User Manual

Sensor Board description
Wireless Sensing Triple Axis Reference design, Rev. 0.9
18 Freescale Semiconductor
is measured 128 times. By default, POWSUM is 0, for 1 measurement of each channel. Before result values
are provided, the accumulated values are scaled back to the 8-bit range and inverted where necessary
(may be required depending on the physical MMA7260Q device orientation relative to the Earth gravity).
Raw (i.e. not calibrated) values are actually sent, the calibration and calculation of an exact g value is
done internally in the PC software.
3.2.2 ADC measurement
The following routine is used for accelerometer measurement:
unsigned int xx = 0;
unsigned int yy = 0;
unsigned int zz = 0;
unsigned char xxx, yyy, zzz;
#define POWSUM 0
for (i = 0; i < (1 << POWSUM); i++)
{
ADCSC1 = 0x01; //read X channel
while(!ADCSC1_COCO);
xx += ADCR;
ADCSC1 = 0x02; //read Y channel
while(!ADCSC1_COCO);
yy += ADCR;
ADCSC1 = 0x00; //read Z channel
while(!ADCSC1_COCO);
zz += ADCR;
}
xxx = ~(unsigned char)(xx >> POWSUM);
yyy = ~(unsigned char)(yy >> POWSUM);
zzz = (unsigned char)(zz >> POWSUM);
3.3 Power management
A CR2032 (or CR2477) Lithium battery provides a fairly limited charge for such a realtime-like demo that
demands frequent transmissions. Some sort of power management has to be implemented in order to
keep the current consumption at a reasonable level.
Typically, current consumptions of Sensor board components are as follows:
• 2.4GHz transceiver MC13191
– in Hibernate mode, 2.3µA
– in Doze mode, 35µA
– in Idle mode, 500µA
– in Transmit mode, 30mA
– in Receive mode, 37mA