Liapac RLP wrecks PIC16f877a UART?

Wireless & IoT
1

I’m trying to build a simple wireless project that involves one part with 8 switches that transmits the status of the switches to another part that turns 8 leds on and off. I’ve been testing this on two seperate breadboards on the same 5V power supply sitting side by side. I got the serial link working just fine using the hardware UART’s running at 1200 baud. I simply directly connected the TX of one 16f877a to the RX of another. However when I tried to put the Liapac TLP/RLP434’s in the circuit the wireless modules worked fine, but the RX port of my PIC chip was wrecked! The circuit connects the Data Out port of the RLP directly to the RX line, pin 26, of my PIC. I also included a 10k pull-down resistor on that pin and every other IO pin. Before using the RLP the pin acts as a normal, high-impedence IO pin. After using it the pin seems to be a short to ground. Interestingly this is only in hardware UART mode, if the UART is not enabled it returns to normal high-impedence.

The reciever module is still working fine, though the 4 pics I’ve tried with it sure aren’t… The last PIC I tried it with I connected to the RLP through a 10k resistor and included a zener diode and 1uF capacitor to try to avoid a voltage spike from wrecking the pin. Still wrecked it. I monitored the voltage with a digital and analog meter during this, neither detected anything unusual. On powerup the voltage goes up to about 4.5V and then settles down to about 1.5V. Of course, this is an average, who know’s what the voltage actually is. Unfortunately I don’t have an oscilloscope, though I will be getting one in about a week or so.

Suggestions? Anyone run into this before?

2

try a non inverting buffer inbetween ther receiver and pin.

3

if you are running at a 5V supply for the RLP and the PIC, I don’t think you are getting voltage spikes over 5V. Also, the spec shows the rlp sources 200 microamps and sinks 10 microamps so that’s unlikely to blow RC7.

Is it possible you zapped the chip with some sort of esd? That might be more likely to take out the port though I would expect the mosfets at the pin to blow rather than the USART circuitry.

Phil

4

Well I tried using a very simple non-inverting buffer made out of a transistor… Blew the chip again! The buffer by itself with a dummy input, IE I just tried hooking it to 5V and then GND, didn’t wreck anything. I also tried wearing a cheap ESD wrist strap while I did this, still no luck.

Very bizzare! Hopefully I’ll have my new oscilloscope arrive soon to figure this out further…