KEYPAD USING STM 110C – A Wireless, Batteryless
Contents
1. APPLICATION NOTE 309 Green Smart Wireless enocean EnOcean STM 110C is extreme low power sensor radio working at 315 MHz frequency It can be powered by ambient energy like thermal light etc This application note describes how to build a wireless and batteryless multi button keypad using STM 110C STM 110C Operation STM 110C has three 8 bit A D channels and four digital inputs It is preprogrammed in fac tory and once power is applied switches itself between sleep mode and operating mode in order to save power STM 110C can be waked up immediately from sleep mode by logic state changes on hard ware pins or periodically by a configurable timer STM 110C always sends 4 byte data tele grams if it is waked up by the hardware pins However for the periodic wakeup the micro processor samples all analog and digital input pins after each wakeup and determines if the 4 byte data should be transmitted through the radio according to the following criteria Exception trigger Periodic trigger m Data will be transmitted if either one of the Exception trigger and the Periodic trigger becomes true The Exception trigger becomes true if Logic state on at least one of the four digital inputs changes m Sampled data from at least one of the three A D channels has significant change This is triggered by the exception threshold and it is configurable Please see the STM 110C user manual The periodic trigger becomes true2. Green Smart Wireless enocean The sampled voltage when a key K i 1 8 is pressed can be calculated From figure 2 as follow V_REF V R R R R 1 R R R R R s V 1 Ry Ry S If the resistance of the key contact can be ignored then 1 becomes V_REF __V 2 Ge R R R R V L STM110C Sampled Voltage vs Divider Resistor STM110C Sampled Voltage at STM110C A D Channel Volt oO 10 20 30 40 50 60 70 80 90 100 Resistance Ri K Ohms Figure 2 The equivalent circuit for A D channel Figure 3 Sampled voltage vs Resistors Assume V 0 5V R 100KQ and R 0 The plots of sampled voltage against divider re sistor R with three different values of R are given in Figure 3 It is obvious that the smaller R results in bigger span of data range The sampled data close to the full span of the reference voltage 2 05V when R 2K in the scope of R lt 100K R must be kept smaller than 10K APPLICATION NOTE 309 Green Smart Wireless enocean Because V_REF is only available during STM 110C wakeup 2 6 ms so the maximum energy assume R 0 consumed on the keypad circuit for a key push is pr AUN REP AV E 2 6m If take R 2K then E 4 13uWs This amount of energy is acceptable for ambient energy powered device The average en ergy consumed will be lower than this because of the resistors R 0 Figure 4 shows an ex
3. periodically based on a configurable redundant transmis sion counter This will define the maximum transmission period of the module Most keypad applications require immediate action after one or more keys are pressed So the hardware wakeup feature of STM 110C should be used for keypad interface APPLICATION NOTE 309 Green Smart Wireless enocean Keypad I nterface Variety of interface methods exists between keypad and MPU The following few keypad interface methods with STM 110C can be considered due to the fact that the STM 110C is pre programmed in factory and the operating mode is fixed User cannot reprogram it m Use WAKE WAKE and digital input pins m Use WAKE WAKE and A D input pins m Use WAKE WAKE and combination of digital input and A D input pins The key is that when any key is pressed down the STM 110C will be waked up and the unique information associated with the key digital or analog or both should be sampled by STM 110C The information associated with the key will become part of the 4 byte data payload and will be transmitted to the remote receiver The key decoding can be done at the receiver side One more thing we have to keep in mind is that this is low power design The circuit should not draw excessive current when no key is pressed Example Picture 1 gives an example of keypad interface circuit to STM 110C It uses the WAKE WAKE pins for wakeup and one A D input pin for key sensing SOL1 and SO
4. L2 connect to solar cells See user manual of STM110C V_REF is the reference voltage out put of STM 110C 2 05V which is only available during wake up 2 6 ms The gate of Q1 is driven high by V_SC1 through Rio when no key is pressed Thus the STM 110C pin WAKE is high and the pin WAKE is low When any of the keys is pressed done the gate of Q1 will be pulled low so the STM 110C pin WAKE will be low and WAKE will be high The change of state on the differential pins WAKE WAKE will cause STM 110C to wake up enable V_REF output and sample all digital and analog inputs In this example the analog channel AD_O is connected to a resistor divider network The re sistance of R1 to R9 is carefully chosen so that the voltages appeared at AD_O when differ ent keys are pressed have the optimal spacing which will minimize the error rate at key decoding The equivalent circuit is shown on Figure 2 if only one key is pressed each time V is the equivalent forward voltage drop across a diode V _REF is the reference voltage 2 05V outputted from STM 110C during wake up APPLICATION NOTE 309 Green Smart Wireless enocean Q1 BSS119 Figure 1 An example keypad circuit interface to STM 110C R is the input impedance of A D channel of STM110C which is 2 100 KQ R is the contact resistance of the push button switches keys R is the divider resistors R toR shown in Figure 1 t APPLICATION NOTE 309
5. ample how to do key decoding Because of the noise generated by the push button contact variation in R the sampled data and key value will not be one to one mapping It is necessary to make a mapping table to map each key value to non overlapped small regions on Y axis The red lines are the centers of such regions for each key from bottom up correspond to keys K1 to K8 Because the curve rise fast at the low R the sampled voltage is more sensitive to the resistor value R Considering the Key Value Mapping And Decoding R9 2K K mapping region ee Ri Yalu f r K7 Sampled Voltage at STM110C AYD Channel Volt n 1 0123 45 6 7 8 9 10111213 14 15 20 25 30 Resistance Ri K Ohms Figure 4 Key value decoding APPLICATION NOTE 309 Green Smart Wireless enocean variation of the resistor values due to tolerance and the temperature variation the voltage spacing is chosen to be wider at the low side of resistor R The decoding error occurs when the sampled data falls in the regions in the neighborhood So a guard band between two regions next to each other can help reducing decoding error If a value falls into the guard band then it should be considered as invalid key stroke As mentioned above noisy key pad contact can generate noise on the sampled voltage A simulated noisy contact with R varies randomly between 0 to 500 Ohms is shown in Fig ure 5 It is obvious that the sampled voltage is more sensiti
6. hannels or by the combination of analog and digital channels Decoding error is a very important factor for a keypad There are many trade offs that will affect the decoding error User should consider the specific application and the reliability requirement when design a keypad The following few areas should be always considered carefully for this matter Use Digital less error or analog interface or combination of two How many keys are assigned to each analog channel the less key the wider spacing and the less decoding error m How a channel is segmented through choosing the divider resistors R9 and R How large is the mapping regions and guard band the wider the guard band the less the decoding error but more invalid key strokes Quality of the keypad contact Tolerance of the resistors used It is also recommended for the user to measure the accurate A D channel input impedance because user manual of STM 110C gives only the range 2100K Disclaimer The information provided in this document describes typical features of the EnOcean radio system and should not be misunderstood as specified operating characteristics No liability is assumed for errors and or omissions We reserve the right to make changes without prior notice For the latest documentation visit the EnOcean website at Www enocean com
7. ve to the contact Ohmic noise to the keys with lower R This is why wider voltage spacing and voltage guard band should be allocated at the lower R side Please note that the Ohmic noise R always shift the curve up This implies that an asymmetric mapping region will give a better performance Deviation on Sampled Voltage with keypad contact noise Se See ee See eee ee eee eee eee eee eee TELET escqdonosoposcossopossodosssodosssspocss Sampled Voltage at STM110C A D Channel Volt eee See See ee ee Se ed ee r iP k r r L 1 r L No noise With Rk noise 0 500 Ohm I E 0 5 10 15 20 25 30 Resistance Ri K Ohms Figure 5 Keypad contact noise Another very important issue is key debounce It is standard practice to give 20ms delay when detect and read a key stroke from keypad to MPU interface to skip the noisy key de bouncing period However STM 110C only gives 2 6 ms during wakeup Because all analog and digital channels are sampled during this time period so much noisier signals are ex pected for the keypad interface that is discussed here APPLICATION NOTE 309 Green Smart Wireless enocean Conclusions Wireless keypad interface to the ambient energy powered EnOcean STM 110C is discussed and a simple example is given Keypad with more keys can be constructed the same way by using more A D c
Download Pdf Manuals
Related Search