Technologic Systems’ ARM single board computers for embedded systems, the 7500 series, may be ordered with a battery backed up real time clock. However, unlike RTC chips used for PC’s these come with no static RAM. I missed this RAM for an application I was working on and therefore modified the board. This made this computer board a perfect fit for my client.
This application frequently needs to store a few bytes of state information in a way that lets them survive reboots and power outages. I could have stored these bytes in the FLASH or microSD, but I like to be conservative when it comes to NOR and NAND FLASH wear. I therefore prefer an SRAM with backup battery for such use.
The hardware modification is simple: replace the original RTC chip with a Maxim DS1338Z (SO8). This is a functionally identical chip, but adds 56 bytes of NVRAM. As the RTC parts of the two chips are identical there is no need to modify the RTC driver supplied with the board.
The RTC/NVRAM chip is accessed by a bit-bang driven I2C bus via the on-board FPGA. The nvram.c source code provides four functions: to open, close, read and write to the NVRAM. The source code illustrates how to use the driver.
Technoloic Systems supplies hardware documentation and all related software sources for their products. Thanks to this I’m able to easily do such minor modifications and thereby adapt a mass produced board to my client’s needs. This openness is not typical amongst computer board manufacturers.
Wow! I was using the TS7250 which had the PC-style RTC, and I used the NVRAM to regularly store a few bytes of vital information – for example, the current time and date and the last recorded meter reading. The time and date allowed the system to produce a message at startup time ‘Starting at 11:30 – last active at 11:20’ so you could see how long it had been down for. (Power outages).
I did complain to Technologic when I moved over to the TS7550, and removed that functionality from the software. At the time, they said they were unaware that customers were using the NVRAM in the RTC.
I will go back to TS with this, since I am buying between one and two hundred boards per month.