7/12

ACCURATE TIMEBASE FOR LOW-COST ST7 APPLICATIONS WITH INTERNAL RC...

TR =Transmission rate divisor

ETPR = Extended Transmit Prescaler Division

The above equation is used normally without the ETPR selection. Due to the flexibility of

ETPR range (1 to 255), this software solution calibrates the SCI baud rate using the ETPR

register. Software keeps PR=1 and TR=1. Again the fraction is applied to the numerator of the

equation as the variation affects the fcpu only. The equation becomes,

To calibrate the baud rate, software loads the value of 9600 in Tx. This gives the value in

terms of (PR * TR * ETPR). Software loads TR=PR=1, So the value is directly assigned to the

ETPR register. This value is loaded into the PR, TR and ETPR bits. For the 9600 baud rate,

this calibration gives ~9300 baud rate (where internal RC varies (-25%)). This software is able

to communicate with the PC hyperterminal after using this calibration.

Here for the 9600 baud rate calculation, ETPR = 13, PR = TR = 1, for fcpu = 2 MHz. When you

convert into Hex value from decimal, resolution will be of 0.5 count. So the accuracy is [(0.5/

13)*100] = 3.84%. Accuracy will be proportional to variations in Baud rate and same as that of

the ETPR. For higher baud rate values, you have to go for a resolution 0.25 (logic can be ex-

tended as shown in software) to achieve better accuracy.

If SCI doesn’t have the ETPR prescaler then we can load PR=1 and assign the counted value

to TR directly.

2 CONCLUSION

This software solution allows you to work with any time critical applications irrespective of the

variations of the internal RC oscillator frequency. Software is calibrated with reference to the

base of 4 MHz.

Software is developed using the ST7 software library functions. It takes less than 30ms for the

simple calibration version. With the averaging and glitch algorithms, it takes less than 325 ms.

Software includes correction for the PWM counter value and the SCI baud rate. The PWM

counter value gives the frequency with an accuracy of 1-2% and the SCI baud rate gives an

accuracy of 3-4% after using this calibration software, when internal RC oscillator varies by (-

25%).

Tx

fcpu

fraction

×

16

PR

×

() TR

×

ETPR

×

---------------------------------------------------------------

=