Issue 133 August 2001
MSP430 News Flash:

Recognizing the Flexibility of Reprogramming

Start • MSP430F1121 • Comparatively Speaking • Battery Monitor • Dynamic Inputs • RC To The Rescue • E(OR I)IN • It's Only The Beginning • Sources & PDF

RC TO THE RESCUE
Use a single analog input and a single digital input to set up the circuit in Figure 3. Choose values for the RC components such that their time constant, R*C, is less than the time it takes for the micro’s timer to roll over. For instance, the DCO can be bumped up to ~3 MHz from the default of ~750 kHz. The 16-bit timer will roll over after counting 65536 tics of the 3-MHz clock. That’s ~20 ms. A 10-kW resistor and a 1-µF capacitor has a time constant of ~10 ms. This is less than the timer overflow time so you won’t need to worry about the timer overflowing. The digital bit is used to either apply VCC to the resistor (to charge the capacitor), or ground the resister (to discharge the capacitor). This can easily be done under program control.

Software is written to set the digital output bit high for five time constants (tc) to assure a full charge (> 99%). When the digital output is switched to ground, the timer is cleared and enabled. Because the (–) input of the comparator is configured with a relative internal voltage of 0.25*VCC, the comparator will change states with the discharging capacitor. At change of state, the timer is read. The count is the reference count and can be equated to the reference resistor’s value. In simplest terms, if the timer has 10,000 counts and the reference is a 10k resistor, then 1 count = 1 W.
If you’ll be using a resistive sensor, make sure your reference resistor’s value is higher than the highest value of your sensor. If you’re using a capacitive sensor, make sure the value of your reference capacitor is smaller than the smallest value of your sensor. To improve result accuracy, use a close tolerance reference device (< 1%), as all calculations are based on this known value. Note that this will be more of a problem in selecting a reference capacitor, as finding one with < 5% tolerance may be difficult.
Now that you have a baseline reference where a count is equal to so many ohms of a reference, you need a way of comparing this to a sensor. See Figures 4 and 5 for examples of how a resistive or a capacitive sensor may be connected to the original base design from Figure 1. This is accomplished by using an addition digital I/O pin.
Referring back to the reference circuit (see Figure 3), the digital I/O pin was used to alternately apply VCC or Gnd to one end of the reference resistor to allow the reference capacitor to alternately charge and discharge. To allow multiple resistive (or capacitive) sensors to the design, each digital I/O pin must also take on another state. Multiple sensors connected to VCC and Gnd will interfere with one another by either being in parallel with one another or forming a divider. In addition to the two output states, the pin must be able to be placed into a high impedance (input) mode. This action will essentially disconnect them from the circuit.

Photo 1—This scope shot shows the analog input (P2.3) on the MSP430F1121 running on the flash memory emulator. The executing program charges and discharges a capacitor, first through the reference resistor and then through a resistive sensor.

Now each component can become the device being tested. First, the reference device is connected and goes through a charge and discharge cycle. The timer count forms a reference value (counts/ohm). Next the sensor is connected and goes through an identical charge and discharge cycle. This time the discharge time (timer count) will be somewhat less (depending on the sensor’s value). Because the reference discharge time (RREF * C = COUNTREF) and sensor discharge time (RSEN * C = COUNTSEN) both use the same capacitor, the capacitor’s value in both equations cancels out and you have a direct relationship between RREF, RSEN, and the timer counts.

    [1]

Using the values from Figure 1, you get

      [2]

If the COUNTSEN value is determined to be 5000, then

     [3]

Or
[4]

Actually, the discharge time is from VCC down to 0.25 VCC, which is slightly more than one time constant (63.2%), but everything here is ratio-metric. As long as VCC stays constant for a complete cycle (reference and sensor) and as long as the capacitor used is stable for the same period, these do not affect the result. You can see that two things are important to the result, the actual value of the known reference and the maximum number of counts for the reference discharge time.

Figure 4—Adding a second resistive element (in this case, a resistive sensor) allows the micro to compare how each R interacts with the C. The R not being tested must be removed from the circuit by setting its control pin as a high-impedance input.

Take a look at Photo 1 for a scope shot of the signal at the RC junction (comparator input) of the circuit from Figure 4. The internal RCO was used as the system clock and was set to ~ 3 MHz. This clock was routed to P1.4 (alternate I/O function of the pin) so I could physically measure the clock speed with a scope. This was a sanity (or confidence) check. The specs say 2.7–3.65 MHz and I measured 3.4 MHz. The flash memory emulator allows breakpoints to be set. After halting execution after each discharge cycle, I was able to check the timer counts for the reference and the sensor. The reference count was 0B65Bh (46683). With a 0.1% reference of 10,000 W, the counts/ohm is

            [5]

The sensor count was 5DC3 (24003). This means that the sensor resistance is

 
                    [6]

Referring back to Photo 1, notice that the sensor’s discharge time is about twice as long as the reference discharge time (vertical measurement bars showing ~ 10-ms discharge time).