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USCI module current consumption in LPM4?

Doug Carlson
Prodigy 70 points
Other Parts Discussed in Thread: MSP430F235

Howdy. I'm looking at the msp430f235 datasheet for the current consumption of the USCI module (specifically as I2C slave). All that I see are the timings. Is this information readily available anywhere? I didn't see it in any of the application notes on the product page, either.

Here's the datasheet: http://www.ti.com/lit/gpn/msp430f235

Thanks for any advice.

-Doug

  • 0
    Expert 1170 points

    Well, I assume that the current that is seen in the external components (pull-ups of the I2C bus) is much higher than the USCI current itself. When the USCI is not clocked you should only see a leakage that is included in the LPM currents listed in the datasheet.

  • 0
    Guru 227245 points

    Doug Carlson said:
    All that I see are the timings.

    Well, the static current consumption for the USCI is negligible.
    And the dynamic current consumption depends on: supply voltage, bus clock frequency, strength of the external pullup, number of peers on the bus, bus parasitic capacitance etc.
    It has too many external influences to be stated, not even a 'typical' value, even less a min/max bounding box.

  • 0 in reply to Jens-Michael Gross
    Expert 1230 points

    Hi all,

    I am wondering the same. Isn´t there anything about current consuption of usci with different frequencies/voltage?

    I need to know wheter it's better to run a high baud rate in my uart. I need to transmit/receive 12 bytes and i need to know if it's better to have higher baud rates (smaller transmitting time) or smaller ( less current consuption).

    What should i do?

    I am working with lpm3 and waiting for interrupts in the usci A module.

    The same applies to my i2c line... Which is the better method?

    I guess i need more information in order to be sure about the difference and not to apply trial and error method...

    Thanks!

  • 0 in reply to leinho
    Guru 227245 points

    Leonardo Mart��nez said:
    Isn´t there anything about current consuption of usci with different frequencies/voltage?

    I explained it above.
    You have the idle current and the operational current. The idle current is same for all the tiem the USCI is active (which it is all the tiem if you want to receive anything).
    The operational current depends on the nubmer of transistor switch operations per second. And for sending (or receiving) a given number of bytes, the number of transistor switching operations is the same, independent of the baudrate. At twice the baudrate, you have twice the current for half the time. Same energy for the same amount of data.

    Leonardo Mart��nez said:
    I am working with lpm3 and waiting for interrupts in the usci A module.

    So the USCI is active and the static current always flows. Independent of the baudrate.

    Leonardo Mart��nez said:
    The same applies to my i2c line... Which is the better method?

    On I2C, things are a bit different. Here you have pullups on the line and pull the line down while sending. A higher baudrate means less time the master or slave pulls the clock and data lines low and therefore less current is drawn from the pullups. However, the current is not drawn by the USCI itself, but externally, through the pullup resistors. If you only look at the MSP, here again no difference with different baudrates.

  • 0 in reply to Jens-Michael Gross
    Expert 1230 points

    Ok, thanks Jens-Michael.

    I have read that i could wait in lpm4 for interrupts in rx of my uart, is that correct?

    Does the SMCLK keeps alive if I set that clock to my usciA?

    Is it correct if I see 150 Khz in my i2c?

    Is there any limitation with working with my mcu at 1.8 V and dco = 1 Mhz?

    Thanks!

  • 0 in reply to leinho
    Expert 1230 points

    I haven't any prescallers and UCBRx = 4...

  • 0 in reply to leinho
    Guru 227245 points

    Leonardo Mart��nez said:
    Does the SMCLK keeps alive if I set that clock to my usciA?

    Depends ont he MSP you use (see users guide). On some families, SMCLK is switched off, on some it is only switched off if not needed by a peripheral.

    Leonardo Mart��nez said:
    Is it correct if I see 150 Khz in my i2c?

    On 1;Hz and /4 divider, it should rather be something like 250kHz. However, the timings are not fixed. Due to clock stretching and start/stop conditon, a scope might see an average frequency of 150kHz.

    Leonardo Mart��nez said:
    Is there any limitation with working with my mcu at 1.8 V and dco = 1 Mhz?

    Yes. With VCC=1.8V, the maximum voltage on any MSP pin is 2.1V. However, the I2C slaves won't likely accept 2.1V as high voltage. And if your pullups go to 1.8V (VCC) as you have it available, then the slaves will nly see 1.8V, which even less likely will be accepted.
    And since I2C is bi-directional and also open-collector, it is quite difficult to do any level translation on the lines. So unless your other I2C peers are 1.8V devices too, I don't think it will work. Independent of the frequency.

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