Comparison of single ended and differential signalling for wired biomedical implants using SPI communication with Reed Solomon Error Correction codes

Abstract

For an implantable system for the recording of brain signals from neonatal mice the design specifications for the weight and the size of a implantable system are very tough. The animals are very small and light weight (1–3 cm, 2– 3 g) and the recording data rate is very high (3,5 Mbit/s). Thus, the system has to be extremely small. With state of the art technique it is not possible to set up a wireless implantable system that is suitable for a neonatal mouse. Thus a wired system is developed. For the wired system the connector is a size limiting factor. In wired transmission systems single ended and differential signalling are available. Differential signalling is more robust against noise disturbances, single ended transmission is beneficial with respect to a minimum number of wires and chip area. A detailed comparison of the suitability of both transmission types for wired implantable systems has been performed. A Serial Peripheral Interface connection with Reed Solomon Encoder connection has been implemented. Reed Solomon Error Correction is used to correct the errors occurring on the wired transmission line. Measurements of data rate and error rate for single ended and differential signalling have been performed for long cables (up to 1.8 m). It could be shown that single ended transmission is favourable for the desired application. For the detection and correction of errors occurring on high speed Serial Peripheral Interface Reed Solomon decoding on FPGA was used. This particular decoder design has capability of correcting up to 2 symbol errors on a packet of data composed of 9 symbols where each symbol is 4 bits long. Complete error correction takes about 65 clock cycles on a speed up to 100 MHz.