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2 MSP430 devices comm over RS-485, LPM3 and DCO accuracy question

PaulSchoenke
Genius 3290 points
Other Parts Discussed in Thread: MSP430FR5729, MSP430F2232, MSP430F2274

Hello:

I would like an MSP430FR5729 to talk to an MSP430F2232 over RS-485 at a baud rate faster than 9600, but I was planning on using a 32kHz crystal on both in order to go into LPM3.  Some MSP430s have an XT2 connection for two crystal inputs, but I don’t think these have that option.  Is the DCO accurate enough over temperature (-40C to+85C) to support a UART?  If not, what are my options for supporting LPM3 and a UART?  Accuracy of the clock for waking up from LPM3 is important.

Thank you for your help!  Any feedback or input is welcomed and appreciated!

Best regards,

Paul

  • 0
    Guru 58965 points

    FR5729 DCO has only 6 different fixed frequencies to pick, but all six are factory trimmed. Normally (at room temperature for home or office), you do not need a crystal or calibrations to support decent UART BAUDRATE. If you do use a 32.768 kHz crystal, you can use one of the timers to measure the DCO frequency more precisely at the temperature and Vcc you are using. But I do not see why you need that to go into LPM3.

    F2232 DCO has thousands of settings (and 4 factory calibrated settings for 4 frequencies) to pick. But the stability is marginally and even the calibrated frequencies are barely good enough to support decent UART BAUDRATE. A 32.768 kHz crystal can be used the same way to measure the DCO frequency more precisely at the temperature and Vcc you are using. Likewise, it is not needed to go into LPM3.

  • 0 in reply to old_cow_yellow
    Genius 3290 points

    I suppose I can source ACLK from an 8 MHz crystal instead of a 32.768 kHz crystal in order to wake up from LPM3, correct?

    Then I can also use the 8 MHz crystal for baud rate generation.

  • 0 in reply to PaulSchoenke
    Guru 58965 points

    I suppose you could. But I do not understand why you would do such a thing.

    People normally use LPM3 to save power. VLO can be used as ACLK and no external component is needed. If VLO is not accurate enough, a 32.728 kHz crystal can be used. Either way, current consumption is reduced down to 3.5 uA at -40 C or 48 uA at +85 C.

    8 MHz crystal probably needs an extra 300 uA,

  • 0 in reply to old_cow_yellow
    Intellectual 310 points

    I am using ez430-RF2500T, MSP430F2274 running on internal DCO (8MHz).

    According to your reply, (if I understand correctly) F2232/F2274 share same silicon platform, it can hardly support high data rate with internal DCO, can it? However someone claims they reach 100K+ data rate on RF2500T with UART/USB bridge like FTDI/Profilic/CP and etc. (off course, in room temperature range). 

    Why RF2500T didn't offer XT2 as an option? It misled too many people. TI should offer an upgrade version of that kit.

  • 0 in reply to allankliu
    Guru 227245 points

    This is the fourth thread where you repeat the same question. See the other ones.

    Kai Liu101519 said:
    Why RF2500T didn't offer XT2 as an option?

    The RF2500 is just for evaluation of MSP/RF combo, no a 'one-for-all' hardware. If you need XT2 for a specific applicaiton, feel free to build your own PCB and populate it with any MSP you want. There are >400 different available. But don't expect TI to provide 400 versions of the EZ430-RF2500.

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