Raymarine Limited. Registered in England. Company no. 1177969
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Registered Office: Anchorage Park, Portsmouth P03 5TD
Seatalk Interface:
The interface that the system uses to communicate to the main processor is based on the Seatalk
interface that is widely used in Raymarine equipment. The hardware configuration has been based
very closely on that used on many other Raymarine equipment, with some slight resistor changes to
ensure correct operability with a 3.3V processor supply.
Backlighting.
Backlighting is split into 3 different sections, which can be independently controlled. The display
backlighting comprises 4 green LED’s situated behind the LCD display, the keypad backlighting
comprises 24 green LED’s and the backlighting for the distress button comprises a single green LED.
The output from the micro-controller is a PWM signal that adjusts the brightness of the LED’s. The
supply to the LED chains comes from the +12V(nom) battery supply, and so the circuit limits the
current flowing through the LED’s to a safe level if the signal from the micro-controller is at a
constant high level. Because of the constant current drivers, the LED’s do not change their brightness
appreciably when the battery supply fluctuates over the range 10.8 to 15.6 Volts. The PWM signal is
driven at approximately 20kHz to ensure that there are no audio artifacts from the backlighting
apparent in the handset speaker.
Microphone Pre-amplifier
The microphone pre-amplifier is a 2-stage circuit. The first stage amplifies the small signal levels
from the electret-condenser microphone (approx 4mV for 94dBA spl) to a level sufficient for the
second stage. The first stage also applies pre-emphasis to the signal as required for transmission of
audio signals over an FM radio link. The second stage comprises an automatic level control circuit
that soft-limits loud audio signals to prevent over-modulation of the output signal. RPT1 sets the gain
of the first stage to ensure uniform output signals for differing microphone characteristics. RPT2 is
used to set the level at which soft-limiting occurs. The gain of the first stage is relatively low to
ensure that the output of the first stage does not clip for large audio input signals.
Audio Amplifier.
The audio amplifier is a Philips TDA8552 1W Bridge tied load amplifier with integral volume
control. A single pin on the device is used to control the volume. The volume is adjustable in 64 steps
from –60dB to +20dB in 1.25dB increments. On power up, the amplifier volume control setting is set
at –20dB. A positive pulse on the volume control pin increments the volume by 1, whilst a negative
pulse on the volume control pin decrements the volume control by 1. The volume control is driven by
a single pin on the processor. The output of the amplifier is fed into an 8R 1W miniature 13mm
diameter speaker .
Key-beep.
A key-beep facility is provided, whereby every press of the keypad can be accompanied by a beep
from the handset speaker. Because of the integral volume control within the amplifier chip, the
volume of the key-beep is dependent on the setting of the volume control. The key-beep is provided
as a square-wave signal from the microprocessor pins at the frequency of the beep required.
Adjusting the mark-space ratio of the signal waveform will alter the apparent loudness of the signal.
In this way, the microprocessor can produce a key-beep that apparently sounds at the same set level
