APPLICATION NOTE 701

Using the DS32kHz with Maxim Real-Time Clocks


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 first-generation 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.


Table 1. N-WELL (First Generation)
Mode DS32kHz IBAT (µA) DS1306 IBAT (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 IBAT (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.
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  
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  
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 Free Samples  
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  
DS17885 3V/5V Real-Time Clocks Free Samples  
DS32KHZ 32.768kHz Temperature-Compensated Crystal Oscillator  


Next Steps
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© Feb 15, 2002, Maxim Integrated Products, Inc.
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APP 701: Feb 15, 2002
APPLICATION NOTE 701, AN701, AN 701, APP701, Appnote701, Appnote 701