Cobra MRF55 Theory Of Operation

• Functional Description
The Cobra models MRF55 and MRF75 are Marine band FM Radio transceivers operated in the
frequency range 156.025MHz to 163.275MHz with WX and DSC. Functional as follows:
1. MRF55: Channel Up / Down, Emergency channel key CH16/9, WX/UIC key, Channel Scan key, TX
power HI/Low key, Dual watch channel key, Call/set key, Distress key, Squelch control volume, DC power
ON/Off and Audio Volume control, External Speaker Jack and GPS Jack, PTT key.
2. MRF75: Channel Up / Down, Emergency channel key CH16/9, WX/UIC key, MEMO Channel Scan key,
TX power HI/Low key, Tri watch channel key, Call/set key, Distress key, Squelch control volume, DC
power ON/Off and Audio Volume Up/Down control, PA Function, External Speaker & PA Jack and GPS
Jack, PTT key.
• MCU Description
The MCU uses Toshiba’s TMP86CM47U micro-controller, there are 32k X 8bits ROM and 1k X
8bits RAM, LQFP44 pins package. These pin assignment as below table:
PIN Description Model PIN Description Model
1 Vss = GND 55/75 23 Control LCD SDI data pin 55/75
2 X-in, crystal 8 MHz 55/75 24 WX alert data pin detect 55/75
3 X-out, crystal 8 MHz 55/75 25 TX Hi / low power control 55/75
4 Test, connect to GND. 55/75 26 Keys scan pin 55/75
5 Vdd, +5VDC input 55/75 27 Keys scan pin 55/75
6 DC power on control F75 28 Beep tone Hi/low control 55/75
7 TX on/off control (on = low) 55/75 29 Digital volume data pin F75
8 Reset pin 55/75 30 Digital volume clock pin F75
9 DC power on key scan F75 31 Open 55/75
10 LCD backlight control 55/75 32 EEprom clock pin 55/75
11 LCD backlight control 55/75 33 RX/TX signal indication 55/75
12 NMEA (GPS data) input 55/75 34 Pull high to +5VDC 55/75
13 EEprom data pin 55/75 35 AVDD (connect +5VDC) 55/75
14 Volume down key scan F75 36 AVSS (connect to GND) 55/75
15 Volume up key scan F75 37
16 Squelch output control 55/75 38
17 Inhibit audio when DSC on 55/75 39
18
Control LCD driver A0 data pin
55/75 40
19 PLL STB. HI=F75, Low=F55 55/75 41
Pin 37 to 41 for 1300Hz and
2100Hz FSK generator. 55/75
20 PLL clock data output 55/75 42 PLL data output 55/75
21 Beep tone generator 55/75 43 DSC detect pin 55/75
22 Control LCD CS1 pin 55/75 44 Squelch input detect 55/75
Remarks: MCU pin19 (BUS14) for MRF55 and MRF75 models select by resistor R371 and
R340 control. R371 open for MRF55 model, R340 open for MRF75 model selection.

Cobra MRF55 Specifications

General

Model	MRF55
GPS	No
NOAA Weather Channels	Yes
Instant Channel 16/9	Yes
Voltage Range	13.8 VDC
Type	Marine Radio
Frequency Range	156.025-163.275 MHz
Power Output	25 Watts
Display	LCD