Using the DS32kHz with Dallas RTC's

Using the DS32kHz with Dallas RTC's
Maxim > Design Support > Technical Documents > Application Notes > Real-Time Clocks > APP 701
Maxim > Design Support > Technical Documents > Application Notes > Time and Temperature Recorders > APP 701
Keywords: timekeeping accuracy, temperature compensation, crystal, real-time clocks
APPLICATION NOTE 701
Using the DS32kHz with Maxim Real-Time Clocks
Feb 15, 2002
Abstract: This application note describes how to reduce current consumption when using the DS32kHz
with Maxim real-time clocks (RTCs).
Overview
This application note is intended to answer some frequently asked questions about using the DS32kHz
temperature-compensated crystal oscillator (TCXO) and Maxim real-time clocks (RTCs).
Using the DS32kHz
The DS32kHz has four pins that are required for operation: VCC, VBAT , GND, and 32kHz_OUT. The
VCC, VBAT , and GND are power-supply connections and must either be connected to a positive supply
or grounded. The 32kHz_OUT signal is intended to drive the X1 input of the RTC. The X2 pin of the
RTC should be allowed to float when driving the X1 input with an oscillator.
The DS32kHz ICC and IBAT specifications are measured with no output load. The input characteristics of
the oscillator on a RTC determine how much additional current the DS32kHz will consume. The
additional current can significantly increase the size of the battery needed for operation.
Choosing an RTC
Maxim Integrated has a wide selection of RTCs from which to choose. Some of these devices were
designed using a P-WELL process while the more recent devices have been designed using the firstgeneration N-WELL process. The second-generation N-WELL RTCs incorporate improvements that
reduce the current consumption of the TCXO/RTC combination. The following data shows the current
consumed by typical devices from each process. The data were taken at a battery voltage of 3.5V at
+25°C.
KEY:
Ind = Individual Currents; DS32kHz with output open, RTC with crystal attached and running.
Direct = DS32kHz driving the RTC X1 input directly.
RC = DS32kHz driving the RTC with a 1MΩ resistor in series with a 100pF capacitor to the X1 input.
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Table 1. N-WELL (First Generation)
Mode
DS32kHz IBAT (µA)
DS1306 I BAT (nA)
Ind
1.60
518
Direct
11.9
386
RC
2.14
584
Table 2. N-WELL (Second Generation)
Mode
DS32kHz IBAT (µA)
DS1337 ICC (nA)
Ind
1.59
612
Direct
3.73
626
RC
2.11
622
Table 3. P-WELL
Mode
DS32kHz IBAT (µA)
DS1202 I BAT (nA)
Ind
1.63
625
Direct
87.1
3410
RC
2.17
685
Note the RTC in each of the aforementioned tables are different RTCs. This accounts for the difference
in the clock currents when comparing the tables.
If the RTC has an oscillator-enable bit, the oscillator must be enabled. If the bit is not enabled, additional
current is drawn and the clock may not operate.
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Table 4. RTC LIST
P-WELL
Devices
First Generation N-WELL
Devices
Second Generation N-WELL
Devices
DS1202
DS12885
DS1337
DS12885
DS1302
DS1338
DS1283
DS1305/DS1306
DS1339
DS1284
DS1307
DS1672
DS1384
DS1315
DS14285
DS1500/DS1501
DS1384
DS1315
DS1602
DS1670/DS1673/DS1677
DS1685
DS1688/DS1689
DS17285/DS17485/DS17885
Summary
When using P-WELL and first-generation N-WELL RTC devices, use an RC circuit to achieve the
minimum possible timekeeping current.
The second-generation N-WELL devices use only slightly more current with an RC circuit.
Related Parts
DS1284
Watchdog Timekeepers
DS12885
Real-Time Clocks
Free Samples DS12R885
RTCs with Constant-Voltage Trickle Charger
Free Samples DS1302
Trickle-Charge Timekeeping Chip
Free Samples DS1305
Serial Alarm Real-Time Clock
Free Samples DS1306
Serial Alarm Real-Time Clock
Free Samples DS1307
64 x 8, Serial, I²C Real-Time Clock
Free Samples DS1308
Low-Current I²C RTC with 56-Byte NV RAM
Free Samples Page 3 of 5
DS1315
Phantom Time Chip
Free Samples DS1337
I²C Serial Real-Time Clock
Free Samples DS1338
I²C RTC with 56-Byte NV RAM
Free Samples DS1339
I²C Serial Real-Time Clock
Free Samples DS1340
I²C RTC with Trickle Charger
Free Samples DS1341
Low-Current I²C RTCs for High-ESR Crystals
Free Samples DS1342
Low-Current I²C RTCs for High-ESR Crystals
Free Samples DS1371
I²C, 32-Bit Binary Counter Watchdog Clock
Free Samples DS1374
I²C, 32-Bit Binary Counter Watchdog RTC with Trickle
Charger and Reset Input/Output
Free Samples DS1384
Watchdog Real Time Clocks Controller
DS1390
Low-Voltage SPI/3-Wire RTCs with Trickle Charger
Free Samples DS1391
Low-Voltage SPI/3-Wire RTCs with Trickle Charger
Free Samples DS1392
Low-Voltage SPI/3-Wire RTCs with Trickle Charger
Free Samples DS1393
Low-Voltage SPI/3-Wire RTCs with Trickle Charger
Free Samples DS14285
Real-Time Clock with NV RAM Control
DS1500
Y2K-Compliant Watchdog RTC with NV Control
Free Samples DS1501
Y2K-Compliant Watchdog Real-Time Clocks
Free Samples DS1558
Watchdog Clocks with NV RAM Control
Free Samples DS1602
Elapsed Time Counter
DS1670
Portable System Controller
Free Samples DS1672
I²C 32-Bit Binary Counter RTC
Free Samples DS1673
Portable System Controller
Free Samples DS1677
Portable System Controller
Free Samples DS1678
Real-Time Event Recorder
DS1685
3V/5V Real-Time Clock
Free Samples DS1688
3 Volt/5 Volt Serialized Real-Time Clock with NV RAM
Control
DS1689
3 Volt/5 Volt Serialized Real Time Clock with NV RAM
Control
DS17285
3V/5V Real-Time Clocks
Free Samples DS17485
3V/5V Real-Time Clocks
Free Samples Page 4 of 5
DS17885
3V/5V Real-Time Clocks
Free Samples DS32KHZ
32.768kHz Temperature-Compensated Crystal Oscillator
Free Samples More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 701: http://www.maximintegrated.com/an701
APPLICATION NOTE 701, AN701, AN 701, APP701, Appnote701, Appnote 701
© 2013 Maxim Integrated Products, Inc.
Additional Legal Notices: http://www.maximintegrated.com/legal
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