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DS1305 NV RAM read/write

 
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TimothyCarter



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DS1305 NV RAM read/write
PostPosted: Tue Jun 22, 2021 9:35 am     Reply with quote

I may be missing something and wanted input here before reporting as a bug, but in the DS1305 driver that is included with PIC-C (using 5.104 compiler)

The functions rtc_write_nvr and rtc_read_nvr appear to OR a 0x20 (write does 0xA0) - the upper most bit (0x80) should be high for a write and low for a read, but why does bit 5 need to be high? This excludes address 0x40 to 0x5F is there a reason or was this just an oversight?
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PCM programmer



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PostPosted: Tue Jun 22, 2021 10:17 am     Reply with quote

It looks like someone was trying to put in a safety feature to prevent the user from over-writing the 0-0x1F block. They just did it wrong.
They should have tested the address.
Ttelmah



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PostPosted: Tue Jun 22, 2021 11:59 am     Reply with quote

If you look the address byte is 0x20 to 0x7F for a read, and 0xA0 to 0xFF
for a write. Figure 2 Hex Address. So the OR makes sense. If this is on a
numeric value for the address to access.
TimothyCarter



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PostPosted: Tue Jun 22, 2021 3:27 pm     Reply with quote

Ttelmah, but if you OR 0x20 then you will skip addresses 0x40 to 0x5F which are still valid addresses; as PCM programmer points out, it should be an add 0x20, rather than an OR. That way you could start with address "0" which would get added to 0x20.
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TimothyCarter



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Work-around
PostPosted: Tue Jun 22, 2021 3:34 pm     Reply with quote

If anyone else has an issue with this, the work around is to not use the rtc_read_nvr and rtc_write_nvr functions and use the read_ds1305 and write_ds1305 respectively. The caveat is that you need to make absolutely sure that your code does not write to addresses 0-0x1F.
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Ttelmah



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PostPosted: Tue Jun 22, 2021 10:52 pm     Reply with quote

That makes total sense.
The error should be pointed out to CCS
I'd say that the rtc_read_nvr should use something like a test for the incoming
address being>0x5F (returning an error if it is), and adding 0x20.
The write should do the same and add 0xa0.
TimothyCarter



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PostPosted: Wed Jun 23, 2021 12:41 pm     Reply with quote

and just like that CCS emailed me a fix this morning...
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Ttelmah



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PostPosted: Thu Jun 24, 2021 12:11 am     Reply with quote

Don't get ne wrong, but this is the key to why the 'old hands' here, like
CCS.... Very Happy
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