Interface IC for 125 kHz Wake-up Function

Features • • • • • • • • Wake-up Function for a Microcontroller with Preamble Detection 1 mVrms Sensitivity 1 µA Standby Current Power Supply: 2 V to 3.8 V Baud Rate: up to 4 kbps (ASK Modulation) Operation Temperature: up to 125°C Withstands +175°C Few External Components Application • Tire Pressure Monitoring (TPM) Description The ATA5283 is a 125 kHz ultra-low power receiver used for the wake-up function of Tire Pressure Monitoring (TPM) application. The sensitive input stage of the IC amplifies and demodulates the carrier signal from the antenna coil to a digital output signal for a microcontroller. During the standby mode the preamble detection unit monitors the incoming signal and activates the wake-up output and the data output, if the IC receives a proper 125 kHz carrier signal. By combining the IC with an antenna coil, a microcontroller, an RF transmitter/transceiver, a battery, temperature- and pressure sensor, it is possible to design a complete Tire Pressure Monitoring system (TPM). Figure 1. Block Diagram Interface IC for 125 kHz Wake-up Function ATA5283 Preliminary Battery VDD ATA5283 Lx COIL Amplifier with AGC RESET Vref Conditioner Preamble check N_WAKEUP N_DATA TST1 TST2 GND Rev. 4598D–AUTO–03/04 Pin Configuration Figure 2. Pinning TSSOP8L COIL TST1 TST2 VSS 1 2 3 4 8 7 6 5 VDD N_WAKEUP N_DATA RESET Pin Description Pin 1 2 3 4 5 6 7 8 Symbol COIL TST1 TST2 VSS RESET N_DATA N_WAKEUP VDD Function Antenna coil input Test pin (reserved) Test pin (reserved) Signal ground External reset input Data signal Low active wake-up signal for microcontroller Battery voltage 2 ATA5283 [Preliminary] 4598D–AUTO–03/04 ATA5283 [Preliminary] Functional Description The ATA5283 is an ultra-low power ASK receiver. Without a carrier signal it operates in the standby listen mode. In this mode it monitors the coil input with a very low current consumption. To activate the IC and the connected control unit, the transmitting stage must send the preamble carrier burst. After a preamble is detected the IC is activated. It adapts the gain of the input stage and enables the wake-up and the data output. The first gap at the end of the preamble generates a wake-up signal for the microcontroller. After that the receiver outputs the data signal at N_DATA. To return the IC into the standby listen mode it must be reset via the RESET input. The input stage contains an Automatic Gain Control (AGC) amplifier to amplify the input signal from the coil. The gain is adjusted by the automatic gain control circuit if a preamble signal is detected. The high dynamic range of the AGC enables the IC to operate with input signals from 1 mVrms to 1.1 Vrms. After the AGC settling time the amplifier output delivers a 125 kHz signal with an amplitude adjusted for the following evaluation circuits’ preamble detection, signal conditioner, wake-up. Before data transmission the IC stays in standby listen mode. To prevent the circuit from unintended operations in a noisy environment the preamble detection circuit checks the input signal. A valid signal is detected by a counter after 192 carrier periods without interrupts. Short interrupts which are suppressed by the signal conditioner are tolerated. When a valid carrier (preamble) is found the circuit starts the automatic gain control. It requires up to 512 carrier periods to settling. The complete preamble should have 704 carrier periods minimum. The preamble is terminated and the data transfer is started with the first gap (Start Gap) in the carrier (see Figure 3). AGC Amplifier Preamble Detection Figure 3. Communication Protocol Procedure 192 Periods of LF Preamble >5.64 ms >512 Periods of LF Start gap Data Signal N_DATA N_WAKEUP RESET Gain control Current profile No gain control AGC adjustment Gain control active No Gain Control 1 µA 2 µA 0.5 µA 3 4598D–AUTO–03/04 Automatic Gain Control For a correct demodulation the signal conditioner needs appropriate internal signal amplitude. To control the input signal the ATA5283 has a build in digital AGC. The gain control circuit regulates the internal signal amplitude to the reference value (Ref2, Figure 4). It decreases the gain by one step if the internal signal exceeds the reference level for two periods and it increases the gain by one step if eight periods do not achieve the reference level. In the standby listen mode the gain is reset to the maximum value. If a valid preamble signal (192 valid carrier clocks) is detected the automatic gain control is activated. Note: With the variation of the gain the coil input impedance changes from high impedance to minimal 143 kΩ because of the internal regulator circuit (see Figure 10). Figure 4. Automatic Gain Control Transmitted signal Coil input Gain control reference Ref.2 100% Gap detection Ref.1 reference 50% Gain controlled signal Internal comparator singal N_DATA 4 ATA5283 [Preliminary] 4598D–AUTO–03/04 ATA5283 [Preliminary] Signal Conditioner The signal conditioner demodulates the amplifier output signal and converts it to a binary signal. It compares the carrier signal with the 50% reference level (see Ref1 in Figure 5) and delivers a logical 1, if the carrier signal stays below the reference and a logical 0, if it exceeds the reference level. A smoothing filter suppress the space between the half-waves as well as a few missing periods in the carrier and glitches during the gaps. The output signal of the signal conditioner is used as the internal data signal for the data output, the wake-up logic and the preamble detection. The timing of the demodulated data signal is delayed related to the signal at the transmitting end. This delay is a function of the carrier frequency, the behavior of the smoothing filter and the antenna Q-factor. The smoothing filter causes a delay of 3 to 6 periods (see tb and td in Figure 5). The rest of the delay is caused by the build-up time of the antenna signal and is conditioned on the Q-factor (see ta and tc in Figure 5). Figure 5. Output Timing Ref.2 Ref.1 Coil input 100% 50% Comparator output N_DATA ta tb tON tc td tOFF 5 4598D–AUTO–03/04 The following diagrams show the delay of the data signal as a function of the antenna Q-factor. Figure 6. Turn On Delay Time (tON) versus Antenna Q-Factor 250 ffield = 125 kHz 200 Typ. Max. ton (µs) 150 100 Min. 50 0 0 10 20 30 40 50 Q-factor Figure 7. Turn Off Delay Time (toff) versus Antenna Q-Factor 200 180 160 140 120 Max. Typ. ffield = 125 kHz toff (µs) 100 80 60 40 20 0 0 10 20 30 40 50 Min. Q-factor Data Output The data output N_DATA outputs the demodulated and digitized LF signal according to the envelope of the antenna input signal. In the standby mode the N_DATA output is disabled and set to level 1. It is enabled by the wake-up signal and it outputs 1 level if the IC detects the carrier signal and a 0 level during the gaps (see Figure 3). As the circuit does not check the received data (except the preamble), it is up to the user to choose the kind of encoding (pulse distance, Manchester, bi-phase...) wanted. Wake-up Signal The wake-up signal (N_WAKEUP) indicates that the ATA5283 has detected the end of a preamble signal and has left the standby mode. It can be used as a wake-up or a chip select signal for an external device (see Figure 3). After a preamble is detected the first valid gap (Start Gap) sets the N_WAKEUP output to low and enables the data output N_DATA. The N_WAKEUP holds the low level until the IC is reset to the standby mode by a reset signal. 6 ATA5283 [Preliminary] 4598D–AUTO–03/04 ATA5283 [Preliminary] Reset The IC is reset either by the internal POR circuit during a power on sequence or by a high pulse at the RESET pin. After the reset all internal counters are in the initial state and the IC is in the standby listen mode. The POR circuit generates a reset while the supply voltage VDD is below the power on reset threshold VPOR and release the function of the IC if VDD exceeds this threshold. A high signal at the RESET pin resets the complete circuit. If the IC is activated a reset signal is necessary to activate the standby listen mode. The RESET pin can also be used to hold the IC in a power down state. In this state the the IC is out of operation and the current consumption is below the standby current. Note: The RESET pin is high impedance CMOS input. To avoid floating effects like undefined input states and malfunctions it should not be open. Standby Listen Mode In the standby listen mode the IC monitors the coil input with a very low current consumption. The automatic gain control is switched off and the gain is set to the maximum value. The N_DATA and the N_WAKEUP output are set to a high level. Before the controller enters its standby mode after the communication, it should activate the standby listen mode of the ATA5283 with a reset signal. This measure ensures that the IC enters the power saving standby mode and that the IC wakes the controller correctly with the next preamble signal. Applications Figure 8 shows a typical TPM application of the ATA5283. Combined with the antenna resonant circuit the ATA5283 is used as wake-up receiver for the microcontroller and the connected temperature- and pressure-sensor. Note: To avoid supply voltage ripples to affect the microcontroller, an RC filter (R1 = 100 Ω, C1 = 10 nF) is recommended. Figure 8. Application R1 C1 8 Amplifier with AGC 125 kHz ATA5283 5 RESET N_WAKEUP N_DATA Temp. Sensor Micro controller Central Board Controller Antenna Driver ATA5275 LA CA 1 ∫ Vref 7 6 2 3 4 Pressure Sensor UHF - Rx T5743 433 MHz UHF - Tx ATAR862 7 4598D–AUTO–03/04 Figure 9. Pin Connection and Pin Protection ATA5283 COIL_X 1 Divider impedance 143 kΩ ... 5 MΩ 8 VDD VDD VDD TST1 2 2k 7 N_WAKEUP VDD VDD VDD TST2 3 2k 6 N_DATA VDD VSS 4 1k 5 RESET Figure 10. Coil Input Impedance 10000 max. typ. min. Z