Power On I Off; Microprocessor Circuit - Motorola CDM1550 LS+ Detailed Service Manual

2-32
Theory of Operation
2.1 0.2 Power On I Off
The on/off/volume knob when pressed switches the radio's voltage regulators on by connecting line
ON OFF CONTROL to line UNSW 5V via D0821 . Additionally, 5 volts at the base of digital transistor
Q0822 informs the control head's IJP about the pressed knob. The IJP asserts pin 62 and line CH
REQUEST low to hold line ON OFF CONTROL at 5 volts via Q0823 and D0821. The high line ON
OFF CONTROL also informs the host radio, that the control head 's IJP wants to send data via SBEP
bus. When the radio returns a data request message, the IJP informs the radio about the pressed
knob. If the radio was switched off, the radio's IJP switches it on and vice versa . If the on/off/volume
knob is pressed while the radio is on, the software detects a low state on line ON OFF SENSE, the
radio is alerted via line ON OFF CONTROL and sends a data request message. The control head
IJP will inform the radio about the pressed knob and the radio's IJP switches the radio off.
2.1
0.3
Microprocessor Circuit
The control head uses the Motorola 68HC11 E9 microprocessor (IJP) (U0831) to control the LED's
and the LCD and to communicate with the host radio. RAM and ROM are contained within the IJP
itself.
The IJP generates it's clock using the oscillator inside the IJP along with a 8 MHz ceramic resonator
(U0833) and R0920.
The tJP's RAM is always powered to maintain parameters such as the last operating mode. This is
achieved by maintaining 5 volts at IJP, pin 25. Under normal conditions, when the radio is off, USW
5V is formed by FLT A+ running to D0822. C0822 allows the battery voltage to be disconnected for
a couple of seconds without losing RAM parameters. Diode D0822 prevents radio circuits from
discharging this capacitor.
There are eight analog-to-digital converter ports (A/D) on the !Jp. They are labeled within the device
block as PEO-PE7. These lines sense the voltage level ranging from 0 to 5V of the input line and
convert that level to a number ranging from 0 to 255 which can be read by the software to take
appropriate action .
Pin VRH is the high reference voltage for the AID ports on the IJP. If this voltage is lower than +5V
the AID reading is incorrect. Likewise pin VRL is the low reference for the AID ports. This line is
normally tied to ground. If this line is not connected to ground, the AID readings could be incorrect.
The IJP determines the used keypad type and the control head ID by reading the levels at ports PCO
-PC?. Connections JU0852/3/4 are provided by the individual keypads.
The MODB
I
MODA input of the IJP must be at a logic 1 for it to start executing correctly. The XIRQ
and the IRQ pins should also be at a logic 1.
Voltage sense device U0832 provides a reset output that goes to 0 volts if the regulated 5 volts goes
below 4.5 volts. This is used to reset the controller to prevent improper operation.
2.1 0.4 SBEP Serial Interface
The host radio (master) communicates to the control head IJP (slave) through its SBEP bus. This
bus uses only line BUS+ for data transfer. The line is bi-directional, meaning that either the radio or
the control head IJP can drive the line. The IJP sends serial data via pin 50 and D0831 and it reads
serial data via pin 47. Whenever the IJP detects activity on the BUS+ line, it starts communication.
When the host radio needs to communicate to the control head IJP, it sends data via line BUS+ . Any
transition on this line generates an interrupt and the IJP starts communication. The host radio may
send data like display information , LED and back light status or it may request the control head ID or
the keypad ID.