In SX Microcontrollers, SX/B Compiler and SX-Key Tool, g_daubach wrote:

Yes, I will also need power down the modules while the SX sleeps, and you are right, the SX can't pull something low or high while sleeping. I have just checked the TX module, and found out that, although pin 8 (PDN) of the LINX TXM-433-LR-S is directly connected to the PDN header pin, you can't measure a meaningful resistance between the PDN header pin and the GND or the 5V header pins. On the other hand, there is a 4.7 kOhm resistor connected between pins 8 (PDN) and 7 (Vcc) of the TXM-433-LR-S. Without checking in detail, I assume that one of the on-board transistors is used to completely cut off the supply voltage of the TXM-433-LR-S. This is why the PDN header pin has a relatively high impedance. IOW, pulling this line low with something like 10 kOhm would nicely keep the Tx quiet while the SX sleeps, and pulling this line high with an SX output would not consume much power, so this is exactly what you and I are looking for. Nice design, Parallax!!!

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