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Original question:

CC1101: Noise in Async mode RX without TX connected

CC1101-Q1: Noise Detected on GDO1 During Asynchronous Mode RX Despite Absence of Corresponding TX Transmission

tech15
Prodigy 120 points
Part Number: CC1101-Q1
Other Parts Discussed in Thread: CC1101

I configure the GDO1 as the Serial Data Output, that is :

CC1101_Write_Reg(CC1101_IOCFG1, 0x0D );

and then I can detect the signal by a oscilloscope.

With the TX transmission present, the signal waveform received looks normal, just the same as what I transmit in Tx:

but when I powered off the transmission device, the siganl received on GDO1 looks like below:

I have read some posts, which mentioned Carrier Sense Threshold,  and I tried,but cannot eliminate the noise signal.

Can anyone help me with some insights? many thanks in advance.

  • 0
    TI__Mastermind 38960 points

    Hi there, 

    We will look into your posts and give follow up by early next week. Please note some of the team who are experts on this are on vacation and will be back next week. 

  • 0
    TI__Expert 3280 points

    Hi,

    Could you tell more about your setup? What mode are you operating in, packet mode, serial mode? What data rate and modulation are you using?

    Regards,

    HG

  • 0 in reply to Haiatullah Gholami
    Prodigy 120 points

    Thank you for your response. Currently, I'm testing with a data rate of 1.2 kbps and GFSK modulation. If you need more specific parameters, I can provide them tomorrow when I return to the office as I'm currently at home.

    Regarding your mention of packet mode and serial mode, I am a bit confused. I have only encountered the term "asynchronous serial mode" in the datasheet, and assumed that only serial mode applies to asynchronous mode, not packet mode. Could you please explain the difference between packet mode and serial mode, and how they relate to asynchronous mode?

    Thank you for your assistance.

  • 0 in reply to tech15
    TI__Expert 3280 points

    Hi,

    The normal/packet mode is the recommended mode to operate the device. In this mode you have more packet handling features available. For instance, the data will be set to the pin only when the device receives the correct sync word or when the signal is above carrier sense threshold. 

    In asynchronous mode most of the the features are not available. This mode is used mostly when the transmitter uses old packet formats that do not have preamble+sync+data+crc format or if the packet format is not flexible. 

    Could you provide the packet format for your transmitter so that I can guide you further? Is the preamble and sync word adjustable?

    Is the transmitter also a CC1101?

    Regards,

    HG

  • 0 in reply to Haiatullah Gholami
    Prodigy 120 points

    Hi HG,

    As you mentioned, our development involves some older packet formats, and it's possible that the transmitter may not be CC1101-based (although I don't have this exact information yet). This could be why my client specifically stated that Asynchronous mode should be used.

    Regarding the packet format, it encompasses various static codes from different types of encoders, each potentially having a distinct code format. Additionally, the preamble and sync word bits are not adjustable and may not even exist, differing across various formats.

    I hope this information clarifies some confusions, which can help you guide me further.

    Thanks

  • 0 in reply to tech15
    TI__Expert 3280 points

    Hi,

    As I mentioned, most of the packet handling features are unavailable in asynchronous mode. If you do not want to process the data when the incoming signal level is below a given level, you can monitor carrier sense. You can monitor carrier sense by configuring CC1101_IOCFG1 to 0x0E. This pin will be low until the incoming signal level goes above the carrier sense threshold. When this signal goes high, the MCU can start processing the data.

    Be aware that when using carrier sense, the sensitivity of the receiver will be limited to carrier sense threshold.

    Regards,

    HG

  • 0 in reply to Haiatullah Gholami
    Prodigy 120 points

    Thank you, HG. Since configuring CC1101_IOCFG1 to 0x0E makes GDO1 no longer serve as a "Serial data output Used for asynchronous serial mode," does that mean I should designate another GDO as the asynchronous serial data output? For example, if I configure GDO2 as the data output, would I then need to wait for GDO1 to go high before using the MCU to process the data from GDO2?

  • 0 in reply to tech15
    TI__Expert 3280 points

    Hi again,

    Correct, the carrier sense pin have to be different from the serial data output. My recommendation is to use GDO0 and GDO2 for carrier sense and serial data output respectively or vice versa. The MCU have to wait for the carries sense signal to go high before processing the data on the serial data output pin.

    Regards,

    HG

  • 0 in reply to Haiatullah Gholami
    Prodigy 120 points

    Hi, again,

    I'm encountering an issue where the received signal exhibits instability in the pulse width. I created another post due to its complexity (link: CC1101-Q1: Need Help with Noise Reduction and Pulse Width Stability for CC1101-Q1 Transmitter Development in Async mode - Sub-1 GHz forum - Sub-1 GHz - TI E2E support forums), but I haven't received a reply so far. This issue might overlap with the current discussion, so I apologize if it causes any confusion.

    The main problem is that this variability seems to increase the difficulty for the MCU to process the data correctly, potentially leading to errors. According to the datasheet, in asynchronous serial mode, the MCU should be capable of handling jitter of ±1/8 of a bit period, as the data stream is time-discrete using 8 samples per bit.

    However, the observed instability seems to exceed this tolerance, making reliable data reception challenging.

    I have also tried adjusting the carrier sense threshold, specifically using the absolute threshold (CS Absolute Threshold), but this has not eliminated the noise issue. I have some questions about this part, which I plan to discuss later for simplicity..

    I came across another post by Siri, which provided some insights:

    "There are no ways of not receiving noise when the radio is in RX. I do not see why this would even be a problem. When the 'real' data comes along, they would have a signal that is stronger than the noise, and you should be able to receive it anyway (unless there are some interferers transmitting on the same frequency that you are operating at)."

    CC1101EM868-915_REFDES: CC1101 Asynchronous Communication Mode Receive Issue - Sub-1 GHz forum - Sub-1 GHz - TI E2E support forums

    This suggests that noise is unavoidable and should not be an issue unless there is interference from other transmitters on the same frequency as the desired signal. Unfortunately, this might be the case for my situation. What steps should I take next to mitigate this issue?

    Thanks in advance.

  • 0 in reply to tech15
    Prodigy 120 points

    Any one could have a comment on this?