Remote Processing RPC-220 - Manual (Page 28)

REAL TIME CLOCK SECTION 7
Page 7-3
Step by step instructions are:
Screen Description
MP a00a Your entr y to access
register A. Response is
A00A 20 31 Indicates oscillator off. Enter
31 for 256 Hz, select extended
registers. Press space bar.
A00B 03 0a Set register B to 0A.
This enables square wave
and 24 hr mode.
MP a04b Select extended control
regist er B.
A04B 40 20< Enter> Indicates 327 68 output.
Enter 20 for crystal bit at
12.5 pF , disable 32Khz
output, enable square
wave.
Other registers are accessed similarly. See RTC-1. C for
program example that accesses m ost of the registers.
OTHER REGISTERS
The D S1689 has a fe w reg isters that m ay be of inter est.
See demonstr ation code in RTC -1.C for access
examples.
There a re two kinds of elapsed tim e counters. The first
one, located at 0xa054 - 0xa057, bank 1, counts elapsed
time power has been applied. The second counter
located at 0xA058 - 0xa05B records the total number of
seconds the clock has been running, both battery and
external powered supplied.
These counters may be useful for determining when
equipment needs maintenance, end of battery life, or
other similar situations. Since these are easily read and
reset, they can be u sed as second ary tim ers for relatively
long events.
A power cycle counter at addr esses 0xa05c-0xa05d, bank
1, c ounts the num ber of tim es power is applied. This
can also be useful for m aintenance alerts.
Each card with a RTC has a unique 64 bit serial number
located at addresses 0xa040 - 0xa47, bank 1. You can
embed this serial num ber into yo ur code to uniquely
identify a system and prevent unauthorized copying.
EXTERNAL BATTERY
The batter y included on tw o RPC -220 ver sions is, in
theory, good for 10 years of power off backup.
1 In
practice , yo u will achieve 4 to 6 year s of life. This is
true of an y lithium button cell. F actors tha t cause life to
decrease are elevated temperatur es, high and low
humidity. A batter ies' self dischar ge doubles w hen its
en vir on me nt t em pe ra tur e c ha ng es fr om 25 °C t o 5 0°C .
This alone reduces battery life from 10 years to 5.
Lower temperatures reduce self discharge rate.
Low h umidity pr omotes the evapor ation of electr olyte
from the battery . H igh humidity helps dischar ge it.
If battery life is not long enough, you may connect an
external 3 volt battery at W7. You m ust remove the
existing battery first. A Tadiran
2 model T L-5276/ SL is
a utility meter battery and will, in theory, pr ovide
backup for over 100 years.
Battery current is about 1.5 micro-amps with board
power off.
Remote Processing can build versions without the battery
and with W7 installed. Minimum quantities will apply.
STABILITY AND TEMPERATURE
R e al ti m e c lo c k' s c r y st a l i ni ti a l t ol e ra n ce is ±2 0 pp m .
(Toleranc e is specified as
) f/f.) This tolerance changes
due to the following factors: Temperatur e, hum idity, and
operating environm ent.
Reference temperature is 25 °C. F requency drift is an
additional 40 ppm at -10°C and + 60°C. Peak
t e mp e r at ur e c oe f fi c ie n t i s - 0 . 03 5 pp m / °C .
Another factor that causes the clock to run faster is very
high humidity. When m oisture condenses on the board,
there is an increase of apparent capacitance. Increased
capacitance causes the crystal to oscillate at a higher
frequency.
When the operating environment is dirty, or contains
conductive "dust", the crystal can change frequency.
A way to prevent these problems is to coat the areas
around the clock cr ystal with a non -conductive sealant.