Real Time Clock; Rtc Backup Battery Charging; Security; Baseband Emc Strategy - Nokia NSB-7 Series Service Manual

NSB-7
System Module
ramp and the in-phase and quadrature signals. A slow speed digital to analogue con-
verter will provide automatic frequency control (AFC).
COBBA is at any time connected to MAD asic with two interfaces, one for transferring TX
and RX data between MAD and COBBA and one for transferring codec RX/TX samples.

Real time clock

Requirements for a real time clock implementation are a basic clock (hours and minutes),
a calender and a timer with alarm and power on/off -function and miscellaneous calls.
The RTC will contain only the time base and the alarm timer but all other functions (e.g.
calendar) will be implemented with the MCU software. The RTC needs a power backup to
keep the clock running when the phone battery is disconnected. The backup power is
supplied from a rechargable Li-ion battery that can keep the clock running some ten
minutes. If the backup has expired, the RTC clock restarts after the main battery is con-
nected. The CCONT keeps MCU in reset until the 32kHz source is settled (1s max).
The CCONT is an ideal place for an integrated real time clock as the asic already contains
the power up/down functions and a sleep control with the 32kHz sleep clock, which is
running always when the phone battery is connected. This sleep clock is used for a time
source to a RTC block.

RTC backup battery charging

CHAPS has a current limited voltage regulator for charging a backup battery. The regula-
tor derives its power from VOUT so that charging can take place without the need to
connect a charger. The backup battery is only used to provide power to a real time clock
when VOUT is not present so it is important that power to the charging circuitry is
derived from VOUT and that the charging circuitry does not present a load to the backup
battery when VOUT is not present.
It should not be possible for charging current to flow from the backup battery into VOUT
if VOUT happens to be lower than VBACK. Charging current will gradually diminish as the
backup battery voltage reaches that of the regulation voltage.

Security

The phone flash program and IMEI code are software protected using an external secu-
rity device that is connected between the phone and a PC. The security device uses the
phone given IMEI number, the software version number and a 24bit hardware random
serial number that is read from the COBBA and calculates a flash authority identification
number that is stored into the phone (emulated) EEPROM.

Baseband EMC Strategy

The baseband EMC strategy is divided into electrical and mechanical items. As electrical
guide lines, clocks and high speed signals should be routed in inner layers and away from
the PCB edges. Clock signals distributed to other circuits should have series resistors
incorporated to reduce rise times and reflections. Slew rate controlled buffers should be
used on custom components wherever possible to reduce the EMC produced by the cir-
cuit. Separate power supplies for digital, analog and rf-blocks should be used as much as
possible. Baseband and RF supply power rails should be isolated from each other by
Page 40 ãNokia Mobile Phones Ltd.
PAMS Technical Documentation
Issue 1 06/00