Replacing the Maxim DS1337 RTC with the ISL12057 ® Technical Brief June 9, 2009 TB480.0 Introduction VPULL-UP = 5.5V VDD 0.1µF 10kΩ 3.3V Zener VDD = 3.3V 10kΩ VIN = 5.5V 55kΩ 10kΩ Intersil has recently introduced the ISL12057 Real Time Clock (RTC) devices. The device is pin-to-pin (8 Ld SOIC and 8 Ld MSOP package) and software compatible to Maxim DS1337 RTC. In fact, the ISL12057 is an enhancement because it provides: SDA SCL IRQx GND • Fixed Default Register Values at Power-up • Flexible Alarm Selection FIGURE 2. EXAMPLE CIRCUIT WITH ZENER DIODE ON VDD TO REDUCE INPUT VOLTAGE TO 3.3V • Stable Frequency Output vs VDD • Smaller Package Option - 2mmx2mm µTDFN (Tiniest RTC available in the market) The features are detailed in the data sheets. This Technical Brief provides information for an engineer wishing to use the ISL12057 to replace the DS1337 in their system. Replacing the DS1337 with the ISL12057 The ISL12057 can drop into a DS1337 socket with no hardware and software changes required. There is one difference that may affect hardware requirements. The DS1337 can operate at VDD up to 5.5V, while the ISL12057 has an operating range up to 3.6V. The ISL12057 allows SDA, SCL, IRQ1 and IRQ2 to be pulled up to 5.5V. A system that has the I2C lines pulled up to 5.5V (or other voltage higher than 3.6V), but has a 3.3V regulator on board requires a simple modification to connect the VDD to 3.3V instead of 5.5V. Figure 1 shows an example circuit with VDD to 3.3V and I2C lines and IRQx pulled up to 5.5V A system that cannot connect VDD to voltage lower than 3.6V can use a Zener diode on VDD pin to lower the VDD to 3.6V or below. Figure 2 shows an example circuit with a Zener diode to lower the 5.5V input voltage to 3.3V for VDD. VPULLUP = 5.5V 10kΩ VDD 10kΩ 0.1µF 10kΩ VDD = 3.3V SDA SCL Advantage on Using ISL12057 vs DS1337 The ISL12057 has a fixed default register value at power-up so a system can easily check for a power failure condition by examination the register value versus the default value. The ISL12057 has a flexible alarm selection for the two alarms versus the fixed alarm selections for the DS1337. For Alarm1, the ISL12057 has a total of 24 alarm selections. For Alarm2, the ISL12057 has a total of 12 alarm selections. The Alarm can be set to any Clock/Calendar value for a match. For example, every minute, every Tuesday or at 5:23 AM on 1st day of a month. The ISL12057 has more stable frequency output versus VDD than the DS1337. Figure 3 shows the 32.768kHz FOUT vs VDD for ISL12057 and DS1337. As the graph shows, the ISL12057 32.768kHz FOUT is flat across VDD but the DS1337 has a linear slope on the 32.768kHz FOUT across VDD. The ISL12057 has the 8 Ld SOIC and MSOP package that are pin-to-pin compatible to the DS1337. In addition to the SOIC and MSOP packages, the ISL12057 offers the tiniest 2mmx2mm µTDFN package. A standard 8 Ld SOIC package occupies approximately 29.4mm2 (4.9mmx6mm) of board space, and a standard 8 Ld MSOP package occupies approximately 14.7mm2 (3mmx4.9mm) of board space. The ISL12057 2mmx2mm µTDFN package only occupies 4mm2 (2mmx2mm) of board space, which is equal to 86% space saving versus the 8 Ld SOIC package and 72% space saving versus the 8 Ld MSOP package. Figure 4 shows the package dimension for µTDFN, MSOP, and SOIC. IRQx GND FIGURE 1. EXAMPLE CIRCUIT WITH VDD AT 3.3V AND I2C AND IRQX PULLED UP TO 5.5V 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2009. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Technical Brief 480 32768.0 FOUT (Hz) 4.9mm 3mm 2mm 32767.8 6mm 4.9mm 2mm 32767.9 µTDFN MSOP 32767.7 32767.6 SOIC ISL12057 FIGURE 4. PACKAGE DIMENSION (NOT ACTUAL SIZE) 32767.5 DS1337 32767.4 32767.3 1.4 1.9 2.4 2.9 3.4 VDD (V) FIGURE 3. FOUT vs VDD Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 2 TB480.0 June 9, 2009