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TCA9548A: Using TCA9617B in series with TCA9548A switch

Dung Huynh
Prodigy 170 points

Part Number: TCA9548A
Other Parts Discussed in Thread: TCA9617B, P82B96

Will 100kHz I2C bus with the following components work?

I2C master 1.5V interfaces with 2 TCA9617B buffers in parallel, each TCA9617B buffer in series with 3.3V TCA9548A switch, each port of TCA9548A switch connect to one 60pF slave device with the same I2C address.  There could be scenarios which all 16 paths (all ports of 2 TCA9548A switches) need to be on at the same time.

Regards,

Dung Huynh

  • 0
    TI__Expert 8870 points

    Hi Dung,

    Do you have a block diagram of this use case?

    The 100kHz I2C configuration seems to not be a issue but operating with all 16 paths active simultaneously does seem like a possible issue. Can you elaborate on this?

    I am also adding the device owners of the TCA9617B for there input on this scenario.

    Thank you,
    Arya

  • 0 in reply to Arya Hussein
    TI__Guru 60998 points

    Hi Dung,

    A block diagram would help to understand the application. TCA9617B buffers have design requirements on the B-side where only A-A, A-B, B-A, connections of the buffer are allowed with series and parallel applications of multiple TCA9617B's. B-B connection of two or more TCA9617B's is problematic. 

    Switching on all 16 paths of the TCA9548A can be problematic depending on your parasitic bus capacitance. 16 paths, or 8 paths per SCL and SDA can easily accrue past the 400pF limit required by the I2C standard. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 170 points

    Hi Tyler and Arya,

    Please see diagram below.

                                                                                        ___________ 3.3V PU at each port
                                                                                        |                    |-----------3.3V Slave, Add 0x21, 60pF
                                                                                        |                    |-----------3.3V Slave, Add 0x21, 60pF
                                                                                        |                    |-----------3.3V Slave, Add 0x21, 60pF
                                                  __________                  | TCA9548A |-----------3.3V Slave, Add 0x21, 60pF
                                                 |                    | 3.3V PU  |                    |-----------3.3V Slave, Add 0x21, 60pF
                                        ____ | TCA9617B |________ |                    |-----------3.3V Slave, Add 0x21, 60pF
                                        |        |__________|                 |                    |-----------3.3V Slave, Add 0x21, 60pF
                                        |                                               |__________|-----------3.3V Slave, Add 0x21, 60pF
                                        |
    __________                |
    | 1.5V          | 1.5V PU |                                               ___________ 3.3V PU at each port
    | I2C Master|------------ |                                              |                     |-----------3.3V Slave, Add 0x21, 60pF
    |_________ |               |                                              |                     |-----------3.3V Slave, Add 0x21, 60pF
                                        |        __________                  | TCA9548A  |-----------3.3V Slave, Add 0x21, 60pF
                                        |        |                    | 3.3V PU  |                     |-----------3.3V Slave, Add 0x21, 60pF
                                        |____| TCA9617B |________ |                     |-----------3.3V Slave, Add 0x21, 60pF
                                                 |__________|                |                     |-----------3.3V Slave, Add 0x21, 60pF
                                                                                       |                     |-----------3.3V Slave, Add 0x21, 60pF
                                                                                       |__________ |-----------3.3V Slave, Add 0x21, 60pF

    Master VCC = 1.5V, Vil = 0.525 V, VoL = 1.125 V, Iol = 6 mA, Mode 100kHz
    Slave VCC = 3.3V, Vil = 0.8 V, VoL = 0.4 V, Iol = 1.6 mA, Mode 100kHz

    For normal operation, only 1 of 16 port is on. There could be scenarios which all 16 paths (all ports of 2 TCA9548A switches) need to be on at the same time.

    Regards,

    Dung Huynh

  • 0 in reply to Dung Huynh
    TI__Guru 60998 points

    Hi Dung,

    A couple questions about this implementation. 

    Is TCA9617B only used for level translation purposes, or do you need buffering? In this orientation, the TCA9617B VCCA = 1.5V, while VCCB = 3.3V. The VOLB of the TCA9617B is 0.58 V. This is compatible with the VIL of the TCA9548A and the 3.3V slave(target) devices. The slave's VIL = 0.8 V which is sufficient margin. 

    Please adjust the pull-up resistors on the slave side and the side connecting B-side of TCA9617B to TCA9548A. The VOL = 0.4 V of the slave device is good for the B-side input of the TCA9617B. 

    Dung Huynh said:
    For normal operation, only 1 of 16 port is on. There could be scenarios which all 16 paths (all ports of 2 TCA9548A switches) need to be on at the same time.

    Please adjust the pull-up resistors to ensure it is compatible in all states. For example, if all 16 paths are enabled, this will look like 9 pull-up resistors in parallel = 1 from each enabled channel, and 1 pull-up on the main I2C bus of the TCA9548A. All 9 of these pull-up resistors will appear in parallel because the TCA9548A is a switch based architecture and not a buffer. The slave side sees other slaves, pull-up current will appear in parallel. 

    Dung Huynh said:
    Master VCC = 1.5V, Vil = 0.525 V, VoL = 1.125 V, Iol = 6 mA, Mode 100kHz

    The VOL of the master is concerning. Is it possible to adjust the pull-up resistors so that the VOL < 30% of VCCA = 0.45V? 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 170 points

    Hi Tyler,

    TCA9617B is used for level translation purposes and to isolate the top 8 slave from the bottom 8 slave if all 16 paths are enabled.
    Questions:
    VOLB of the TCA9617B is 0.58V, VOL of TCA9548A is 0.4V with Iol min of 3mA. Will Iol at slave side of TCA9548A be 0.58V + 0.4V = 0.98V when TCA9617B is in series with TCA9548A?
    If the capacitance for each slave is 100pF, is there any TI buffer which can replace TCA9617B and support 8x 100pF load (all 8 paths of each TCA9548A are enabled)?

    Regards,

    Dung Huynh

  • 0 in reply to Dung Huynh
    TI__Guru 60998 points

    Hi Dung,

    Dung Huynh said:
    VOLB of the TCA9617B is 0.58V, VOL of TCA9548A is 0.4V with Iol min of 3mA. Will Iol at slave side of TCA9548A be 0.58V + 0.4V = 0.98V when TCA9617B is in series with TCA9548A?

    No. The VOL = 0.4 V at iol = 3mA is when the TCA9548A is pulling the bus low either through an ACK bit or read data transaction. 

    The output to the slave from the TCA9617B buffer will only increase by an rds_on voltage drop across the enabled channel passFET. See diagram: 

    Where VOL2 > VOL1 = 0.58V + small voltage drop across passFET

    Dung Huynh said:
    If the capacitance for each slave is 100pF, is there any TI buffer which can replace TCA9617B and support 8x 100pF load (all 8 paths of each TCA9548A are enabled)?

    Our buffers are designed to drive cap loads specified by the maximum range of the i2c standard. TCA9617B supports up to 550 pF loading on both sides of the buffer. 

    An 800pF load is out of spec. There are devices such as P82B96 which are long distance I2C buffers that can buffer up to 4000pF cap loads, but the VOL levels will not work for your system and the P82B96 is usually used in a pair rather than a standalone device. 

    It is recommended to not have all 8 channels enabled in your case. IF possible, can you only enabled < 5 channels at a single time to keep the cap load from exceeding 550pF? 

    Regards,

    Tyler