Sbep Serial Interface; Keypad Keys - Motorola CDM Series Service Manual

Chapter 2: 2-2
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 µP. They are labeled within the device
block as PE0-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 A/D ports on the µP. If this voltage is lower than 5V the
A/D reading is incorrect. The VRL signal is the low reference for the A/D ports. This line is normally
tied to ground. If this line is not connected to ground, the A/D readings could be incorrect.
The µP determines the used keypad type and the control head ID by reading the levels at ports PC0
– PC7. Connections JU0852/3/4 are provided by the individual keypads.
The MODB / MODA input of the µP must be at a logic "1" 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 0V if the regulated 5V goes below
4.5 V. This is used to reset the controller to prevent improper operation.
2.4

SBEP Serial Interface

The host radio (master) communicates to the control head µP (slave) through its SBEP bus. This
bus uses only line BUS+ for data transfer. The line is bi-directional, that is, either the radio or the
control head µP can drive the line. The µP sends serial data via pin 50 and D0831 and it reads serial
data via pin 47. Whenever the µP detects activity on the BUS+ line, it starts communication.
When the host radio needs to communicate to the control head µP, it sends data via line BUS+. Any
transition on this line generates an interrupt and the µP starts communication. The host radio may
send data (for example, LED and back light status) or it may request the control head ID or the
keypad ID.
When the control head µP wants to communicate to the host radio, the µP brings the request line
CH REQUEST to a logic "0" via µP pin 62. This switches Q0823 on, which pulls line ON OFF
CONTROL high through diode D0821. A low-to-high transition on this line informs the radio that the
control head requires service. The host radio then sends a data request message via BUS+ and the
control head µP replies with the data it wants to send (for example, which key has been pressed or
that the volume knob has been rotated).
The control head µP monitors all messages sent via BUS+, but ignores any data communication
between the host radio and the CPS or the universal tuner.
2.5

Keypad Keys

The control head keypad is a six-key design. All keys are configured as two analog lines read by µP
pins 13 and 15. The voltage on the analog lines varies between 0V and 5V depending on which key
is pressed. If no key is pressed, the voltage at both lines is 5V. The key configuration can be thought
of as a matrix where the two lines represent one row and one column. Each line is connected to a
resistive divider powered by 5V. If a button is pressed, it will connect one specific resistor of each
divider line to ground level and thereby reduce the voltages on the analog lines The voltages of the
lines are A/D converted inside the µP (ports PE 0 - 1) and specify the pressed button. To determine
which key is pressed, the voltage of both lines must be considered.
An additional pair of analog lines and A/D µP ports (PE 3 – 2) are available to support a keypad
microphone, connected to the microphone connector J0811. Any microphone key press is
processed the same way as a key press on a control head.
6881091C63-A
Control Head (CDM750 / PRO3100)