Abstract: An embedded clock features temperature measurement circuit design. Temperature measurement circuit using temperature sensors to monitor the outside temperature, the temperature sensor by the resistance of the oscillator changes frequency signals into changes in the analog signal to digital signal conversion, and then use digital signal processing methods calculated temperature value, realize the temperature measurement. Keywords: temperature sensor FPGA oscillator
  The traditional mercury or alcohol thermometer to measure temperature, not only to measure a long time, reading is not convenient, and single function, can no longer meet the requirements in the digital age. This paper presents a new type of digital temperature measurement circuit design program, which integrates a temperature measurement circuit and real-time calendar clock circuit.
  Temperature measurement circuit of the temperature range between -20 ~ 50, a resolution of 1, temperature less than 1 second. Circuit resistance using Linear Technology's LT1799 programmable oscillator to achieve resistance to frequency conversion, and then in the pre-stored in the ROM mapping parameter values are tested to compare the temperature values. Real-time calendar clock circuit can show year, month, day, week, hours, minutes, seconds, seven kinds of clock signals, users can set or modify the time. The whole circuit with Altera's FPGA ACEX1K series hardware simulation carried out to achieve, circuit design flexibility, easy to modify.
  1 Principle of temperature measurement
  Temperature monitoring is mainly achieved using temperature sensors. The design of the temperature sensor NTC thermistor is used, which has a negative temperature coefficient thermistor, its resistance (RT) with temperature (T) decreases rapidly with the increase. Temperature resistance is expressed as:

Where, A, B by the semiconductor materials and processing technology of the two constants are determined, B is the thermal index. Design choices are the MF58 R25 ? to 100kO thermistor temperature measurement accuracy, thermal index for the 3 650K.
  LTC1799 is a resistor programmable oscillator [1], can produce a 50% duty cycle square wave, and has a temperature stability characteristics and power supply voltage stability, is a low-power devices, only one external component, that is, setting resistors and bypass capacitors. LTC1799 standard circuit shown in Figure 1, the figure 0.1µF capacitor connected to the power pin to ground, power supply noise can be reduced to a minimum. The first set resistor connected between pins 1,3, to control the output frequency, this design alternative settings using thermistor resistance. 4 pin is a tri-state frequency pin, the master clock in the output before the decision was 1,10 or 100 frequency, the design of the grounding pin that the output frequency division factor of 1. 5 pin as an output pin, the output frequency and the relationship between the set resistor is:

Since the thermistor resistance changes with temperature changes, so that we can through the LTC1799 to establish the relationship between temperature and frequency in order to achieve the temperature measurement.
  By (1), (2) shows that there are two kinds of circuit design, nonlinear relationships: First, the thermistor resistance and temperature of the nonlinear relationship between the second frequency conversion is nonlinear resistance . For nonlinear problems, you can use mathematical methods for processing, but more complicated algorithm, but also to take up a lot of hardware resources. Therefore the design of another method used to process, namely the use of ROM pre-stored frequency - temperature data, by look-up table for temperature mapping. This not only avoids the problem of nonlinear and save the hardware resources.

Figure 1 LTC1799 standard circuit
2 Hardware Design
From the division of functions, the hardware circuit is divided into real-time temperature measurement circuit and the calendar clock circuit of two parts.
2.1 Temperature measurement circuit
2.1.1 Frequency Measuring Circuit
Frequency measurement circuit mainly uses the frequency counting method, divided by the external crystal oscillator circuit is obtained by gating the sampling period 2T reference signal count_en and count reset signal count_clr, in the sampling reference signal positive half cycle counter is enabled, the counting module began to measure the frequency of the input signal, measuring the time exactly T, and the falling edge of the sampling reference signal the results of the sampling data latch. If the counter at time T, the number of measured signal pulse is N, then measured the frequency of the signal: FX = N / T.
Counter reset signal for the start of each measurement module to reset the count to clear the last measurement results. The timing relationship between the various signals shown in Figure 2.

Figure 2 sampling control signal timing relationships between
In addition, because the measurement process the measured signal and the sampling frequency is not synchronized between the lock control signals between the end of the count will produce an additional error of ± 1, namely, reference [2] mentioned in the quantization error. Therefore, sampling time T and the counter digits must be a reasonable choice. In this design, the measured frequency range 10kHz ~ 300kHz changes in the temperature range tested for each change of temperature 1 , the resistance minimum change 0.5828kO. When taking T = 20ms, R = 100kO time, N = 1000, then the quantization error of ± 1's impact is almost negligible.
2.1.2 Data processing circuit
As the frequency measurement circuit, there is bound to measurement error, so the results of these measurements must be processed. The design for data processing, a new idea, the data processing flow shown in Figure 3. Shown in the figure, the frequency measurement data results in the circuit N 'first shift register through a number of registers, and then use a comparator to compare the data on the storage, removal of the maximum and minimum values ??after the rest of the intermediate phase plus, finally get through the divider, the average N.

Figure 3 Flow chart of data processing
  The approach used for data processing advantages:
  (1) to reduce the measurement error. By comparing the device can be the occasional error data rule out the unexpected. Compensation under the principle of random error, for the limited measurements, multiple measurements can be regarded as the arithmetic mean is the best estimate of the true value.
  (2) can dynamically reflect the trend of the measured signal. This method is suitable for many demanding real-time measurement occasions.
  2.1.3 Temperature mapping circuit
  Temperature mapping circuit diagram shown in Figure 4. From Figure 4, the address counter and temperature through a finite state machine counter to control. State 1: Clear counter; State 2: counter; state 3: Latch count results; state 4: the results of the counter output. Repeatedly execute the above four states can be realized real-time measurement of temperature control.