Ambulatory Blood Pressure Monitor 90217 Service Manual
3-7
Theory
+5 VA Supply
+5 VA is a secondary power source for the analog circuitry. It can be disconnected to reduce
power consumption when the analog circuits are not being used by asserting the PWR_DWN line.
CONV_VREF
The converter reference (CONV_VREF) is created by a band gap reference (about 1.25 volts) to
produce +5 volts (adjusted to + 5 volts) at the +5 VA supply.
Backup Battery Circuit
A 3-volt rechargeable lithium cell provides backup power to the clock and RAM circuits when the
main batteries are removed or exhausted. This lithium battery receives its charge from the three
AA batteries and has a 20 - 35 ma/hrs capacity with sufficient charge to last approximately
4 months without the main batteries installed.
Main Battery
The main battery voltage is monitored by reading the BAT VOLTS line with the A/D converter. The
voltage at this point is a divided down sample of the +VSW voltage.
Wake-up Interrupt
The power converter is activated when one of its three wake-up sources generate an interrupt and
trips the 5 msec one-shot. The microprocessor must assert the PWR_HI line during this 5 msec
period to place the power converter in its high power mode.
Three sources can place the power converter into high power mode:
• An RS-232 serial cable is connected
This cable connect is detected when the reed switch is activated by an external magnet. When
the reed switch closes, it causes an interrupt to the processor on the WAKE_UP line.
• A clock wake-up
The real-time clock produces an output that changes state every 30 seconds. These
transitional states are shaped into pulses that create interrupts to the processor.
• The START/STOP button is pressed
When the START/STOP button is pressed, the WAKE_UP line is asserted.
Once an interrupt request has gone to the processor by asserting the WAKE_UP line, the
processor looks to see which of the three sources asserted the WAKE_UP line.
Power Switch
When the power switch is ON (closed), the +VSW supply comes up and provides power to all the
circuitry in the ABP unit. The +VBB supply receives power directly from the main batteries to
ensure that the lithium battery will not be drained when main batteries are available.
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