DS1227 Datasheet, PDF (15/20 Pages) Dallas Semiconductor – KickStarter Chip

DS1227
The DS1275 Line Powered RS232 Transceiver allows
the instrument to communicate with the RS232 port on a
host computer (e.g., COM port on an IBM PC). It oper-
ates from a +5V supply and draws no power from the
instrument’s main energy source to create negative
voltages. Instead, it steals power from the incoming
RXD line to generate the negative voltages needed dur-
ing transmission.
INSTRUMENT OPERATION
A common requirement of instruments is event logging
with time stamp and date. The Dallas chip set provides
this capability using the DS5000 and DS1283. The
DS1283 interfaces directly to the DS5000FP embedded
bus, and may be accessed by CE2. In this way, valuable
port pins are conserved. Events can be recorded by the
microcontroller and logged in RAM with the date and
time. In the absence of V_CC, the data will be retained in
RAM by the backup lithium cell. The same energy cell
provides backup to the DS1283, so that timekeeping is
maintained in the absence of a primary energy source.
Therefore, events may be time stamped and dated with
confidence that the correct time has been maintained.
Backup lithium current is managed by the DS5000FP
and is distributed from the V_CCO line in the absence of
V_CC.
PERMANENTLY POWERED OPERATION
In order to achieve permanently powered operation,
Dallas Semiconductor uses several techniques which
conserve the life of a primary energy source. First, the
illustrated chip set operates at extremely low power.
These components are also capable of very low power
data retention. Second, the crashproof circuitry of the
DS5000 allows V_CC to be removed and restored without
disruption. This allows the energy management circuits
of the Kickstarter to power down the microcontroller dur-
ing periods when it is unused. Since the DS1227 can
monitor external events and wakeup the DS5000 as
necessary, the microcontroller and other circuitry may
remain in low power data retention mode until needed.
The DS5000, RAM, and DS1283 will be backed up via
the button cell as show in Figure 1. Finally, the Kickstart-
er allows software-controlled powering of auxiliary cir-
cuits when tasks require them.
Low operating power is a basic requirement of battery-
operated systems. The illustrated Dallas chip set can
perform most instrument functions using minimal pow-
er. Using a 3.57 MHz crystal, the circuit in Figure 1 will
draw approximately 8 mA during microcontroller opera-
tion. When the Kickstarter turns off the DS5000, the cir-
cuit draws approximately 5 µA from the primary energy
source. If a similar configuration were created with an
ordinary CMOS microcontroller in stop mode, the cur-
rent could be as high as 55 µA. Idle mode operation
would consume approximately 3 mA, which would ex-
cessively drain a primary power source over extended
periods. The Dallas low power chip set provides a ten-
to-one improvement over previously available alterna-
tives.
Achieving the lowest power instrument requires the
DS1227 Kickstarter. Using the Kickstarter, low power
operation is achieved by powering down the microcon-
troller. When this occurs, the DS5000 effectively con-
sumes zero power. RAM and key registers are backed
by the lithium button cell, with no power draw from V_CC.
When a task must be performed, the Kickstarter powers
up the DS5000 to execute a function and powers it down
when the function is complete (under software direc-
tion). The period for which power remains on is mini-
mized in this way. Since most tasks require minimal pro-
cessing time with long periods of waiting, the instrument
may remain in a low power data retention mode for the
majority of time. Therefore, even if an operator interface
is necessary, the microcontroller can remain on for milli-
seconds (or microseconds) to perform a task, and re-
main off for the seconds between operations.
The ability to react to external stimuli allows the instru-
ment to operate autonomously for many applications.
Fundamental to this operation is the kickstart caused by
external stimuli. The following section describes the op-
eration of the Kickstarter with respect to four different
stimuli.
KICKSTARTING OPERATION
The DS1227 receives primary power from a +3V lithium
battery. Prior to a kickstart, battery voltage is present on
V_DCO, which is the main voltage output. When the sys-
tem receives a kickstart stimulus, an on-chip boost reg-
ulator raises V_DCO to +5V. Upon completion of power
up, +5V is switched to the DS5000 on V_CC01. Prior to
kickstart, no power was supplied to this line.
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