30770【PDF 43/74 页】SET DERFNODE FOR AVR HOUSING

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User Manual

Version 1.1

2011-07-15

deRFnode and deRFgateway

dresden elektronik

ingenieurtechnik gmbh

Enno-Heidebroek-Str. 12

12 7 Dr

rm n

Tel.: +49 351 31 85 00

Fax: +49 351 3 18 50 10

wireless@dresden-elektronik.de

www. r

n- l k r nik.

Page 43 of 56

Assumed that you use an ARM MCU, the initialization might look like:

#define PINS_TWI { ((1<<10) | (1<<11)), AT91C_BASE_PIOA,

AT91C_ID_PIOA, PIO_PERIPH_A, PIO_DEFAULT }

static Twid twid; // managing datastructure

const Pin pins[] = { PINS_TWI }; // SDA/SCL pins (PA10, PA11)

PIO_Configure(pins, PIO_LISTSIZE(pins));

PMC_EnablePeripheral( AT91C_ID_TWI); // enable twi peripheral

TWI_ConfigureMaster(AT91C_BASE_TWI, 100000, BOARD_MCK); // 100kHz

TWID_Initialize(&twid, AT91C_BASE_TWI); // initialize datastructure

AIC_ConfigureIT(AT91C_ID_TWI, 0, ISR_Twi); // configure and

AIC_EnableIT(AT91C_ID_TWI); // enable twi interrupt

During the next step the devices are configured. This includes activity intervals, resolution/

sensitivity, triggers when exceeding/falling below given limits, etc. Usually the sensors power

up idle and must be explicitly started. Additionally the acceleration sensor includes a configu-

ration EEPROM in which an overriding startup-configuration may be saved. Configuration is

usually done by writing to device registers and incorporates:

(1) initiate a TWI start condition,

(2) write the configuration register address,

(3) write the configuration register value,

(4) send a TWI stop condition.

So if you i.e. want to activate the TMP102 temperature sensor measuring temperatures only

upon request, select the configuration register MSB (0x01) and write in 0x80 to shut down

the device. Here TWID_Write() encapsulates all the required steps in one function:

#define BOARD_SENS_ADDR_TEMP (0x48) // sensor address, 1bit shifted

unsigned char ucBuf[2]; // buffer for twi transmissions

ucBuf[0] = 0x01; // configuration register, MSB

ucBuf[1] = 0x80; // shutdown-mode

TWID_Write(&twid, BOARD_SENS_ADDR_TEMP, 0x00, 0x00, ucBuf, 0x02, NULL);

As like as configuration is performed, sensor values are read from device registers. Depend-

ing on the device, you may either read the current register value directly or must send a start

command first and wait a certain time until measurement is available (otherwise you would

read outdated values). To continue with the temperature sensor, a code snippet looks like:

ucBuf[0] = 0x01; // configuration register, MSB

ucBuf[1] = 0x81; // shutdown mode | one-shot

TWID_Write(&twid, BOARD_SENS_ADDR_TEMP, 0x00, 0x00, ucBuf, 0x02, NULL);

// wait at least 26 ms (depends on selected resolution), then

// select temperature register (MSB) and read 2 bytes from it

ucBuf[0] = 0x00;

TWID_Write(&twid, BOARD_SENS_ADDR_TEMP, 0x00, 0x00, ucBuf, 0x01, NULL);

TWID_Read(&twid, BOARD_SENS_ADDR_TEMP, 0x00, 0x00, ucBuf, 0x02, NULL);

// convert value to a human-readable format ...

Besides the I

C communication lines, the acceleration sensor includes an interrupt line which

may trigger under certain circumstances, i.e. acceleration increases above/decreases be-

low/changes relatively to a configured threshold. These features might be used to detect the

device falling or its motion at all. For all these cases, the sensor might drive its INT line high,

as long as the condition is met. For detailed information, please refer to the BMA150

datasheet. Using this feature requires JP1 to shortcut pins 2-3 and configuration of an inter-

rupt trigger on the MCU side.

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