FAST PROTOTYPING OF
IMPLANTABLE SYSTEMS FOR STIMULATION AND RECORDING BASED ON A BI-DIRECTIONAL AND
RF POWERED TELEMETRY INTEGRATED CIRCUIT
David
Marín*, Oliver Scholz**,
Jordi Parramon*,
Tere Oses*, Jörg-Uwe Meyer**, Elena Valderrama*
* Centro Nacional
de Microelectrónica (CNM-IMB), Universitat
Autònoma de Barcelona, Bellaterra,
Spain.
** Fraunhofer-Institut für Biomedizinische Technik (IBMT), St. Ingbert,
Germany.
dmarin@cnm.es
SUMMARY
The use of
implantable systems is required for biomedical studies about neuromuscular and
functional electrical stimulation and biosignal
recording. Such systems have to send and receive information from external
units that manage and store the data controlling the stimulation, the recorded
data and the status of the implanted device. A new RF powered telemetric
system, incorporating a Telemetric IC (TIC) and one or more application
specific chips, is presented and described in this paper. The main objective of the
former is to provide multiple methods of communication between the specific
application circuitry of the implant and the external unit. It also generates
the regulated supply voltage for the rest of the implanted unit and provides a
controlled global reset of the system.
The TIC system
offers a wide range of possibilities to be connected to common microcontrollers
(e.g. Microchip’s PIC controllers) using different bus architectures such as
serial or parallel. Also, the TIC provides a number of different transmitters to
choose from in order to transmit the data from the implant using different carrier frequency and
modulation schemes. In-link and Out-link data transmission channels allow a
bi-directional transmission with bit rates higher than 100Kbps. The TIC
practically does not
depend on the microcontroller or ASIC used which can focus on the
stimulation/recording process. Two initial applications of the TIC system are
presented: a stimulation system for the use in neural prostheses and a 5
channel micro-stimulator.
STATE OF THE ART
The initial stages
of biomedical studies requiring implantable systems for neuromuscular
electrical stimulation and biosignal recording often
have a certain number of non well-known characteristics: the definition of
stimulation waveforms, the control of the stimulation/recording process and the
number of channels required. In these cases, the use of microcontrollers within
the implanted stimulation unit is a good solution in the early stages for the
digital control because of their flexibility and low cost. The main effort can
thus be centred on the development of the customised application circuitry. A
full duplex telemetry system with an adequate bi-directional bit rate and an interface for
various microcontrollers or ASICs could be used
during both initial and final stages of prototyping. This way, the effort to control the telemetry
can be reduced: demodulation and decoding to receive data and coding and
modulation to send the data.
MATERIAL AND METHODS
The designed
telemetry unit (TIC system) becomes a complete interface between the external
unit and the implantable application unit. The TIC was designed to be well
adapted to PIC microcontrollers. As a consequence, the final result is a common
architecture which allows easy connection to other devices using multiple
methods. In any case, this paper refers to PIC as a generic name for the
customised part of the application, which provides or uses information to be
transmitted or received from the ETU. This section describes the main blocks
and modules of the TIC, giving detailed information on its configuration.
An overview about
the complete TIC system is shown in figure 1. The external transceiver unit
(ETU) is based on a high efficient class-E driver generating a carrier frequency between 6 MHz to 12 MHz,
depending on the application, providing enough energy to power the implanted
system /1-2/. An accurate coil design maximises the coupling coefficient for
the range of distance between the coils. The antenna system has to be tolerant
to lateral and angular misalignment, keeping the Bit Error Rate (BER) as low as
possible but taking into account the restrictions imposed by the maximum
dimensions of the coils and data bandwidth /3-4/[OS1]. Data from the ETU
to the implant is provided by means of amplitude shift keying the class-E
driver supply voltage. Information is restored by passing on the received
signal to the TIC’s onchip
envelope detector, filtering stage and decoder. On the other side, the TIC
transmits data to the external receiver by using BPSK or OOK. Special effort
was done to minimise the crosstalk between the In-link (ETU to TIC) and the
Out-link (TIC to ETU).
|
|
Figure1: TIC scheme
TIC configuration
and global reset definition
Data from the ETU
is coded using a variable duty cycle to determine the changes of the ASK
signal. A duration about 14.8µs and duty cycle of
3.2µs defines the global reset of the TIC, which can be used to reset the rest
of the implanted system. The start up sequence of the TIC is defined by the
initial reset (external reset) and 5 bits (Prog1 to Prog5) which program the TIC’s configuration register. Figure 2.
shows an example of start up sequence and table I.
shows the data flow for the different possible transfers from the ETU to the
PIC. The data bit rate for the Out-link channel is configurable using Prog2 bit
selecting 234 Kbps or 460 Kbps. The TIC generates a system clock with
frequencies of 234KHz, 468KHz, 3.75MHz or 7.5MHz
(using Prog4 and Prog4 bits).
Two additional
lines (CS1 and CS2) manage the information through the different busses of the
TIC system. These lines can be set statically if only one communication channel
is used.
Table I: Main data
flow programming
|
CS1 |
CS2 |
Prog1 |
Data flow |
Comment |
|
0 |
0 |
X |
ETU Þ TIC Þ PIC |
Data reception by the 4 bits
parallel bus |
|
0 |
1 |
X |
PIC Þ TIC Þ 4bits
Command |
Control another
device using 4 lines |
|
1 |
0 |
X |
PIC Þ TIC Þ ETU |
Data transmission
using the 4 bits parallel bus |
|
1 |
1 |
X |
PIC Þ
TIC Þ 8bits
parallel Output |
Parallel data
transfer from PIC |
|
X |
X |
0 |
Serial Þ TIC Þ ETU |
Data transmission
serial |
|
X |
X |
1 |
ETU Þ TIC Þ Serial |
Data reception
serial |
|
|
Figure1: Start up
sequence and first data frame.
- Data frame
definition: Each data frame is defined by an initial symbol called internal
reset which is succeeded by eight data bits. The internal reset is defined with
a duration of about 12µs and a duty cycle of 3.2µs.
The data is sent to the controller by using either the serial or parallel bus,
depending on the programming and the state of the CS lines.
- Power supply
regulation system: The TIC provides a stable 5V based on an internal
bandgap voltage reference.
- Transmitters:
Seven different transmitters are included into the TIC in order to use the most
adequate in each application. Only one of them can be used at a time by
hardwiring. Table II summarises the characteristics of each transmitter.
Table II: Transmitter
description.
|
Power supply pad |
Modulation scheme |
Carrier freq. |
Notes |
|
Vdd1 |
BPSK |
20 MHz |
|
|
Vdd2 |
BPSK |
25 MHz |
|
|
Vdd3 |
BPSK |
30 MHz |
|
|
Vdd4 |
BPSK |
30 MHz |
*manages two
times more current than BPSK(Vdd3). |
|
Vdd5 |
BPSK |
40 MHz |
|
|
Vdd6 |
OOK |
25 MHz |
|
|
Vdd7 |
LC ** |
- |
**Peak generation
at the free frequency fixed by the LC tuned circuit. |
Receiver: A single ASK
demodulation scheme based on filtering and triggering of the RF incoming signal
is used to provide a digital signal to the decode circuitry.
RESULTS
Current
applications
At present, the TIC
system is working in some applications where a generic telemetric system is
required. The TIC provides the interface between an external neural prosthesis
control unit and the implantable stimulator/recorder system /5/. In this
application, the internal control unit is a PIC16C71, which manages the TIC. Photos
1 and 2 show the top and bottom of a [OS2]PCB carrying the
TIC and PIC, both as die, and the transmitter coil using the BPSK-30MHz. The
communication between the TIC and the controller is provided by the 4 bit
parallel bus. The system clock, too, is generated by TIC. The external reset
line is used to initialise the microcontroller. Another application for the TIC
system is the control of a five-channel micro-stimulator. In this case, only
serial communication between the TIC and PIC is possible because of the number
of signals that have to be controlled by the microcontroller: amplitude and
duration of pulse, charge recovery and channel selection.
