Re: Real Time Clock on Cassette lines

From: Jim Brain <>
Date: Sun, 12 Feb 2012 21:53:20 -0600
Message-ID: <>
On 2/11/2012 11:11 AM, Gerrit Heitsch wrote:
> On 02/11/2012 07:33 AM, Jim Brain wrote:
>> Anyone see issues with wiring up a I2C-based (DS1307-type) RTC to the
>> cassette port with SCK on WRITE and SDA (data) on sense? Looking at
>> schematics, READ is often hooked to CA1 or similar read-only pin, but
>> SENSE appears to be on a GPIO on all CBM machines I found. Obviously,
>> the RTC would not work if the buttons were pushed on the player, but are
>> there other concerns?
> In case you built that setup, it would be worth thinking about 
> including a space/socket for a 24Cxxx I2C-EEPROM. They are small (8pin 
> DIP or 8pin SMD) but give you up to 256KByte (24C2048) memory. They 
> only need +5V and GND besides the I2C connection so the only 
> additional cost for quite a bit nonvolatile storage would be the PCB 
> space.
>  Gerrit
>       Message was sent through the cbm-hackers mailing list

  * I decided to try the MOTOR/WRITE idea, as some suggested
  * I ditched the 2 resistor voltage divider on MOTOR in favor of an NPN
    NOT setup. That should allow faster signal response.  That resolved
    one of my major concerns with using MOTOR.
  * I checked, and 24CXXX I2C EEPROMs use the same pinout as the 1307
    RTC, so the same PCB will work for both.  Since it's a pass-through
    PCB, they can be chained.
  * I mapped the EEPROM on the cassette side of the pinouts, so the SMD
    jumpers can be cut and the uC could have total control of the
    EEPROM.  I thought that might allow a 256kB "tape" emulator, dunno.

Still worried about accidentally writing to a tape since I'm using 
MOTOR, but I guess the code can check sense before it starts and fail if 
sense is active. (low).

Here are the various options:

v2: SCK on WRITE, SDA on SENSE (original posting)

v2a: SCK on MOTOR, SDA on WRITE, MOTOR fed through inverting NPN setup. 
(Ruud et al suggestion)

v2b: Remapped some uC pins to Cassette port to allow for better routing

v2c: Moved RTC footprint closer to XTAL for RTC.  Rerouted some of the 
lines for better layout and less EMI

Disclaimer:  Last minute changes, like enlarging PWR trace width and 
MOTOR trace width, were only done on 2c, but would be done on any 
production board.



Jim Brain

       Message was sent through the cbm-hackers mailing list
Received on 2012-02-13 04:00:03

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