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I need to gate a TTL Tx pin @115000bps as the chipset that drives the RxTx lines do not implement UART BREAK for arbitrary time periods.

Thanks to this community's feedback (Selecting proper vales of Rb, Re and and Rbe in a NPN transistor emitter follower configuration?) I have come up with the following configuration that I will connect to the TTL Tx pin of the said chipset, at TTL.TX.In.

enter image description here

The Tx output from the whole device will be tapped from TTL.TX.Out

BREAK is the gating input that controls whether TTL.TX.Out will be in BREAK condition or not.

BREAK is an output pin of a typical FET Microcontroller - say AVR, MSP430 or PIC.

The TTL levels can be ~5V DC or ~3v3 DC.

My questions are:

  1. What should be the proper value of R1 and D1? (I am thinking R1 = 1K and D1 = 1N4148 but that is very arbitrary)

  2. Is this a proper design that will allow me to implement BREAK for arbitrary time periods without affecting the "usual" transmission (When BREAK is off and all signals should go through)?

My fears are:

  • i. The Vf drop, typically 0.7V can degrade the output signal quality

  • ii. The 1N4148 is speced to work upto 4ns but it's capacitance is around 1pF and taking stray capacitance into account, I am not sure how the output signal would look like @115000bps.

Would you have reason to believe that the UART signal outputs could become severly degraded?

I have shared the schematic here. Feel free to make copies and modify it: https://www.circuitlab.com/circuit/gh3g6z/ttl_uart_gating_public/

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2 Answers 2

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This circuit should work but I would make two serious recommendations.

1) Purge that 1N4148 diode and replace it with a Schottky type, such as a BAT54A, to get a much lower Vf.

2) What is the aversion to using a single logic gate part? Put in a single 2-input gate device to give you the ability to override the UART TTL output to generate the break condition.

If your project is a one-off type or hobby project you should be just fine with approach 1. On the other hand if you are making a commercial product I really recommend approach 2.

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  • \$\begingroup\$ Besides the lower Vf of the BAT54A is there a reason to purge that 1N4148 diode? Will the 1N4148 fail miserably @115000bps? \$\endgroup\$
    – vsmGuy
    Dec 18, 2012 at 19:43
  • \$\begingroup\$ @vsmGuy - I do not think that the diode will have a problem switching fast enough for 115.2k baud. Note that the BAT54A (or other similar Schottky diode) will typically switch faster than a normal silicon diode so you have two reasons to toss out the 1N4148. \$\endgroup\$ Dec 19, 2012 at 4:14
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Unless your microcontroller has fixed-function special purpose pins, you would likely find it much easier to temporarily reconfigure the TX pin as a GPIO, drive it low for the desired period of time, then reconfigure it as the UART output.

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  • \$\begingroup\$ Correct - however, the UART periperal controller in the microcontroller "takes" over the Tx and Rx pins so that the firmware cannot explicitly disable the Tx pin. \$\endgroup\$
    – vsmGuy
    Dec 18, 2012 at 19:41
  • \$\begingroup\$ Then you disable the UART and re-enable it. \$\endgroup\$ Dec 18, 2012 at 21:37
  • \$\begingroup\$ True. However, if this can also be done with a DRL why not try that approach? :-) \$\endgroup\$
    – vsmGuy
    Dec 18, 2012 at 22:21
  • \$\begingroup\$ Because a software only solution is cleaner and cheaper where it is workable. \$\endgroup\$ Dec 19, 2012 at 12:32

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