Figure 1 shows the interface between MSP430F2274 and TMS37157. The TMS37157 is connected to the MSP430F2274 through a 3-wire SPI interface. To simplify communication between the MSP430F2274 and TMS37157, the BUSY pin of the TMS37157 is connected to the MSP430. The BUSY pin indicates the readiness of the TMS37157 to receive the next data byte from the MSP430F2274. The PUSH pin is used to wake up the PaLFI from standby mode so that the MSP430F2274 can access the EEPROM of the PaLFI. CLKAM is used for the antenna automatic tune feature of the PaLFI target board.
Interface Between MSP430F2274 and SHT21
Figure 2 shows the interface between MSP430F2274 and SHT21. I2C is used to connect both devices. The MSP430F2274 contains two communication modules. One is used as a UART connection to a host PC, the other one is used to communicate to the TMS37157. Therefore, the I2C interface has been implemented completely in the software.
Hardware Changes to Original PaLFI Board
Several changes were made to the standard PaLFI board to implement the wireless sensor application. The most important change is to use an external DC/DC converter attached to VCL to generate a VBAT/VCC voltage out of the 134.2-kHz RF field. Figure 3 shows the basic principle of this circuit.
The input of the dc/dc converter TPS71433 is connected to VCL via diode D1. D1 prevents the resonance circuit (consisting of LR and CR ) from any disturbances coming from the dc/dc converter. Capacitor CBAT stores the energy derived from the RF field.
Using an external dc/dc converter instead of the internal of the TMS37157 overcome two issues. The first advantage of an external dc/dc converter is that it can provide higher output currents in comparison to the internal regulator (80 mA compared to 5 mA). The second advantage of using an external regulator is the simpler flow for the application and the firmware (see Table 1).