ST AN2326 Application note

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AN2326

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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 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 volatile memory and load it in the RCCR/SICSR registers after each reset. However, if any of the external conditions (temperature or voltage, for instance) changes too drastically, the stored value may no longer produce the required accuracy. One solution is to recalculate the RCCR/SICSR register values after each reset, based on an external reference.

The purpose of this application note is to present 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).

The basic software takes less 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 application 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 input connected to the mains.

April 2006 Rev 1 1/20

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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 Timebase interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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

4.2.1 Main program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.2.2 Input capture interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.2.3 Timebase interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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AN2326 Calibration software

1 Calibration software

1.1 Software 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:

cpu

countedvalue

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 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.

----------------------------=

32 f

mains

Figure 1. Dichotomous analysis of RCCR value

increase oscillator

frequency

RCCR Register

The RCCR 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.

decrease oscillator

frequency

80h

Start Value

FFh0h

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Calibration software AN2326

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

Mains

F9h

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:

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 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.

Overflow

Free-running Counter

Capture 1 Capture 2

time nbover F9h capture2 capture1–+×=

1.2 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.

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

1.3 Average version

This version uses the method described in Section 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.

This version 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 completed.

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

smaller

Dichotomization

finished?

yes

Clock is set to 1 MHz

Compare result with

reference

greater or equal

Increase RCCRDecrease RCCR

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Calibration software AN2326

Figure 4. Average software flowchart

Initialization of Lite Timer

4 measurements and

calculation

Average

smaller

Decrease RCCR

Clock is set to 1 MHz

Dichotomization

finished?

yes

Compare result with

reference

greater or equal

Increase RCCR

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AN2326 Power supply and timebase delivery circuit

2 Power supply and timebase delivery circuit

The following figures show circuits which will provide 5V DC to the ST7ULTRALITE and protect the input capture from overcurrent. If no power supply is needed, the only component to keep is the resistor on the LTIC input, which is mainly to protect from overcurrent.

2.1 Basic circuit

This circuit contains a capacitive power supply which converts the 220V/50Hz of the mains, as well as the 110V/60Hz of the US mains, into 5V DC.

Warning: be aware that this kind of power supply cannot be used if there are

big current variations.

It also inputs 220V/50Hz to the Lite Timer Input Capture pin (LTIC/PA2) protected by resistor R2.

The incoming alternating signal on the LTIC input pin is 220V/50Hz. Because of the clamping diode on the input of the ST7ULTRALITE, the input signal can be considered as a 0-5V square signal.

Figure 5. Power supply and timebase delivery circuit diagram

220µF/16V

ST7ULTRALITE

GND

LTIC

Mains

(220V/50Hz)

R1 47R/0.5W

zener

5.6V

1N4148

220nF/400V

R2 470k/0.5W

The maximum current available in the microcontroller depends on the C2 value. Table 1 gives the maximum average current versus the capacitor value. The average current follows the equation below:

Imax Vmax 2 f C⋅⋅⋅=

In the case above, C2 is equal to 220nF so the available current is limited to 4.9 mA in the case of a European mains. To have the same current levels in the case of the US mains

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Power supply and timebase delivery circuit AN2326

(110V/60Hz), C2 must be multiplied by two. A 440nF capacitor will limit the current to 4.9 mA.

For the US standard, R2 must be divided by two in order not to limit the current too much on the LTIC input. A 220kΩ resistor is enough in this case.

Table 1. Maximum MCU current

Capacitor C2 Maximum Current

220nF 4.9mA 330nF 7.3mA 470nF 10.4mA 680nF 15mA

1µF 22.1mA

2.2 Hardware protection

To prevent bad measurements due to noisy mains, a filter can be added between the mains and the input capture of the ST7ULTRALITE. The following figure shows one example of a filter. This filter is a pass band centered on the mains frequency in order to reject all frequency which could be understood by the microcontroller as a mains edge.

Please note that this is just a second order filter and that this may not be enough if the mains is really noisy. Any kind of filter can be added on the LTIC.

Figure 6. Band pass filter

mains

470nF/400V

The pass band filter above must be tuned to mains frequency. The value of the resistors for this filter is given in the table below.

Table 2. Resistor values

resistors 50Hz/220V 60Hz/110V

R1 6.8K/0.5W 5.6K/0.5W R2 6.8K/0.5W 5.6K/0.5W R3 470K/0.5W 220K/0.5W

R1 R2

C2 470nF/400V 470nF/250V

LTIC

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AN2326 Conclusion

3 Conclusion

This system allows you to have a power supply for the microcontroller and an auto adjustable clock set to 1, 2, 4 or 8MHz, selected by prescaler, with the required accuracy whatever the external conditions.

This solution also offers the advantage of being less expensive than a solution with a transformer and requires less space.

It requires a small amount of space in program memory (less than 128 bytes) in its smallest version.

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Software examples AN2326

4 Software examples

A zip file attached to this application note contains the complete software of this calibration method.

4.1 Single alternance

This version performs only one count between two edges and changes the value of the RCCR/SICSR registers according to this measurement. This can lead to bad tuning if there is noise on the reference signal.

4.1.1 Main program

;All the bytes from locations 80h to 85h are used by this software to store values or as control registers but they can be reused safely after the clock has been set.

;dichotomy value .value equ $81 ;this byte contains the value which will be added or subtracted to/from the RCCR last value at the end of each round ;capture values .capture1 equ $82 .capture2 equ $83 ;these two bytes contain the two values of the counter captured on the edge of the mains, they are used to calculate the time elapsed between the two edges ;number of overflows .nbover equ $84 ;this byte contains the number of counter overflows during the measurement ;control register .cr equ $85 ;this byte is used as a control register for the measurement. Its bits allow or not the interrupts and show which step of the count is the current one. .strtstp equ 1 ;this is set to start the count and reset to stop it .lsb_RCCR equ 7 ;this bit is set when the first capture has occurred. It allows the overflows to be counted ;address to program in NVM (optional) .E2ADDR equ $86 .RAM equ $88

.main

ld A, #$63 ld AVDTHCR, A ; set internal clock to fRC/8 meaning 1MHz targeted

clr SICSR ;

ld A, #$80 ;value containing the value which will be ld value, A ;add or subs to/from RCCR during the dichotomy ld RCCR, A ;RCCR is set to the middle of its range of value

clr cr ;clear the byte use as control register for the count

next clr nbover ;clear the byte containing the number of timer overflow ld A, LTICR ;clear the ICF bit

count btjt cr, #strtstp, count; wait for the end of count

rim ;interrupts enable bset LTCSR, bset cr,#strtstp ;set the start-stop bit of cr: count can start

clr LTCSR ;lite timer interrupts disable

srl value ;dichotomy value divided by 2

ld A,#$F9 ;these lines calculate this equation: ld X,nbover ; mul X,A ;(nbover*$F9)+ capture2 - capture1 add A, capture2 ; jrnc nocarry ;this equation is calculated with 16 bits inc X ;

clear the 2 lowest significant bits located in this register

#7 ;enable input capture interrupt

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AN2326 Software examples

nocarry sub A, capture1 ;MSB are in register X

noneg cp X, #$01 ;if mains frequency is 50Hz the reference value is $138

compare cp A, #$38 ;value is smaller than the reference the program jump to

plus

plus_lsb btjt value, #5, p_lsb0 btjt value, #4, end ld A, value ld SICSR, A ;add value if counted value is greater than ref jra next p_lsb0

minus btjt cr,#lsb_RCCR,minus_lsb ld A, RCCR

jra new minus_lsb btjt value,#5, m_lsb0 btjt value, #4, clr_lsb0 bres RCCR,#0 ;reset the LSb of RCCRH ld A, value ld SICSR, A ;substract value if counted value is smaller tha n ref jra next m_lsb0

clr_lsb0

new

bset cr, #lsb_RCCR ld A, #$80 ld value, A jra next

.end_calibration ; end of the calibration ; at this point the Fcpu is set to 1MHz ; the last operation is to set the clock to the targeted value by setting the prescaler register

jrnc noneg ; dec X ;and LSB in register A.

jrmi minus ;if it is 60Hz the reference is $104.the program first jreq compare ;compares MSB with $01 and then compare LSB with jp plus ;$38 for 50Hz and $04 for 60Hz. if the calculated

jrmi minus ;minus to decrease RCCR else it increase RCCR

btjt cr,#lsb_RCCR,plus_lsb ld A, RCCR add A, value ;add value if counted value is greater than ref jra new

bset SICSR,#5 ;add value if counted value is greater than ref jra next

sub A, value ;subtract value if Y

bres SICSR,#6 bset SICSR,#5 ;subtract value if Y is smaller jra next

bres SICSR,#5 ;reset the LSb of RCCRL jra end

ld RCCR,A ;enter the new value in RCCR btjf value, #0,

ld A, #$03 ; set clock to fRC meaning 8MHz ; ld A, #$23 ; set prescaler to fRC/2 ; ld A, #$43 ; set prescaler ; ld A, #$63 ; set prescaler to fRC/8 ld AVDTHCR,A ; in the three cases AVD is set off

next;stop after 7 rounds

is smaller

to fRC/4

4.1.2 Input capture interrupt

ld A, LTICR ;load captured value in A btjt LTCSR, #4, finish ;test if it is first or second capture

overflows

finish ld capture2, A ;if it is the second capture, captured value is stored in capture2

bset LTCSR, #4 ;allow timebase interrupt in order to count the number of

ld capture1, A ;captured value is stored in capture1 jp endit1

clr cr ;clear cr to end the count

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Software examples AN2326

endit1

iret

4.1.3 Timebase interrupt

ld A, LTCSR ;clear TB bit

endit2

inc nbover ;increment number of overflows

iret

4.1.4 Writing in non volatile memory for products without Data EEPROM

To store the final value of RCCR/SICSR registers in non volatile memory, the following lines must be added after the timer interrupts have been disabled in the main program. As Data EEPROM is not embedded in the ST7ULTRALITE, the storage is done in program memory, so the sector 0 size must be

0.5k (selected by option byte). The programmation routine must be executed from RAM, so it is first loaded in RAM and then called. Both RCCR and SICSR registers are saved.

; Content of RCCR and SICSR is saved at FC00h and FC01h

; < RESET_FCSR >

LD A,#$56 ; Enter RASS keys to unlock FCSR register LD FCSR,A LD A,#$AE LD FCSR,A

; < LOAD_RAM >

LD X,#$3F ; Copy programming software driver 32 bytes = 4 lines .RAM_Copy LD A,(RAM_Driver,X) LD (RAM,X),A DEC X JRPL RAM_Copy

; < USER_APPLICATION_PROGRAM >

; < FIRST_PROG >

LD A,#$FC ; High address LD X,A ; clr Y ; define FC00 as destination address

LD {E2ADDR},X ; Address high to be programmed (0081h) is in X LD {E2ADDR+1},Y ; Address low to be programmed (0082h) is in Y

CALL RAM ; Call the programming driver located into RAM

JP USER_APPLICATION ; ----------------------------------------------------------------------------; ROUTINE: XemulE2_ByteProg ; DESCRIPTION: Emulated data EEPROM byte ; BEFORE: A = data to be programmmed ; X:Y = address where it has to be programmed [FC00h..FDFFh] ; -----------------------------------------------------------------------------

; into RAM from address 0083h

programming driver routine

.RAM_Driver BSET FCSR,#LAT ; Enable Emul. EEPROM latches LD A,FCSR CLR X

LD A, RCCR LD ([E2ADDR.w],X),A; Set address/data to be programmed inc X

A, SICSR

LD LD ([E2ADDR.w],X),A; Set address/data to be programmed

BSET FCSR,#PGM ; Launch the Emul. EEPROM programming

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AN2326 Software examples

.EEPROM_Prog BTJT FCSR,#PGM,EEPROM_Prog; Wait end of programming (~5ms) RET

4.1.5 Writing in non volatile memory for products with Data EEPROM

To store final value of RCCR/SICSR registers in EEPROM, add theses lines after disabling the timer interrupts in the main program.

ld RCCR, A bset EECSR,#1 ;start to enter value in the EEPROM ld $1003,A ;load value of the RCCR in EEPROM ld A, SICSR ld $1004,A ;load value of the SICSR in EEPROM

wait btjt EECSR,#0,wait ;wait for the end of writing in EEPROM

bset EECSR,#0 ;start to write in the EEPROM

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Software examples AN2326

4.1.6 Detailed basic version software flowchart

STORE STARTING VALUE

80H --> VALUE 80H --> RCCR

CLEAR RAM BYTES

ENABLE INPUT CAPTURE INTERRUPT

(NBOVER, CR)

EDGE

ON LTIC

INPUT

YES

STORE CAPTURED VALUE IN CAPTURE 1

ENABLE TIMEBASE INTERRUPT TO

START TO COUNT TIMER OVERFLOWS

EDGE

EDGE ON LTIC ON LTIC

INPUT

STORE CAPTURED VALUE IN CAPTURE 2 DISABLE TIMEBASE AND INPUT CAPTURE

NBOVER X F9 + CAPTURE 2 - CAPTURE 1

GREATER

SUBTRACT VALUE TO RCCRADD VALUE TO RCCR

YES

INTERRUPTS

DIVIDE VALUE BY 2

CALCULATION OF:

COMPARE

RESULT WITH

138H

SMALLER

RC OSCILLATOR IS TRIMMED TO 1 MHZ

IS THE

DICHOTOMY

FINISHED?

YES

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AN2326 Software examples

4.2 Average version

This version performs the count between two edges four times and changes the value of the RCCR/SICSR registers according to the average of these measurements. This method allows to perform a better tune of the RC oscillator especially in noisy environment.

4.2.1 Main program

;All the bytes from locations 80h to 8Bh are used by this software to store values or as control registers but they can be reused safely after the clock has been set.

;dichotomy value .value equ $81 ;this byte contains the value which will be added or subtracted to/from the RCCR last value at the end of each round ;capture values .capture1 equ $82 .capture2 equ $86 ;these bytes contain the values of the counter captured on the edge of the mains, they are used to calculate the time elapsed between the two edges ;number of overflows .nbover equ $8A ;this byte contains the number of counter overflows during the measurement ;control .cr equ $8B ;this byte is used as a control register for the measurement. Its bits allow or not the interrupts and show which step of the count is the current one. .strtstp equ 1 ;this is set to start the count and reset to stop it .lsb_RCCR equ 7 ;this bit is set when the first capture has occurred. It allows the overflows to be counted

;address to program in NVM (optional) .E2ADDR equ $86 .RAM equ $88

.main

next clr nbover ;clear the byte containing the number of timer overflow ld A, LTICR ;clear the ICF bit

capture rim ;interrupts enable

count

calcul add A, (capture2,Y) ;

nocarry sub A, (capture1,Y) ;MSB are in register X

noneg inc Y

ld A, #$63 ld AVDTHCR, A ; set internal clock to fRC/8 meaning 1MHz targeted

ld A, #$80 ;value containing the value which will ld value, A ;add or subs to/from RCCR during the dichotomy ld RCCR, A ;RCCR is set to the middle of its range of value

clr cr ;clear the byte use as control register for the count

clr Y

bset LTCSR, #7 ;enable input capture interrupt bset cr,#strtstp ;set the start-stop bit of cr: count can start

btjt cr, #strtstp, count; wait for the end of count

clr LTCSR ;lite timer interrupts disable inc Y cp Y,#4 ;repeat the capture four time to make an average jrne capture

srl value ;dichotomy value divided by 2

clr Y ld A,#$F9 ;these lines calculate this equation for the four ld X,nbover;measures: mul X,A ;(nbover*$F9)+ capture2 - capture1

jrnc nocarry ;this equation is calculated with 16 bits inc X ;

jrnc noneg ; dec X ;and LSB in register A.

cp Y,#4

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Software examples AN2326

jrne calcul

srl A ;these lines calculate the average of the last four srl X ;measures by dividing their total by 4. It is done by jrnc carry1 ;two consecutive right shift on the 16 bit result.

carry1 srl A

carry2 cp X, #$01 ;if mains frequency is 50Hz the reference value is $138

compare cp A, #$38 ;value is smaller than the reference the program jump to

plus

plus_lsb btjt value, #5, p_lsb0 btjt value, #4, end ld A, value ld SICSR, A ;add value if counted value is greater than ref jra next p_lsb0

minus btjt cr,#lsb_RCCR,minus_lsb ld A, RCCR

jra new minus_lsb btjt value,#5, m_lsb0 btjt value, #4, clr_lsb0 bres RCCR,#0 ;reset the LSb of RCCRH ld A, value ld SICSR, A ;substract value if counted value is smaller jra next m_lsb0

clr_lsb0

new ld RCCR,A ;enter the new value in RCCR

next_near

.end_calibration ; end of the calibration ; at this point the Fcpu is set to 1MHz ; the last operation is to set the clock to

add A,#$80

srl X jrnc carry2 add A,#$80

jrmi minus ;if it is 60Hz the reference is $104.the program first jreq compare;compares MSB with $01 and then compare LSB with jp plus ;$38 for 50Hz and $04 for 60Hz. if the calculated

jrmi minus ;minus to decrease RCCR else it increase RCCR

btjt cr,#lsb_RCCR,plus_lsb ld A, RCCR add A, value ;add value if counted value is greater than ref jra new

bset SICSR,#5 ;add value if counted value is greater than ref jra next

sub A, value ;subtract value if Y is smaller

than ref

bres SICSR,#6 bset SICSR,#5 ;subtract value if Y is smaller jp next

bres SICSR,#5 ;reset the LSb of RCCRL jra end

btjf value, #0, next_near;stop after 7 rounds bset cr, #lsb_RCCR ld A, #$80 ld value, A

jp next

ld A, #$03 ; set clock to fRC meaning 8MHz ; ld A, #$23 ; set prescaler to fRC/2 ; ld A, #$43 ; set prescaler to fRC/4 ; ld A, #$63 ; set prescaler to fRC/8 ld AVDTHCR,A ; in the three cases AVD is set off

the targeted value by setting the prescaler register

4.2.2 Input capture interrupt

ld A, LTICR ;load captured value in A btjt LTCSR, #4, finish ;test if it is first or second capture

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AN2326 Software examples

overflows

finish

capture2

endit1 iret

bset LTCSR, #4 ;allow timebase interrupt in order to count the number of

ld (capture1,Y), A ;captured value is stored in capture1 jp endit1

ld (capture2,Y), A ;if it is the second capture, captured value is stored in

clr cr ;clear cr to end the count

4.2.3 Timebase interrupt

ld A, LTCSR ;clear TB bit

endit2

inc nbover ;increment number of overflows

iret

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Software examples AN2326

Table 3. Detailed average version software flowchart

STORE STARTING VALUE

80H --> VALUE 80H --> RCCR

CLEAR RAM BYTES

ENABLE INPUT CAPTURE INTERRUPT

(NBOVER, CR)

EDGE

ON LTIC

INPUT

YES

STORE CAPTURED VALUE IN CAPTURE 1.X

ENABLE TIMEBASE INTERRUPT TO

START TO COUNT TIMER OVERFLOWS

EDGE

ON LTIC

INPUT

STORE CAPTURED VALUE IN CAPTURE 2.X DISABLE TIMEBASE AND INPUT CAPTURE

NBOVER X F9 + 4(CAPTURE2.X - CAPTURE1.X)

RESULT DIVIDED BY 4 TO AVERAGE IT

EQUAL OR GREATER

YES

INTERRUPTS

FOUR

MEASUREMENTS

DONE?

CALCULATION OF:

DIVIDE VALUE BY 2

COMPARE

RESULT WITH

138H

YES

ADD VALUE TO RCCR

SMALLER

SUBTRACT VALUE TO RCCR

RC OSCILLATOR IS TRIMMED TO 1 MHZ

IS THE

DICHOTOMY

FINISHED?

YES

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AN2326 Revision history

5 Revision history

Table 4. Document revision history

Date Revision Changes

03-April-2006 1 Initial release.

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Revision history AN2326

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ST AN2326 Application note

Specifications and Main Features

Frequently Asked Questions

User Manual

1 Calibration software

1.1 Software principle

1.2 Basic version

1.3 Average version

2 Power supply and timebase delivery circuit

2.1 Basic circuit

2.2 Hardware protection

3 Conclusion

4 Software examples

4.1 Single alternance

4.1.1 Main program

4.1.2 Input capture interrupt

4.1.3 Timebase interrupt

4.1.4 Writing in non volatile memory for products without Data EEPROM

4.1.5 Writing in non volatile memory for products with Data EEPROM

4.1.6 Detailed basic version software flowchart

4.2 Average version

4.2.1 Main program

4.2.2 Input capture interrupt

4.2.3 Timebase interrupt

5 Revision history