Zigbee Journey (6): Several important CC2430 basic experiments - ADC single sampling

In wireless sensor networks, it is important to convert the analog values of the sensors into digital quantities for easy transmission and processing. ADC (Analog-to-Digital Converter) is used to complete this conversion.

In the previous section, we introduced serial port communication between the CC2430 and a PC. CC2430 has a temperature sensor embedded inside, and this section will implement a simple experiment on on-chip temperature monitoring based on the previous section: use the ADC to convert the voltage value of the on-chip temperature sensor into a digital quantity, use the formula to calculate the temperature value, and then transmit the temperature value to the PC through the serial port and display it.

The ADC is used to convert the temperature value of the CC2430 on-chip temperature sensor, and the temperature value is sent to the PC through the serial port and displayed.

For the code content of serial port communication, please refer to the previous section, which will not be explained here~

ADCs generally involve 6 SFRs:

(Note: Please refer to the CC2430 Chinese manual for the above SFR details)

Next, let's focus on itgetTempuraturefunction, which is the key to getting the temperature value:

(1) First, configure the ADC for a single sampling: set ADCCON3=0x3E, select 1.25V as the system voltage, select 14-bit resolution, and select the CC2430 on-chip temperature sensor as the ADC conversion source

(2) Then set ADCCON1 |= 0x30 to set the ADC trigger method to manual (i.e., when ADCCON.6=1, start the ADC transition)

(3) Then let ADCCON1 |= 0x40 to start the ADC single conversion

(4) Use the statement while(!( ADCCON1 &0x80)) Wait for the end of the ADC transition

(5) The conversion result is stored in ADCH[7:0] (8 bits high), ADCH [7:2] (6 bits lower), and passed:

Save the conversion result in the value

(6) Finally, use the formulatemperature= value*0.06229-311.43, calculate the temperature value and return

CC2430 Tips

You must be puzzled by the last formula, why a one-time function? Why does it have a slope of 0.06229 and an intercept of 211.43? OK, here's the answer:

This temperature sensor is located inside the CC2430 chip, so its description can certainly be found in its manual. Sure enough, I'm hereElectrical specificationsThe relevant content is found in this section, and the screenshot is as follows:

This table describes the temperature (°C) of the temperature sensor in relation to the output voltage (V).

First look at the second red box:temperature coefficient。 "Coefficient"? Doesn't it feel a little? Then look at its unit: mV/°C, and you will suddenly realize that the relationship between temperature and voltage is linear~ That is:

where V is the output voltage value, T is the temperature value, and 2.45 is the slope. The intercept b must be determined below.

At first glance, we will find at the first red box that the voltage at 0°C is 743mV, so b is equal to 743? Otherwise, if you continue to look down, you will find that its absolute error reaches as much as 8°C! Then looking to the right, we will see that it already provides the most suitable intercept, i.e.: b=763, so there is the following formula:

OK, now we already have the temperature sensorEnter temperature TandOutput voltage VThe next step is to find the ADCInput voltage VandOutput value N(i.e., the conversion result of 14 bits), and finally find the conversion formula of N and T.

The conversion result N is 14 bits, and when N=11 1111 1111 1111 (binary), the output voltage should be the maximum value (i.e. reference voltage 1.25V). Therefore, we have the following proportional relationship:

(Note: Since the output of 14 bits is a binary complement, the 14th bit is the symbol bit.) So in absolute terms, the effective value is only 13 bits, so it is 2 to the power of 13)

Combining the two formulas, the relationship between T and N can be derived:

OVER~

Finally, a few words about why 64 cycles are required per sample. Because sensors are inevitably subject to interference or random errors when measuring temperature, the data obtained by sensors is sometimes exaggerated (for example, a sudden change of 10°C, and then an instant return to normal). But we know that temperature change is an integral process, and it is rare for a large jump to occur in an instant). Therefore, we use the average method to reduce such errors.

First, open the serial port debugging tool, then download the program and start it, and the following screen will appear:

The temperature inside the film is about 14.5°C. The author feels the room temperature of the dormitory with his body, which is about a little more than 10°C. The inside of the chip needs to get some heat, so 14°C is basically normal~

This is the end of the experiment.

This article describes the implementation of ADC single-sampling. In the next section, we will introduce a data transfer modeDMA(direct memory access), that is, "direct memory access". Peripheral units such as ADC/UART/RF transceivers and memory devices can be exchanged directly under the control of the "DMA controller"Little CPU intervention is required, which greatly improves the overall efficiency of the system.

Stay tuned!