QED Battery-backup
RAM data retention time for the backup battery and RTC socket
The QED-4 Board may optionally use a real time clock (RTC aka calendar clock) whose battery serves a double purpose — it also backs up RAM so that data is preserved while the board is unpowered. In this case, the expected data retention time for an installed 128 KB RAM chip is 10 years while the board is without power. When the board is powered, the battery quickly recharges, so that when subsequently unpowered it has another 10 years of data retention.
The QED-4 Board may use either an RTC socket with battery (part number DS1216D) into which a RAM chip is installed, or a hybrid chip (part number DS1248) that contains RTC, battery, and RAM in one device. If the RTC socket is used it must be plugged into socket S3 on the board; if the hybrid RAM is used it must be plugged into socket S2 on the board.
Data retention time is the minimum time that the RTC is active and RAM data remains valid while the device is unpowered and relying only on its internal backup battery. For both the DS1216D (RTC socket) and the DS1248 (128x8 RAM with RTC) the expected data retention time is in excess of 10 years at 25°C.
For the DS1216D socket, the specified retention time is based on using an external SRAM with a data retention current of less than 0.5μA at +25°C. Its expected data-retention time for a given RAM battery current can be calculated using the following formula:
For both devices data retention time is specified at 25°C – it will decrease significantly at greater storage temperatures. Our experience with other RAM chips (specifically, the RAM used on the QCard Controller, see QCard RTC and RAM retention time), and the observation that diode leakage currents roughly double every +10°C, both suggest that RAM retention current doubles with every +10°C above 25°C. So, retention time if the product is turned off and stored at a relatively high temperature is reduced, to 5 years at 35°C, 3.5 years at 40°C, and 2.5 years at 45°C.
Both devices are shipped with the battery-backup power internally disconnected for maximum shelf life. On first power-up the backup battery connection is switched ON; thereafter the internal battery powers the RTC and RAM VCC pin upon power-down.
Whenever power is applied the internal battery recharges, re-establishing a 10 year unpowered retention time and allowing very long useful operational life. Based on our experience with other lithium backup batteries the maximum time to fully recharge should be approximately one day.
Devices mentioned in this article: