DS3231 & DS1307 RTC in C64

Update 29.07.2024 - Added version 1.0
- Separate versions for cassette port, UserPort PB0/PB1 and UserPort PB2/PB3 are now included. This includes both GEOS and console apps and drivers.

Updated 20.09.2018 - Added version 0.2

Cheap and easy way to add Real Time Clock for your C64 if you don't yet have one.

Requires DS3231 RTC to be connected to C64 userport:

C64 UserPort Pin Pin Name DS3231 RTC pin

1 GND GND
2 +5V VCC
C PB0 SDA
D PB1 SCL

or

C64 UserPort Pin Pin Name DS3231 RTC pin

1 GND GND
2 +5V VCC
E PB2 SDA
F PB3 SCL

or

C64 Cassette port Pin Name DS3231 RTC pin

A-1 GND GND
B-2 +5V VCC
F-6 SENSE SDA
E-5 WRITE SCL

Module needs a battery backup. There are very cheap ready made modules which already have one

added, for example:

Tiny RTC DS1307:

NOT RECOMMENDED - GET DS3231 BASED MODULE INSTEAD.

Note, that you may need to remove the pullup resistors (R2 and R3) from Tiny RTC DS1307

for proper operation. Also, the battery holder in my unit was not very good and

there was a risk of battery falling out or not having a proper contact.

I added just a bit of folded plastic to force the battery holder's "spring" - so, nothing difficult, really.

The board is designed to be used with rechargeable battery. However, it is recommended to use normal battery (CR1225) and disable the charging circuit by removing R4, R5, R6 and D1. And if there is a resistor installed in R7 - remove that as well.

Short R6, for example, by soldering a short wire between the pads.

DS3231 based RTC module

In general this is probably the version you'll want to use. It is more accurate than DS1307 and also seems to be more reliable.

Just like Tiny RTC (DS1307) this board also is designed to be used with rechargeable battery.

It is recommended to use standard CR2032 instead and remove the charging diode.

Included programs (same SW for both DS1307 and DS3231):

Included native programs:

fetchrtc.prg

- Fetches and shows date and time from DS3231

- Updates basic TI$ variable with current time

- Updates TOD with current time

setrtc.prg

- Same as fetchrtc but also lets you change the date & time

fetchrtc0.prg

- version that is not crunched with exomizer.

This leaves basic in better health after running the prg

setrtc0.prg

- version that is not crunched with exomizer.

This leaves basic in better health after running the prg

Included GEOS programs:

rtcgeosload.prg

- C64 GEOS driver (auto-exec) which loads the time & date from DS1307 during GEOS boot and updates the C64

registers so that GEOS gets the correct time

setrtcgeos.prg

- C64 GEOS utility for setting the correct date & time into DS1307 RTC.

Setter uses the same input method as Maciej Witkowiak's DS12C887 utility

Installing and testing rtcgeosload.prg and setrtcgeos.prg:

1. Copy these files into your GEOS boot floppy

2. Load GEOS: load"geos",8,1

3. Once GEOS has booted - use CONVERT utility to convert rtcgeosload.prg

and setrtcgeos.prg into GEOS format.

4. Now you can run "SetRTC" utility to set correct time & date.

5. Return to basic and switch off C64 for a few seconds

6. Switch on C64 and load GEOS

--> After GEOS has booted, the correct time & date should show in upper right corner

Downloads: