WX200 / WM918 Temp / Humidity Sensor

• Microprocessor        
  • A/D Converters
  • LCD Display Driver (Not used in this sensor)
  • Buzzer Driver (Not used in this sensor)
  • Watchdog Timer
  • Oscillators
  • Time Base Counter
  • I/O Ports
  • Interupts
  • Battery Check Circuits
  • Reset
• Humidity Sensor
• Temperature Sensor
• Unused PCB tracks
• Connections and Wiring
• Schematics

General Temperature / Humidity Sensor Information

• Operating range
  
  • 2 Operating frequencies in unit are 32.768 kHz and 400 kHz.
  • There are 3 operating voltages to choose from -
    1.25 to 1.7 V (1.5 V spec.)
    2.0 to 3.5 V (3 V spec.)
    2.2 to 3.5 V (3 V spec., 1/2 duty).
  • The operating temperature range is rated -40 to +85°C.
• Memory space
  • The internal program memory has a capacity of 2016 bytes of storage.
  • The internal data memory's capacity is 128 nibbles.
• Minimum instruction execution time
  • With the oscillator running at 400kHz it is 7.5ms.
  • and at 32.768 kHz it is 91.6 ms.
• A/D converter
  • There are 2 independant channels of RC oscillation type A/D converters. These have the temperature and humidity sensors attached to them. The A/D converter is composed of a 2-channel oscillation circuit, Counter A (CNTA0-4, a 4.8-digit decade counter), Counter B (CNTB0-3, a 14-bit binary counter), and A/D Converter Control Registers 0 and 1 (ADCON0, ADCON1).
    By counting oscillation frequencies that vary depending on a resistor or capacitor connected to the RC oscillation circuit, the A/D converter converts resistance values or capacitance values to corresponding digital values. By using a thermistor or humidity sensor as a resistance, a thermometer or a hygrometer is constructed.
• LCD Driver
  • There are 24 outputs available, although unused in this sensor, with the duty ratio switchable by software. The LCD driver consists of 21 4-bit display registers (DSPR0-20), the Display Control Register (DSPCON), a 24-output LCD driver circuit, and a bias generation circuit (BIAS).
    There are three types of driving methods: 1/4 duty, 1/3 duty and 1/2 duty. Software selects the duty mode.
    A mask option can select either a common driver or a segment driver for each LCD driver pin.
    A mask option can also specify assignment of each bit of the display register to each segment.
    All the display registers must be selected by a mask option.
    L16 to L23 of the LCD driver can be configured to be output ports by a mask option.
    The relationship between the duty, the bias method, and the maximum segment number follows:
    
  • (1) At 1/4 duty and 1/3 bias, a maximum of 80 segments can be realised.
  • (2) At 1/3 duty and 1/3 bias, some 63 segments are available.
  • (3) At 1/2 duty and 1/2 bias, only 44 segments are available.
• Buzzer Driver
  • The 1 output available is something else not used in this sensor. It has 4 output modes selectable and 2 buzzer output frequencies. The buzzer output is selected by the Buzzer Control Register (BDCON) and the Buzzer Frequency Control Register (BFCON).
• Capture circuit
  • Not sure what this is or does, but there are 2 channels available.
• Watchdog Timer
  • There is a built-in watchdog timer to detect CPU malfunction. The watchdog timer is composed of a 6-bit watchdog timer counter (WDTC) to count a 16 Hz output and a watchdog timer control register (WDTCON) to reset WDTC.
• Oscillator
  • The clock generation circuit contains a 32.768 kHz crystal oscillation circuit built around a 32768 Hz Surface mount crystal (X1), a 400 kHz RC oscillation circuit formed around R1, a 270000 ohm resistor and some capacitance internal to the microprocessor, and a clock control port.
    This circuit generates the system clock (CLK) and the time base clock (32.768 kHz).
    The system clock drives the CPU while the time base clock drives the time base counter and the buzzer driver.
    The system clock can be switched between 32.768 kHz (the output of the crystal oscillation circuit) and 400 kHz (the output of the RC oscillation circuit) via the contents of the frequency Control Register (FCON),
    Note: The oscillation frequency of the RC oscillation circuit varies depending on the value of an external resistor (ROS ), operating power supply voltage (VDD ), and ambient temperatures (Ta).
• Time Base Counter
  
  • The time base counter (TBC) generates clocks signls supplied to internal peripheral circuits. The time base counter is composed of 15 binary counters. The count clock of the time base is driven by the oscillation clock (32.768 kHz) of the crystal oscillation circuit. The output of the time base counter is used for the buzzer driver, the system reset circuit, the watchdog timer, the time base interrupt, the sampling clocks of each port, and the capture circuit.
• I/O Ports
  

  	Input-output ports (P2, P3) (8 bits)	:	Pull-up (pull-down) resistor input or high- impedance input, CMOS output or NMOS open drain output: these can be specified for each bit; external 0 interrupt
  	Input port (P0) (4 bits)	:	Pull-up (pull-down) resistor input or high- impedance input; external 1 interrupt
  	Output port (P1) (4 bits)	:	CMOS output or NMOS open drain output

  
• Power supply voltage
  • 1.5 V/3 V (selectable by mask option)
• Interupts
  
  • 9 interrupt sources (9 vector addresses), are available, of which two are external interrupts from ports and seven are internal interrupts.
    The watchdog interrupt is the only one which cannot be disabled (non-maskable interrupt). The other eight interrupts are controlled by the master interrupt enable flag (MI) and the interrupt enable registers (IE0, IE1 and IE2).
    When an interrupt condition is met, the CPU branches to a vector address corresponding to the interrupt source.
• Battery Check Circuits
  
  • The battery check circuit (BC) detects the level of the supply voltage by comparing the voltage generated by an external supply-voltage dividing resistor (RBLD) with the internal reference voltage (Vrb).
• System reset input pin
  
  • The reset signal input is active low, meaning when it is grounded, the micro is put into a reset state and all activity is stopped. C2, measured at 0.1uF is tied to this line. When power is first applied, C2 is in a discharged state, and the micro is held in a reset state. The uP internal pull up resistor charges up C2, and when the voltage is high enough, the reset state is released and the uP starts executing an instruction from address 000H. The uP's internal firmware then reads the temp and humidity sensors, and sends a burst of data out to the weather station.

• The Yellow wire which supplies +1.5 volts from the regulator IC in the junction box Box 1 ).
• The Blue wire which is the common ground return.
• The Red wire which is the Serial Data output pulsed at 1.5 Volts peak for a short burst about 1 second after power is applied to the sensor.
• The Black wire which is the Serial Clock output pulsed at 1.5 Volts peak for a short burst about 1 second after power is applied to the sensor.

Updated 02:39 17/12/2000.

Schematics