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System Interrupt Timer
Acquisition Module Interface
2-22
Equipment Overview: Theory of Operation
A 1KHz timer generates system interrupts (which may be routed to FIQ or IRQ)
once every millisecond. This interrupt provides the foundation for all operating
system timers.
Overview
The MAC 5500 acquisition module communication protocol is different from
previous generations in several key respects:
1.
Acquisition module timing is synchronized to the system.
There is no longer a need to play synchronizing games to get the system
(especially the display and printer) operating at the same sampling rate as the
acquisition module.
2.
Data is framed and has checksum.
Previous acquisition modules offered rudimentary error detection. This has
finally been done nearly right. Each ECG data packet contains a checksum.
3.
Commands do not interrupt the data stream.
Previous generation acquisition modules required a cessation of sampling to
transmit commands to the module. This cessation of sampling had the
undesirable effect of breaking the acquisition stream for operations as simple as
changing the line filter frequency or enabling or disabling the pace pulse
detector. With the MAC 5500 this restriction is removed.
4.
Buttons are supported.
Button state is communicated to the system in each ECG data packet. This
allows limited operator interaction with the machine via the acquisition module.
Details
A constant reference clock frequency of 1MHz must be provided to the acquisition
module for generation of its internal sampling clocks. To eliminate the need for data
lines, command information is encoded on this reference clock by altering its duty
cycle. The FPGA provides a serializer for the command bytes and clock generator/
modulator to transmit both the clock and command bits from the serializer. The
reference clock duty cycle is nominally 50%. By altering the duty cycle, the DC
content of the clock is changed. The acquisition module detects this change in DC
level. The timing of these shifts in DC offset encode command data bits. A zero is
encoded as a single shift in duty cycle from 50% to 25% lasting 31.25µs, followed
by a refractory period of 468.8µs. A one is encoded as a pair of 31.25µs periods of
25% duty cycle separated by 93.75µs, followed by a 343.8µs refractory period. In
either case the transmission of a single bit takes 500µs. A higher level protocol
organizes commands as groups of 8 bits.
Data from the acquisition module is packed into 257 bit NRZ frames. The receive
line idle state is high. The first bit of each packet is a zero and serves as the packet
start bit. As with a UART, the start bit is discarded. The following 256 bits are
received into a 16-word x 16-bit buffer for use by the ATMEL CPU. The receive
logic then looks for an idle period (analogous to a UART stop bit) of at least 125µs
in length as an indicator that the link is again idle. Special marker words are inserted
into the ECG data packet (words 5, 10 and 15) to guarantee there will never be a run
MAC 5500 resting ECG analysis system
2020299-020
Revision E
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