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Tool/software:
Hi.
We have a scenario where battery life is critical. We are implementing sniff mode/eWor/sleep, minimising the TX power where possible, and minimising packet sizes. We can use: 2-FSK (2 kbps, 2 ksps) or 4-GFSK (4 kbps, 2 ksps) - slow but apparently better for battery life than high data rates.
But Smart Studio 7 does not show any delta in current drains on RX or TX for 4-GFSK compared to 2-FSK, and the user guide for the CC1120 has only a subjective comment that 4-GFSK draws more current. But how much more current? (How long is a piece of string.) Slower date rate means a longer TX time. So might mAh be similar? Maybe there is more processing power by CC1120 for the 4-GFSK compared to the 2-FSK that consumes more power. Maybe Smart Studio is not no smart, or I have missed something.
If there were little difference between these modes on battery life, we would stick with the 4-GFSK. Is there documentation anywhere that details the different modes and the current drain impacts on the CC1120?
Other basic app info:VDD = 3.0V, Freq = 923MHz or 868 MHZ.
(Incidentally, I have test equipment that can accurately measure currents down to 10 picoamps. However I loaned it out to someone and disappointingly it came back inoperable! So until it is repaired, meaningful documentation would help me and help others.
regards,
David
Hi David,
Could you please point me to the section in the user guide where it mentions that it draws more current?
Hi Diego,
Sorry, I was mistaken, it is inferred in the CC1120 datasheet, section 4.4. Power Consumption Summary, not the user guide. TX modes says nothing, but RX modes shows a significant increase in current with a slightly higher speeds in "RX wait for sync". 1.2 kbps = 2mA, 38.4 kbps = 13.4mA. So, from 2 kbps (2-FSK) to 4 kbps (4-GFSK) what is the increase in energy consumption? Same symbol rate but no hard data on how this effects consumption. Also, lower speeds = longer TX/RX time, high speeds = shorter TX/RX time. We are using the Lithium Thionyl Chloride batteries and current consumption must be managed due to relatively high internal resistance within the batteries.
regards,
David
Hi David,
As mentioned in the previous thread, there is no difference in current consumption between 2-FSK and 4-GFSK in continuous TX or RX.
Note that in the datasheet (Section 4.4) it mentions:
Using RX sniff mode, where the receiver wakes up at regular intervals to look for an incoming packet.
and links to the following App Note: SWRA428 (CC112x/CC120x RX Sniff Mode): https://www.ti.com/lit/swra428
You can use the calculator linked in the App Note to calculate the expected average current consumption.
This App Note shows how the current consumption is calculated (and differs between PHYs) in RX when using RX sniff mode. The current consumption in RX is dependent several parameters (discussed in SWRA428) and is a trade-off between these. So, the difference in RX current consumption numbers in the datasheet is due to the use of RX sniff mode.
One difference is the RXBW required for the different data rates. The following thread (and the App Note) points out that the RXBW changes the CS response time and, hence, the current consumption: https://e2e.ti.com/support/wireless-connectivity/sub-1-ghz-group/sub-1-ghz/f/sub-1-ghz-forum/382946/cc1120-rx-sniff-mode-consumption-vs-rx-bw - a higher data rate requires a larger RXBW.
As an aside (but a follow-on point), Siri's reply notes in the following thread that it is recommended to first check your settings work correctly before implementing RX sniff mode: https://e2e.ti.com/support/wireless-connectivity/sub-1-ghz-group/sub-1-ghz/f/sub-1-ghz-forum/728727/cc1200-sniff-mode-with-short-preamble-length
In RX sniff mode the average RX current goes down if you increase the TX preamble length (the wake up interval can then be longer). This means that the device will spend longer time in TX, but in most protocols the RX current is of more concern.
Regards,
Zack
Hi Zack.
Many thanks for the reply. Much appreciated. I used the Smart RF studio to minimise the current drain when sniffing packets, and noted the preamble length has a big impart on current drain as does the RX filter bandwidth. I believe we can get this optimal. I will read the links you sent too. I will use 4-GFSK.
Once again, thanks. You have been most helpful.