Nuts and Volts - November 2013

As discussed previously, we will fill up our SRAM and then transfer the values to EEPROM, giving us three separate sets of data. Since each of these sets is structured the same, we can benefit from defining a single data structure. We will first fill the SRAM instance of this structure and then when it is full, we will copy the raw bytes from it into the first 512 bytes of EEPROM. We will then clear the SRAM memory for the structure and begin our second set of sampling data. When the SRAM fills up again, we will copy the data to the second 512 bytes of EEPROM, and again clear the SRAM and start sampling over. We will use three instances of the following data structure: Remember that the 126 bytes will store twice that — 252 nibbles.

Let's Use a Data Structure

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struct fac_log {
datetime origin_datetime
uint8_t origin_intemp
uint8_t origin_outtemp
uint8_t origin_inhum
uint8_t origin_outhum
uint8_t inTemp[NIBBLE_ARRAY_SIZE]
uint8_t outTemp[NIBBLE_ARRAY_SIZE]
uint8_t inHum[NIBBLE_ARRAY_SIZE]
uint8_t outHum[NIBBLE_ARRAY_SIZE]
}

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If the SRAM fills up a third time before we download the data to the PC ... we didn’t plan well, did we? We could figure out a way to trash the oldest data and keep only the most recent stuff until somebody downloads it, but we are already getting too complex. So, let’s just stop sampling data and leave it up to the user to figure this part out.

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So, now we’ve got a handle on the data during normal times, but what about that exceptional case of a storm passing through and making a change outside of our – 8 to + 7 range? Again, we’ll have to make an arbitrary assumption and just declare that if any value falls outside our range, we will declare an exception by setting all four nibbles at that sample time to – 8.

Let’s assume that we will never see all our sensor values drop exactly eight points from one interval to the next under normal conditions, and we’ll use this ‘fact’ to create an exception state that the software can see in the stored data and then compensate for it.

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Even if the rare case happens that all four values do — in fact — drop exactly eight points, we only lose a little data space since the solution to the exception involves storing four new initial bytes. So, really, all we’ve