SPI RTC DS1302 (3 wire not really SPI)

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Hello All,

A revision of a current project with an SPI ASIC is going to include the addition of SPI FRAM EEPROM and an SPI RTC. I'm interested in using the DS1302 but it is slightly different than a standard SPI interface.

My questions are:

1. Do I just connect both the SDA and SDO to the I/O pin on the DS1302?

2. If that is correct, will that cause any complications with the other SPI hardware on the bus? (I assume not because of the ability to address individual hardware with the CS?)

3. I may have enough extra pins to separate the RTC to other I/O pins and Bit-Bang SPI for the RTC. Any need for that based on questions #1 & #2?

Note: Demand for any given piece of hardware on the SPI bus will not occur simultaneously. (i.e. PIC will request values from ASIC, if value meets criteria, PIC will get time from RTC, then PIC will write data to EEPROM, and then will start back with the ASIC.....)

Thanks Again to All,

John

EDIT:

I found some threads that discuss this issue. From what it sounds like it may be simpler (or cause less problems) if I separate DS1302 from the other "true" SPI peripherals.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

If the addition of the RTC is new then why not use the DS1305 which is an SPI device.
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treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

My current project is using fram and rtc on hardware I2C port.

I2C FRAM FM24C256(four of them) and a M41T80 RTC with a 1F cap backup.

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Andrew,

Thanks, I had originally decided on the DS1305. But I ran into to minuses when I evaluated the 1305. I was hoping to go to all SMT parts and the 1305 is a super-shrink package. I can stuff small outline parts by hand, but super-shrink I'm not sure if I'm ready to try. Also, the 1302 already has the driver written and the 1305 has less info available.

I may just go to the 1305 in the DIP and accept the fact I'll have some driver work to do..... it appears as though I may need those extra pins back for some feature bloat.

John