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MAX3232E: Pull-up requirement for UART input and output lines

Vasu
Prodigy 120 points
Part Number: MAX3232E

Tool/software:

Hello Team, 

MAX3232EI line driver is planned to use as line driver device between the Renesas RX series controller SCI pins  and the RS232C serial flash programmer . 

In the Renesas controller datasheet it is mentioned to use the pull-up resistor for TxD and RxD lines for SCI lines, but the necessity of pull-up circuit with RS232  line driver is not mentioned in the data sheet . 

Please clarify if we use the MAX3232E line driver still we need to provide the Pull-up circuit for both RxD and TxD lines. 

In previous version of the ECU circuit  , with different Renesas controller, pull-up circuit is provided only at the ROUT pins. 

  • 0
    TI__Guru 72521 points

    Hi Vasu,

    Can you provide a block diagram of what pins are being connected together?

    Am I to understand you are connecting an MCU pins to the TTL pins of the MAX3232E or are you trying to interface it with the RS232 pins?

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 120 points

    Hello Bobby , 

    Block diagram attached below. 

    Recommendation from the controller data sheet is to use the Pull-up resistor in both TxD and RxD pins when it is directly connected to the Flash programmer .

    Since MAX3232E1 is used in between pls confirm still the pull up is needed or it can be managed in MAX3232EI

  • 0 in reply to Vasu
    TI__Guru 72521 points

    Hi Vasu,

    Thanks for clarifying. Normally you can place resistors on the TTL pins which are the DIN/ROUT pins.

    I've never seen anyone place pull up resistors on the RS232 pins before. I actually do not really understand why you would want to or need to on the DOUT pin. This puts a larger load on the RS232 driver, if the resistor is a small value then it could cause the V+/V- rails to collapse. I wouldn't suggest you place anything lower than 5k ohms if you were to do this. Larger would technically be better to reduce the current load that gets introduced. 

    The RIN pin wouldn't care but you could get funny stuff happening if the flash programmer wasn't driving and the pin sits inbetween Vth- and Vth+. 

    My personal opinion is you shouldn't place a resistor there but I would need to know what the flash programmer's inputs/outputs look like that connect to the DOUT/RIN pins. Either way, I still think you're better off not placing pull up resistors on those lines. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 120 points

    Hello Bobby , 

    Thanks for your feedback. Actually, the query is asked to clarify whether to place pull-up resistor on TxD and RxD lines of MCU means DIN/ROUT pins of MAX3232EI, not on the DOUT and RIN pins

  • 0 in reply to Vasu
    TI__Guru 72521 points

    Hi Vasu,

    That makes more sense. I would say that you should place a pull up resistor (maybe 4.7k or larger) on the DIN pin. 

    The ROUT pin should always be active when our device is powered, if the Renesas MCU is powered by the same power rail as the MAX3232E, then you don't need the external pull up. If they sit on different rails, I would put a pull up resistor there but make it much larger like 47k ohms. 

    -Bobby