## 2 Sensor Platform LSI Network System
### Sensor Platform LSI  
The sensor platform LSI was fabricated by a TSMC (Taiwan Semiconductor Manufacturing Co., Ltd., Hsinchu, Taiwan) 0.13 \(\upmu\)m standard CMOS process technology. As shown in Figure 3, the analog part includes a reset generator, a clock generator, capacitance-to-frequency converters, frequency counters, a resistance-to-voltage converter, an Analog-to-Digital Converter (ADC) and a  
Figure 2: Network system with 48 sensor platform LSIs.  
diode-based temperature sensor. The on-chip clock is generated by a ring oscillator circuit, and its typical frequency is 64 MHz in design (about 61 MHz in measurement). The sensor platform LSI operates according to this internal clock.  
The digital part includes main logic, data transmitting and command receiving circuits, and three function register blocks. The voltage levels of physical address pads can be adjusted to set the physical address of a specific sensor platform LSI. To relieve data collision and congestion, we implemented the collision avoidance, adaptation and event-driven functions [17].  
The nonvolatile One-Time Programmable (OTP) memory stores the configuration information of the sensor platform LSI such as its own ID number, threshold values and sampling time. However, we did not use the OTP memory in this study. Thus, in the network system, there was no 5.9 V (VDD59) power line that was used for writing the OTP memory.  
Each sensor platform LSI can be configured to operate in either capacitive or resistive sensing mode. In capacitive sensing mode, the sensor platform LSI processes signals from eight capacitance channels. The capacitance readout circuit is composed of a capacitance-to-frequency converter and a frequency counter. For precise sensing and fast processing, eight sets of the converter and counter were used for eight capacitance channels, which operate simultaneously. Each capacitance channel can connect a capacitive sensor with a capacitance ranging from 1 pF to 10 pF [15]. The typical frequency resolution of the capacitance-to-frequency converter is 56.6 kHz/ff at 1 pF, 3.8 kHz/ff at 5 pF, 1.0 kHz/ff at 10 pF. The capacitance of our MEMS tactile sensor [18] ranges from 0.9 pF to 1.6 pF, in which the frequency resolution is high. The bus communication speed can be configured to be 1 MHz, 2 MHz, 4 MHz, 8 MHz, 16 MHz or 32 MHz by dividing the on-chip clock (64 MHz in design) for different types of data receiving devices.  
For signal transmitting method, there are single-ended and differential signaling. The latter, which is employed by standards like Low-Voltage Differential Signaling (LVDS), Current Mode Logic (CML), RS485 and so on, is more robust against electrical noise than the former. However, the current sensor platform LSI has adopted a single-ended signaling I/O cell for simplicity.  
For asynchronous communication with the relay node, we used Non-Return-to-Zero Inverted (NRZI) encoding to map a binary signal to a physical signal and 4B5B to avoid a long series of zeros. We also used unique data and command packet formats with variable length. The asynchronous serial bus communication between the sensor platform LSI and the relay node was supported by the delay  
Figure 3: Block diagram of the sensor platform LSI.  
window blind oversampling clock and data recovery algorithm [19; 20]. In the following subsections, the functions that are implemented in the sensor platform LSI and important in this study, i.e., collision avoidance, adaptation function, event-driven function and configuration, are explained.