CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 435

value by the corresponding A/D associated to the shock-box microcontroller. Impedance
limit values are determined within the shock-box microcontroller through a preloaded
lookup table.
To conserve power, the impedance-measurement circuit is powered only when neces-
sary. This is done by supplying the circuit through a switch (MOSFET Q20 in the battery-
charging circuit) whenever deemed necessary by the shock-box controller (through the line

5V_ENABLE). The input of the impedance measurement circuit is connected to the
de

brillation electrode terminals by way of a self-activated isolation circuit. A solid-state
switching circuit that reacts to current

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fl flow protects both the impedance measurement cir-

cuit from de brillation pulses as well as the patient from faults in the impedance meas-

urement circuit.
Referring to the upper half of the circuit of Figure 8.47, the gates of FETs Q29 and Q30
are held at a positive voltage in excess of the conduction threshold by the

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5-V source
through resistor R63. This allows impedance measurement to take place via these FETs
and resistor R70. However, when a de brillation pulse is applied, current increases

through R70 (shunted at the impedance measurement circuit by the back-to-back zener
diodes), and depending on the polarity of the pulse, one of the bipolar transistors (Q7 or
Q28) starts to conduct, placing its associated FET in the high-impedance state.
The bipolar transistor remains conductive until the voltage drops below a safe level for
the impedance measurement circuit, at which time the low-impedance path is reestablished
through the FET. The lower portion of the circuit in the

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figure is essentially the same as
that described above. This portion is used to protect the second terminal of the impedance-
measurement circuit from the high-voltage de

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fi brillation pulses.

Firmware for the Shock Box

The embedded software source code listing includes comments, to make it easy to under-
stand its operation. It is supplied in the topic's ftp site as SHOCKBOX.ZIP. The following
are short descriptions of the modules implemented by this software.

GUI Command Processor This module processes commands sent by the programming
GUI computer to the shock-box microcontroller. There are 15 commands that can be sent
by the programming GUI computer to the shock box via RS232. These commands are sent
in frames with the following format:

Command Number Data Count Data LSB Checksum MSB Checksum

where Command Number is one of 15 possible commands (see the code for a list of com-
mands) Data Count is the number of data bytes in the parameter data message to follow,
Data the Parameter data, LSB Checksum the least-signi

fi cant byte of the 16-bit checksum,

and MSB Checksum the most-signi cant byte of the 16-bit checksum. Every time the

module executes an accepted command, it either echoes back the GUI command (when-
ever data are not expected back as a response for the command), or sends the command
number and the data expected with the same format as for GUI to shock-box communica-
tion (where the data are not a command parameter but rather, the response).

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Main Wake-up Module This module wakes the processor up every 250 ms. Upon wake-

up, it does the following:

1. Checks if front-panel switches (manual charge or manual de fi brillate) have been

pressed.