A Low-Power Remotely-Programmable MCU for Implantable Medical Devices Xiaoyu Zhang 1 , Hanjun Jiang 2 , Binjie Zhu 2 , Xinkai Chen 3 , Chun Zhang 2 , Zhihua Wang 2 1 Department of Electronic Engineering 2 Institute of Microelectronics Tsinghua University, Beijing, China 3 Ecore Technologies Ltd., Beijing, China Email: zhangxiaoyu00@mails.tsinghua.edu.cn Abstract-This paper presents a low-power MCU with remotely- programmable feature which is specifically optimized for implantable medical devices (IMDs). In medical applications the most critical requirements are low-power, energy-efficient, flexible, etc., which are provided by utilizing the techniques such as clock gating, power gating, instruction set improvement, DMA optimizations for data paths. The MCU has been verified correctly and fabricated with a scale of 79.1K equivalent gates, in standard 0.18μm CMOS technology. I. INTRODUCTION For decades researchers have made great progresses in implantable medical devices (IMD) development. Due to the resource-strictly-constrained application environment, the IMDs are always battery-powered and capsulated in a miniature package, avoiding any wire lines (fibers, cables, etc.). As a consequence, low power and high energy efficiency are required exactly in IMDs. [1] Generally, the IMDs can be divided into 2 categories due to different purposes: - “Monitoring”: these IMDs usually (continuously) collect body information as vital signs. They perform the potential application procedure “sensing - processing - radio” during data / information gathering. - “Stimulus”: these IMDs usually give bio-electrical pulse-stimulus or perform drug-delivery. The preferred procedure is usually “radio - parsing - stimulating”. To achieve the best energy utilization from batteries, module-level co-operations of the whole IMD system must be optimized. All these sub-modules are controlled by the central micro control unit (MCU), which schedules each task from both software level and hardware level. For example, according to the general control flow of IMDs, data path and control path should be elaborated specifically to satisfy low- power and adequate performance. For either purpose, energy consumed in the radio phase holds a largest percentage in total. To improve efficiency, it demands not only a low-power MCU core to perform good control flow and communication flow, but also a hardware medium access controller (MAC), which can accelerates link operations and reduce energy wastage (e.g. RF is on but no data to transmit). [2][3] Additionally, to improve performance of critical operations, modifications in instruction set (e.g. for DMA & I/Os) can bring benefits of code compression and accelerated execution. Also, mass data transferring will not need MCU to execute so many operations as previously. Instead, the MCU can stay in idle state while mass data accessing. Remote re-programmable feature is also practical and necessary for IMDs. Wireless field programming arbiter (WFPA) supports remotely on-line configuration, including SFRs and program memory. This provides much flexibility for the IMDs which need software (program) updating after being installed. There are some additional requirements that the implementation contains multi-mode peripheral interface including I 2 C, SPI, and general purpose parallel ports for I/O flexibility. The interrupt controller must be elaborated to ensure the robustness when exception occurs. Since security is notso strict in IMDs, we can implement simple security algorithm in MCU program. This paper is organized as follows. Section II discusses general control flows of IMDs, including communication flows. Section III presents the MCU architecture and implementations of detailed modules. Section IV gives some results and finally the conclusions. II. GENERAL CONTROL FLOWS OF IMDS Generally, the IMDs of monitoring functions have different control flows from those of stimulus functions. Figure 1 shows both control flows. For common cases, we adopt half-duplex channel here for the data communication link. [3][4] There are many kinds of sensor (or transducer) devices for IMDs. For example, we can use single lead electrode to collect heartbeat and ECG/EKG information, only a thermistor for temperature sensing, wheatstone bridge to measure pressure, amperometric sensors for glucose or blood pH, or CMOS image sensor for intestinal examination, etc. 978-1-4244-7456-1/10/$26.00 ©2010 IEEE 28