DS1994L 4Kb, alternative product of clocked memory iButton®
DS1994L is produced by Maxim's 6-inch fab, and the manufacturing process used is outdated and obsolete. Therefore, in order to avoid the high development cost caused by the migration of old devices to new processes, Maxim implemented the last purchase of DS1994L, encouraging all users of the device to adopt alternative products.
DS1994L features DS1994L includes NV SRAM, RTC counter, time interval counter, cycle counter, and alarm registers for RTC, time interval counter, cycle counter. The interval counter can work in manual mode or automatic mode. In manual mode, the counter can be started and paused under software control; in automatic mode, the counter will remain active as long as voltage is applied to the 1-Wire® port. When the 1-Wire port voltage is removed, the cycle counter is incremented. In this way, DS1994L automatically calculates the number of power-on / power-off times and the working time of the device. When any counter of DS1994L reaches the corresponding alarm register setting threshold, DS1994L will generate an alarm.
The non-clearable independent write protection functions of the three counters and their alarm registers make the DS1994L ideal for applications such as software authorization period timers or time-limited access tokens. Both of these applications benefit from user-programmable functions that block access to memory once an alarm occurs. This combination of functions makes DS1994L more widely used, as shown in Table 1. For some applications, alternative products can indeed be adopted by modifying the software.
Table 1. Application of DS1994L
| AlternaTIve Devices | ||||
| DS1994 ApplicaTIon | DS1904L | DS1904L with DS1993L | DS1921G | DS1922L |
| RTC module / token |  | | | |
| RTC with memory module / token | — | | | |
| Data token (NV SRAM) | — | | | |
| Equipment on / off cycle counter with service alarm | — | Extra SW1 | Extra SW1 | Extra SW1 |
| Equipment operaTIng TIme counter with service alarm | Extra SW2 | Extra SW3 | — | — |
| Equipment scheduled service timer with alarm | — | Extra SW4 | | Extra SW4 |
| Secure time-limited access token | — | — | — | Extra SW5 |
| Secure software license token with expiration timer | — | — | — | — |
Legend / Comment
The function replaces the device, but the software needs to be partially modified. — Cannot replace the function of the device.
1. The replacement device needs to create a counter in the memory under software control.
2. Function replacement device, need to control the start / pause of RTC counter through software. The alarm service requires additional storage space for storing reference values. The alarm is generated by comparing the RTC counter value with the reference value stored in the memory.
3. Function replacement device, need to control the start / pause of RTC counter through software. The alarm is generated by comparing the RTC counter value with the reference value stored in the memory.
4. Function replacement device, need to compare RTC value with reference value stored in memory under software control. The program does not use any security measures.
5. Function replacement device, need to compare RTC value with reference value stored in memory under software control. The program uses password protection measures.
Alternative devices Unfortunately, no alternative device can contain all the functions of DS1994L, and some devices can provide some of the functions of DS1994L. In addition, if it is allowed to modify the firmware to support different combinations of functions of the application, many functions of DS1994L can be found by replacing devices. Table 2 lists the functional comparison of these alternative devices with the DS1994L.
Table 2. Comparison of alternative devices
| Alternative Devices | |||||
| Characteristic | DS1994L | DS1904L | DS1904L with DS1993L | DS1921G | DS1922L |
| RTC format | 40b binary counter | 32b binary counter | 32b binary counter | BCD: seconds, minutes, hours | BCD: seconds, minutes, hours |
| RTC resolution | 1/256 second | 1 second | 1 second | 1 second | 1 second |
| Calendar | Software algorithm | Software algorithm | Software algorithm | BCD: day of week, date, month, year | BCD: date, month, year |
| Interval timer | 40b binary counter, resolution 1/256 second | — | — | — | — |
| Cycle counter | 32b binary | — | — | — | — |
| RTC alarm | Yes, using conditional search ROM | — | — | Yes, using conditional search ROM | — |
| Interval timer alarm | Yes | — | — | — | — |
| Cycle counter alarm | Yes | — | — | — | — |
| User memory | 512B | — | 512B | 512B | 512B |
| Security | Irreversible write protection of RTC counter and alarm register, interval timer and alarm register, cycle counter and alarm register | — | — | — | Two 64b passwords (one for read, one for full access) |
| Extras | • User-programmable memory access blocking upon alarm • Interrupt signaling upon alarm (an uncommon feature) | — | — | • Temperature logger with 2KB logging memory, temperature histogram, and temperature alarm recording • Temperature alarm | • Temperature logger with 8KB logging memory • Temperature alarm |
DS1904LDS1904L RTC iButton is a RTC counter in iButton package. Unlike the 5-byte counter that increments by 256 per second, the DS1904L has a 4-byte counter that increments every second. In addition to the time resolution, the function of the DS1904L is equivalent to the RTC counter of the DS1994L. The algorithm for converting the counter reading to the conventional time / date format or the reverse process is the same as DS1994L (see application note 517: "DS1371 / DS1372 / DS1374 32-Bit Binary Counter Time Conversion") The cost of the DS1904L is about half that of the DS1994L, and it is suitable as a real-time counter for the most basic functional replacement. To measure the device's operating time, the DS1904L requires the microcontroller to start the RTC oscillator when the device is powered on and suspend the oscillator when the device is turned off. This method will prevent the RTC counter from displaying the correct time. DS1904L with DS1993L module is a combination of DS1904L and DS1993L 4Kb memory iButton as a 1-Wire device set that can implement the RTC counting and storage functions of DS1994L. The main functions missing are the interval timer, cycle timer and three alarm functions. Depending on the application environment, these functions can be implemented through software. As with the DS1904L alone, this module prevents the RTC counter from displaying the time correctly. In addition to two iButtons, two DS9094-SM5 surface-mount iButton clips and a DS9092R iButton port included in the template are also installed on the printed circuit board. Due to the installation of additional electromechanical components, the cost of the DS1904L / DS1993L combination module is higher than the DS1994L.
DS1921G DS1921G Thermochron® iButton has the same memory, RTC and RTC alarm clock as DS1994L. However, its RTC and corresponding alarm clocks use the BCD format to count seconds, minutes, hours, dates, months, and years. Since there is no need to convert the second count value to a time / date format, this format is convenient for RTC alarms. On the contrary, when trying to use the RTC of the DS1921G as the working time counter of the device, this format is not practical.
In addition to having the same functions as the DS1994L, the DS1921G also provides an independent temperature recorder with temperature histogram and temperature alarm function. These additional functions increase the cost of DS1921G, and the price is higher than DS1994L. Therefore, if you can accept such a cost-effective, you can use DS1921G as a replacement device. In addition, the temperature recorder function can also be used in applications.
DS1922L DS1922L temperature recorder iButton with 8KB data logging memory has the same memory and RTC as DS1994L (without alarm clock). However, its RTC uses the BCD format to count seconds, minutes, hours, dates, months, and years. Since there is no need to convert the second count value to a time / date format, this format is convenient for RTC alarms. On the contrary, when trying to use the RTC of the DS1922L as the device's working time counter, this format is not very practical.
In addition to having the same functions as the DS1994L, the DS1922L also provides an independent temperature recorder with temperature alarm and password protection. These additional functions and large-capacity data recording memory increase the cost of DS1922L, and the price is higher than DS1994L. Therefore, if you can accept such a cost-effective, you can use DS1922L as a replacement device. In addition, temperature recording and / or password protection can be used in the application.
Summary DS1994L does not have a fully compatible replacement device, but it is indeed possible to find a reasonable substitute for DS1994L in its many typical applications. In any case, the software needs to be modified. The most basic alternative product is the DS1904L, which lacks 4Kb of memory. The module including one DS1904L, DS1993L, two DS9094-SM5 clips and one DS9092R iButton port can provide RTC function and 4Kb memory, but the cost is slightly higher than DS1994L. The DS1921G and DS1922L provide almost the same functions as the DS1904L / DS1993L module, but include additional temperature recording functions, making them a more cost-effective alternative.
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