4-14 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.5.2 Internal I
2
C Bus
The internal I
2
C bus consists of four lines: data, clock, interrupt and
data-line enable. The data and clock lines operate as an I
2
C bus; the data line
carries data and address bits serially, with their transfer being synchronised
by transitions on the clock line.
Data, clock and interrupt lines connect the microcontroller with 8-bit I/O
IC’s, IC3 and IC4. Each 8 bit I/O IC converts parallel data on its eight ports
to serial data for application to the bus and conversely, converts serial data
from the bus to parallel data on its ports. Each IC has a separate address set
by the connections to its address pins (1, 2 and 3). When a data transfer is
required, the microcontroller first outputs on the data line the address bits of
the required IC and follows this with a Read/Write command. Data is then
read from, or written to, the selected 8-bit IC.
When an input to any of the ports changes, the IC sends an interrupt to the
microcontroller. As all the interrupts are connected to a single input, the
microcontroller must interrogate each IC in turn until the changed data is
read. When this is done, the IC removes the interrupt and the
microcontroller can resume its other operations.
Part of IC3 is used for connection to selective call options. The remainder of
IC3 and all of IC4 carry data to and from various functions within the
transceiver.
The clock and data lines of the internal bus also connect the microcontroller
with IC5 and IC15 which operate as I
2
C bus receivers. Under control of the
data, each IC accepts serial bits from the bus. A set of bits consists of two
address bits and sixteen data bits. On completion of the transfer of a set, the
IC to which the set is addressed latches the data bits as 16-bit parallel data
on its outputs. These two IC’s provide 32 open drain outputs to various
circuits throughout the transceiver.
The microcontroller is connected via the data and clock lines to the non-
volatile memory, IC6. This stores the channel selection, volume control,
clarifier and scan channel settings. When power is switched off, the data is
retained in the memory by battery G1. Drain on the battery is very low,
therefore the battery life is limited mainly by its shelf life i.e. approximately
10 years. A discharged battery will cause the channel, volume, clarifier and
scan channel settings to require resetting each time the transceiver is
switched on, but will allow normal operation of all other facilities.
Also connected to the microcontroller via the bus are two EEPROMs, IC16
and IC17. These provide the non-volatile memory for assemblies 04–02451
to store data entered on the front panel pads. For the 04-03031 version of
assemblies, IC16 only is shown being of a different component type to the
earlier design.
The clock and data lines are also connected to the synthesizer circuits on the
RF, Mixer and Dual Synthesizer PCB via J4. Data for setting the two
synthesizer frequencies is sent over the data lines and on completion of the
transfer, the synthesizer latch line is pulsed.
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