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View Full Version : Is RESET 5V tolerant?



blittled
04-10-2012, 05:22 PM
I'm hooking up a Propeller to a system with another microcontroller that runs at 5V. That system already has a 10k pullup resistor tied from the reset line to 5V and I was wondering if I could simply put the Propeller's reset on this line? The other lines that interface between the two systems will have 10k resistors inline to protect the Propeller.

Leon
04-10-2012, 05:34 PM
If you look at the data sheet (page 25) you will see that it isn't 5V tolerant!

Mike Green
04-10-2012, 06:01 PM
The Propeller's reset pin isn't 5V tolerant, but the 10K pullup resistor will limit any "fault" current to a tolerable level (< 500uA), so you could tie the Propeller's reset pin to the reset line for the other microcontroller.

Phil Pilgrim (PhiPi)
04-10-2012, 06:34 PM
Some caveats:


1. With the two reset lines tied together and pulled up to +5V, they will only be pulled up to about 3.9V or so, due to the protection diodes in the Propeller shunting the input to its own Vdd.
2. Under no circumstances should the reset line be driven to 5V, for example, via a supervisor chip with a push-pull output.
3. With the Prop's nBOE pulled low, the Propeller will pull nRST low during a brownout condition.

If the other micro will stay out of reset with it's /RST line pulled to 3.3V, I'd be more inclined to do it that way.

-Phil

Leon
04-10-2012, 06:58 PM
Does the reset pin have protection diodes like the I/O pins? I wasn't sure. Some other chips don't have them on reset.

Mike Green
04-10-2012, 07:12 PM
The protection diodes are there for ESD protection. Other microcontrollers should have them since the reset pin is an "external" signal. Crystal oscillator pins (XI / XO) might not have them or might have a more limited type of protection since they're really analog I/O when a crystal is attached.

Phil Pilgrim (PhiPi)
04-10-2012, 07:45 PM
I guess, if you wanted to be absolutely safe, you could connect the two resets together with a diode (e.g. 1N4148), anode end to the Prop's nRES, cathode end to the other micro's reset pin. Since the Propeller has its own pull-up when nBOE is grounded, no other components would be necessary. In this configuration, however, a brownout condition in the Prop would not force a reset in the other micro.

-Phil

hippy
04-10-2012, 07:56 PM
The protection diodes are there for ESD protection. Other microcontrollers should have them since the reset pin is an "external" signal.

Many Microchip PIC's don't have internal diodes to Vcc as their reset (/MCLR) pin is dual or multi-purpose, used for supplying greater than 5V OTP/Flash program enabling voltages.

blittled
04-11-2012, 05:24 PM
Thanks all for the great advice. I'm still in the brainstorming phase of my project and it changes daily :). My plans, if they actually happen, is to interface a Propeller to a Arduino or UNO32. The Arduino being 5 V had me a little concerned but the more I think about it the better the UNO32 with 3.3 V is since it also has a better SPI library which is how I'm interfacing the two.

Another thing that may change the RESET is that I would like a setup where the UNO32 reads a binary file from a SD card then writes directly to the Propeller's EEPROM via I2C. After that it pulls the RESET line low so drivers can be dynamically changed with the UNO32.