Adding a RTC (battery backed)

Date:

2017-08-19

Intro

A battery (or SuperCAP) backed RTC (real time clock) keeps track of time, even if the clock lacks power. Obviously that is a nice thing to have, and obviously this will work only as long as the backup storage of energy provides any.

There is a multitude of such clocks, I have used only two of them:

  1. PCF8583 (Philips)

  2. DS3231 (Dallas Semiconductors)

Both feature a i2c bus interface.

Code Details

i2c bus

AmForth provides sufficient support for the i2c bus (called two wire interface (twi) in Atmel lingo). So we just need to load it:

#include i2c-twi-master.frt
#include i2c.frt
#include i2c-detect.frt

The last file provides a command to scan the bus, helpful while debugging. We then add a function to set the i2c interface up to our liking:

PORTC 0 portpin: i2c_scl
PORTC 1 portpin: i2c_sda

: +i2c  ( -- )
  i2c_scl pin_pullup_on
  i2c_sda pin_pullup_on
  0                                     \ prescaler
  #6                                    \ bit rate --- 400kHz @ 11.0592 MHz
  i2c.init
;

The exact numbers for prescaler and bit rate must be calculated according to the atmega data sheet. I also added my own function to scan the bus — just the output looks different.

: i2c.scan
  base @ hex
  $79 $7 do
    i i2c.ping? if i 3 .r then
  loop
  base !
  cr
;

BCD digits

Probably for historical reasons, the information provided by these RTCs is encoded in binary code decimal format. I have written 2 words to convert the data at hand. These will fail with numbers larger than 99 — you have been warned!

: bcd>dec  ( n.bcd -- n.dec )               $10 /mod  #10 * + ;
: dec>bcd  ( n.dec -- n.bcd )     #100 mod  #10 /mod  $10 * + ;

PCF8583

This clock provides a subsecond counter, namely 1/100 th of a second. However, this clock does not provide the year any better than year modulo 4. This is the absolute minimum to keep track of leap year — with occasional errors (3 times in 400 years).

The speed of this clock can be slowed down by adding load capacitors (a few picofarad) to its clock crystal.

DS3231

This clock provides a temperature compensated crystal oscillator, a 32768 Hz clock output. The year counter provides year modulo 100, which gives a wrong leap year only once in 400 years.