LPC932 EXAMPLE: 	AUTOCALIBRATION OF INTERNAL LOW POWER OSCILLATOR FOR    
                                 	CLOCKING THE UART
Christian Kulig, 12/06/02


INTRODUCTION:
The LPC932 provides a number of features, which help to reduce power consumption as well as
to minimize external components.

For example, it might be an option for applications in battery powered and handheld devices 
to clock the µC with the internal watchdog timer (WDT). (See figure 1)
This lowers the power consumption considerably compared to the use of an external crystal 
or the internal RC oscillator. And of course, there's no need for the crystal and its two 
capacitors anymore. Thus, applications, which don't need a high clock frequency, can be 
developed more economically.


APPLICATION TO THE UART:
Nevertheless, it's possible to achieve a highly accurate time base if required. For example 
to drive the integrated UART, it's necessary to set up a baud rate with a deviation smaller 
than ±2.5%. Since the watchdog oscillators frequency is in a range between -30% and +20% of 
400kHz, the actual frequency must be measured and considered before setting up the 
particular baud rate. This means the watchdog oscillator has to be calibrated. 

Fortunately, the actual frequency is very stable, so that the WDT only needs to be 
calibrated once. The results are written into the EEPROM and can be used from then on.

This description shows in detail how to setup the UART as an example, but the calibration 
itself and its results can be used for any other purpose requiring an accurate time base.

CALIBRATION PRINCIPLE (See figure 2)
A convenient way to generate a baud rate for the UART is to use the internal baud rate 
generator. It divides the CCLK by a factor that is defined by two registers BRG0 and BRG1. 
If CCLK is fWDT =400kHz and the intended baud rate is 9600 the factor should be 42. 
Obviously, this factor is proportional to CCLK and fWDT. 

To get the frequency of the WDT we just have to count watchdog timer events (counts, 
overflows) during a certain period. It's easy to use overflows, because they cause an 
interrupt, which can be used to increment a counter. To maximize the accuracy we set up 
the highest overflow frequency, by writing a 0 into the watchdog timer reload register. 
Now every 80µs an overflow occurs. The resulting count value could be used directly as 
the factor for the baud rate generator, but it's advantageous to count a value that's 
4 times the required factor. By shifting the result by 2 digits to right, the LSB error 
can be removed. 
Now the measurement time is 4x42x80µs = 13.44ms. The result of the calibration is as 
accurate as this time can be measured. 

When the LPC932 is powered up for the first time, it uses the internal RC oscillator as 
its clock source. It has a tolerance of 2.5% over the whole temperature range and is 
better than 0.8% when used at room temperature. This tolerance is well suitable for 
performing the calibration. The RC oscillator has a frequency of 7.373Mhz, this means 
an internal timer (e.g. Timer0) increments every 0.271µs. To get a total measurement 
time of 13.44ms it has to count 49547 times until it overflows and causes an interrupt. 
This interrupt must stop all timers  (T0 and WDT) and write the current count value 
into the EEPROM.

Now that the WDT is calibrated, an IAP routine is called to set up the µC to use the 
WDT as its clock source. The last step is to execute a software reset to let the new 
clock settings take effect. 

On further powerups this calibration procedure can be skipped, since the EEPROM contains 
the necessary information. It can be read, shifted right by 2 digits and used as the 
factor for the baud rate generator to clock the UART.
It can also be used for the prescaler of any other peripheral of the LPC932 that needs 
an accurate time base. 


SOFTWARE EXAMPLE:
The following code implements the calibration of the WDT. It's divided into two major parts, so that it's easy to include it in existing projects. 

The first four files are a simple example of customer software. It uses the UART for receiving a command ('S' = Send Data) from the PC and sending back some data.
The last two files contain the necessary code, which calibrates the watchdog timer. 

To test this example, the LPC932 needs a RS232 interface to a PC, which may be provided by the evaluation board. 
Start the program 'Hyperterminal' (Start --> Programs --> Accessories --> Communications) and set the configuration to 9600 baud, 8 data bits, 1 stop bit, no parity bit and no flow control. Then click on 'Wait for Call'.
After a power up, the programmed device blinks the LEDs on Port 2. That means, it has already calibrated the watchdog timer and is using it as the clock source.
By pressing 'S' in Hyperterminal, the LPC932 sends out some data to the PC, which are displayed by Hyperterminal.