Genesis
01-14-2006, 11:55 AM
Maxim makes a big deal about keeping crystals "as close as practical" to the chip.... you have no idea!
I've been pulling my hair out now for a couple of days on a PCB that has a DS1307 and is using a BS2pe.ˇ Works using the PDB's clock (same chip), but the dang thing would NOT oscillate for love or money when on a PCB build-up with a through-hole (DIP-8) chip on the board.
I'm using the Citizen CFS145, which is a very small crystal and is one of the ones that Maxim shows as being "acceptable."
Well, its twitchy as hell.ˇ The pads for the crystal mounting are literally one hole-width from the IC's pins.ˇ With the crystal in there it will NOT oscillate!ˇ Period!ˇ The chip is working fine otherwise - it saves/restores data, I can write and read it, the battery works, etc - but no oscillation.ˇ There's no problem with stray pulses - I've followed all their rules, there are no traces under the footprint of the crystal or its pins on the board, etc.
If I remove the crystal from the board and physically touch it to the two pins of the chip itself - it oscillates!ˇ If I solder it DIRECTLY to the pins on the back side of the board, it oscillates.ˇ But literally 1mm away - it does not.
Wild stuff.
I'm wondering if anyone else has used any of the Dallas RTC chips in a through-hole config on a PCB and if you've found a crystal that is a bit more tolerant - or if I just need to do it this way and then stick a blob of epoxy over the crystal and two pins to protect it once assembled.....ˇ SMT is not an option for now, although it may be in the future.
Or, for that matter, if anyone knows which crystal is being used on the PDB - since that one's obviously working well .....
I've been pulling my hair out now for a couple of days on a PCB that has a DS1307 and is using a BS2pe.ˇ Works using the PDB's clock (same chip), but the dang thing would NOT oscillate for love or money when on a PCB build-up with a through-hole (DIP-8) chip on the board.
I'm using the Citizen CFS145, which is a very small crystal and is one of the ones that Maxim shows as being "acceptable."
Well, its twitchy as hell.ˇ The pads for the crystal mounting are literally one hole-width from the IC's pins.ˇ With the crystal in there it will NOT oscillate!ˇ Period!ˇ The chip is working fine otherwise - it saves/restores data, I can write and read it, the battery works, etc - but no oscillation.ˇ There's no problem with stray pulses - I've followed all their rules, there are no traces under the footprint of the crystal or its pins on the board, etc.
If I remove the crystal from the board and physically touch it to the two pins of the chip itself - it oscillates!ˇ If I solder it DIRECTLY to the pins on the back side of the board, it oscillates.ˇ But literally 1mm away - it does not.
Wild stuff.
I'm wondering if anyone else has used any of the Dallas RTC chips in a through-hole config on a PCB and if you've found a crystal that is a bit more tolerant - or if I just need to do it this way and then stick a blob of epoxy over the crystal and two pins to protect it once assembled.....ˇ SMT is not an option for now, although it may be in the future.
Or, for that matter, if anyone knows which crystal is being used on the PDB - since that one's obviously working well .....