adds to or subtracts from the current RCCR/SICSR
	register values.	-
1.2	Product(s)
	Basic version
	In this version the measurement is done only once for each dichotomization step. This
	allows the calibration software to be light and fast. It requires only 128 bytes of program
	memory and 5 bytes of RAM during calibration. The calibration takes less than 200 ms to be
	completed.
Obsolete
	The software works as shown in the following flowchart. The assembly code and a more

detailed flowchart can be found in Section 4.

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AN2326

Calibration software

Figure 3.

Basic software flowchart

Initialization of Lite Timer

Measurement and

calculation

smaller

Compare

result with

reference

greater or equal

Decrease RCCR

Increase RCCR

no

Product(s)

Dichotomization

finished?

yes

Clock is set to 1 MHz

1.3Average version

This version uses the method described inObsoleteSection 1.1 except it performs four measurements and uses their average for each dichotomization step. It is useful when the mains is noisy. For instance, when a

motor starts it generates a tension pick-and this can be considered as a mains edge.

pleted.

This versionProduct(s)is safer than the basic one but it requires more resources. It uses 136 bytes of program memory and 11 bytes of RAM during calibration. The calibration takes less than 800 ms to be com-

The average version works as shown in the following flowchart. The assembly code can be found in Section 4.

Obsolete

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Calibration software	AN2326

Figure 4. Average software flowchart

Initialization of Lite Timer

4 measurements and calculation

Average

	smaller	Compare
		result with
		reference

						finished?			Product(s)
									greater or equal
			Decrease RCCR					Increase RCCR
			no
					Dichotomization
			Obsolete
							yes
				Clock is set to 1 MHz
-
	Product(s)
Obsolete

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