Re: Real Time Clock on Cassette lines

From: Jim Brain <>
Date: Mon, 13 Feb 2012 21:22:32 -0600
Message-ID: <>
Any thoughts?

On 2/12/2012 9:53 PM, Jim Brain wrote:
> 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
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> Updates:
>  * 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.
> Thoughts?
> Jim

Jim Brain

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Received on 2012-02-14 04:00:03

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