Nuts and Volts - September 2015

from EEPROM. We kick the read off in a similar way: Start condition, address with write bit set, send memory location. Wait — did that seem counter-intuitive? We set the write bit, but we want to read data!? The write and read bit don’t refer to the operation we’re performing at a module level (i.e., writing to memory or reading from memory), it refers to the operation being performed between the module and MCU. In this case, we need to write to the module in order to tell it which address we want to read from. Carry on reading through the code and it should make sense.

So we send the memory location (high and low bytes), and then we issue another start condition (called a repeated start). This tells the module we’re going to do something different in the same conversation. Then, we re-address the module with the read bit set. This tells the module to get ready to send us the data we requested in our initial write. We read this data and once we’ve got what we want, we terminate the conversation with a Stop condition.

EEPROM is Mastered

You’ve now mastered the EEPROM — and got to grips with understanding how to use read and write. Remember that with most modules, you’ll need to write to a device to tell it what you want to read. All the EEPROM-related code in the project is stored in the files “EEPROM.c” and “EEPROM.h” at the article link.

Your Turn — The RTC

Now that you’ve got a feel for how I2C works, I’m going to give you a turn to interpret the code for the RTC — you’ll find it in “RTC_MCP79400.c” and “RTC_MCP 79400.h” also at the article link. Before I do that, though, there are a couple of principles I want to run through.

Registers

The RTC uses registers — and lots of them! There are registers to set seconds, registers to set and read minutes, hours, days, weeks, months, years, leap years, days of the week ... and then alarm registers, configuration registers ... you get the picture. The only way to master this module is to read the datasheet ... a number of times!

Register Names: As this is an external module, Atmel Studio doesn’t know the register names. This means you’ll need to define these yourself if you want to make your code readable. Take a look at the file “RTC_MCP79400.h” — it contains the address names and bit numbers for the most common registers ... and yes, I had to type them all in manually off the datasheet! Register Values: To set a value in a register takes a couple of writes — one to specify the register you want to write to, and another to specify the value that goes into the register. To read from a register, you’ll follow the same process as you did when reading from the EEPROM — except that the register address is only one byte, so it doesn’t need to be split into high and low.

Changing Register Values: This is an important point. You can’t change single bits in the RTC’s registers like you would in your microcontroller’s registers — if you write a single bit over I2C to a register, you clear all other bits in the register and only set that one bit. Not ideal! If you want to preserve the other bit values in a register, you’ll need to read the register value, perform your bitwise logic on the value you’ve read, and then write the entire register back again.

Binary-Coded Decimal

Binary is binary, right? Why is the datasheet littered with references to “Binary-Coded Decimal?” Binary-Coded Decimal (BCD) is not as common as it was historically, but nevertheless is fairly common in RTCs. It is a way of using binary to represent individual digits in a number — usually four bits to represent a single digit (0-9). Figure 8 shows a representation of the RTCMIN register (which stores the “minutes” portion of the time), and a few BCD examples.

To make things simpler (well, only slightly), there are a couple of helper functions in “RTC_MCP79400.c” that convert between BCD and decimal. Unfortunately, BCD is a reality, and I grumblingly learned to live with it. If BCD sounds like something that interests you, there is a pretty comprehensive article on Wikipedia that you can sink your teeth into.

Configuration Registers

There are a couple of important configuration registers that you need to know about, apart from the main timekeeping and alarm registers:

RTCSEC: Bit 7 of RTCSEC is “ST” — this bit starts the oscillator. Without an oscillator, your clock isn’t ticking, so ensure that you set it on startup.