Ericsson EDACS Monogram Series, Руководство по обслуживанию

Страница: 23 / 56

AE/LZB 119 1647 R1A
23
•
11.0592 MHz clock generation for itself, the micro
controller and the ASP.
High-Speed Data Conversions
Modem U702 converts high-speed EDACS data
between parallel and serial formats. The high-speed rate is
9600 baud for the 800 MHz radio.
When the radio is receiving high-speed data, U702
converts this serial data to parallel data that can be handled
by the micro controller. Limited high-speed data from the
ASP (Audio Signal Processor) U804 feeds U702 pin 23.
When the radio is transmitting high-speed data, the
modem converts the parallel data from the micro controller
to serial high-speed data the can modulate the transmitter.
This data is routed to the transmit audio portion of the ASP
via U702 pin 26.
An interrupt output from U702 pin 32 signals U701 pin
34 that the modem is ready for the next transmit or receive
byte.
Address Demultiplexing
The micro controller has a multiplexed 8-bit wide
address/data bus that transfers both the lower eight address
lines and the 8-bit bi-directional data. Before the micro
controller can read or write data, the lower address byte
must first be externally latched and applied to the memory
chips. Modem U702 contains an 8-bit latch which provides
this demultiplexing function.
The micro controller's ALE output line is applied to
U702 pin 12. The lower address byte (A0-A7) is latched
when the ALE line transitions from high to low. The latched
byte is applied to U703 and U707 via the eight outputs(A0-
A7) from U702.
Address Decoding
Another function of the modem is to provide address
decoding (chip selection) for itself, and the RAM. Four
active low outputs from U702 (combined on Q0 pin 30) are
applied to the RAM (CSI pin 20) and the modem chip itself
(Q3 on pin 27).
Reset Logic
A reset pulse from U702 pin 43 is applied to the micro
controller and the ASP at the following states:
•
at power-up
•
if the watchdog timer circuit in U702 times out
•
if the +5Vdc regulated supply from U801 falls out
of regulation.
This active-high reset pulse is inverted by Q703 and
applied to the active-low reset inputs of the micro controller
(U701 pin 30) and the ASP (U804 pin 9).
A watchdog timer inside the modem must be serviced
by the micro controller at least every two seconds or a 50
microsecond wide reset pulse will be sent to the micro
controller and the ASP. This will occur if a hardware or
software failure develops.
The modem receives a reset signal generated by +5Vdc
regulator U801 when the radio is powered-up and if the
+5Vdc supply falls out of regulation. Transistor Q804
inverts the reset line from U801. This reset input to the
modem on U702 pin 33 (RESIN) is low during normal radio
operation. At power-up, U801 and Q804 pull U702 pin 33
low after the +5Vdc supply becomes stable. U702 then
brings its reset output on pin 43 low and the micro controller
and the ASP begin to operate. If the +5Vdc supply falls out
of regulation (less than 4.75 Vdc), U801 will pull U702 pin
33 high (via Q804) and U702 will then reset the micro
controller and the ASP by pulling its reset output (U702 pin
43) high.
Clock Circuitry
A clock oscillator in U702 generates an 11.0592 MHz
clock for the micro controller and the ASP. Crystal X701 is
the frequency reference component. The buffered clock
signal at U702 pin 15 is sent to the micro controller and the
ASP.
This 11.0592 MHz clock frequency can be slightly
shifted if a clock harmonic or interfering signal ("birdie")
falls on the current receive frequency. This oscillator shift
function is enabled with the PC programmer on a per
channel basis. When the shift is enabled on the current
receive frequency, the micro controller turns Q702 on via an
output from the ASP (U804 pin 15). With Q702 on,
additional capacitive loading is applied to the crystal via
C735.
SERIAL NUMBER ROM U706
The serial number ROM (Read Only Memory) U706
contains a unique 48-bit number which is read by the micro
controller at power-up. A single pin on the device provides
serial communication with the micro as well as +5Vdc
power through pull-up resistor R728.