The Design of a Bi-directional, RFID-based ASIC for Interfacing with SPI Bus Peripherals

The design of a mixed-signal Integrated Circuit (IC) for data logging and wireless telemetry applications is presented. The 3.3mm x 3.3mm IC is integrated into a RFID transponder along with a MEMS pressure sensor and FRAM memory. The sensor and memory ICs are independently addressable via the serial peripheral interface (SPI) standard. The IC was designed for fabrication using a 0.5-mu m CMOS process and implements a RF front-end, SPI Master and digital Finite State Machine (FSM). The wireless front-end operates via a 13.56MHz inductive coupled system for power transfer and bi-directional data transmission to a reader and has on-chip voltage-regulation, data recovery, and load modulation circuits. The digital logic implements a custom developed wireless SPI protocol, allowing for direct communication with SPI peripherals via the RF front-end. Additionally, a digital FSM is responsible for data logging operation, in which pressure data is autonomously measured and stored in memory over constant intervals. Test results are presented for an independently fabricated digital circuit consisting of 4,014 equivalent 2-input NOR gates with a measured current consumption of 12 mu A at V(dd)=2.8V and F(CLK)=140kHz.