Mallinckrodt NELLCOR NPB-4000, Service Manual

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Section 14: Main Color Board Digital Theory of Operation
14-35
14.17.20 SPEAKER HIGH AND LOW CONTROL
The speaker requires tones from about 300 Hz to 1 kHz with a 55 percent duty
cycle. In order to give software full control over both the frequency and duty
cycle, there are 2 software programmable 8 bit up counters, one for generating
the low portion of the TONE_OUT and one for generating the high portion. The
TONE_OUT flip flop is jammed reset when the FREQ_GO bit in the control
register is low. This enables the SPEAKER LOW counter. The software
programs the high and low values into the respective holding registers, then sets
the FREQ_GO bit. The TONE_OUT on pin 93 is low, and remains low until
the SPEAKER LOW counter overflows, which sets the TONE_OUT high and
loads the SPEAKER HIGH counter. This counter now increments until it
overflows, which now sets the TONE_OUT signal low and reloads the
SPEAKER LOW counter. This process continues until the software turns off the
TONE_OUT signal by resetting the FREQ_GO bit in the control register.
14.17.21 CONTROL REGISTER
The control register is an 8 bit register with miscellaneous programmable control
bits. These 8 bits are programmable and can be read back via the read back
multiplexer.
14.17.21.1 PUMP_PWM_GO and VALVE_PWM_GO BITS
The PUMP_PWM_GO is bit 0 and VALVE_PWM_GO is bit 1. These bits have
been mentioned above and are used to control the pump control signal for
inflating the cuff and the valve control signal for deflating the cuff.
14.17.21.2 CLK_FREQ_SEL CONTROL BIT
Bit 2 is the CLK_FREQ_SEL bit which selects the clock frequency for the
speaker counting registers. When low 156 kHz is selected and when high 313
kHz is selected. The 156 kHz is required so we can generate the low tones with
the two 8 bit counters.
14.17.21.3 FREQ_GO BIT
The FREQ_GO is bit 3 and is used to control the TONE_OUT signal to the
speaker. Setting this bit to a high enables the TONE_OUT and setting it low,
disables the TONE_OUT.
14.17.21.4 BCK_LITE_ON
This is bit 4 and is used to turn the LCD display CCF (Cold Cathode
Fluorescent) backlight ON and OFF. Setting this bit high turns the backlight on
and setting this bit low turns the backlight off.