arduinocapacitormicrocontroller
My Arduino AtMega328P board has this circuit for reset. I understand the switch (LTSpice didn't have a switch symbol) pulls the line low, thats obvious, but what does the cap do when reset comes through DTR? Does it invert the signal or only allow a pulse?
IIRC the cap is wired to /RTS not DTR.
When /RTS is high there is 0V across the cap. When /RTS goes low the cap can not change voltage instantaneously so the reset line goes low. The cap charges through the 10K resistor. Once the reset line is above the reset threshold the uC program starts.
@LouisDavis referenced this page, which states
The ATmega168 is reset by pulsing its reset pin to GND. ...by setting the DTR line to LOW ... the reset pin gets sucked to LOW until the capacitor is charged through the internal pull up resistor and R1 - which resets the chip.
Simulated:
Auto-Reset
VDTR 1 0 PWL(0 5 0.5m 5 0.000500004 0 1m 0)
Vcc 3 0 5
C1 1 2 100n
R1 3 2 10k
.control
delete all
tran 10n 5m
plot v(1) v(2)
.endc
.END
V(1) is DTR or /RTS and V(2) is /RESET
In this case, I think you have to disable the reset-on-serial-connect function by modifying the board. Searching for 'arduino disable auto-reset' for how to do this to various arduino boards. It will be board specific but most involve adding a bleed resistor across the reset capacitor, removing a capacitor, or cutting a trace on the board.
Best Answer
Yes.
It converts the level-triggered signal DTR into an edge-triggered signal and has the effect of level shifting it to within the operating voltage range of the MCU (+/- a forward diode drop).
DTR (Data Terminal Ready) is a signal line used for hardware flow-control in various EIA serial protocols (such as EIA-232). In your case, it is being "hacked" to serve as a host-initiated reset of your microcontroller.
Ease-of-use
The host has software control over the state of the DTR line, but implementing a short pulse may not be possible due to scheduling or other tasking in the way.
Speed
Perhaps you want a really fast reset (you don't want a user to notice the micro went through a reset), this wouldn't be possible with software control of the reset line directly through DTR. You'd have to send a low-level, then a high-level via your host software.
Level Translation
The DTR line may be at EIA-232 levels, many volts higher (and lower) than the microcontroller can safely tolerate. AC coupling the reset edge severely limits the current (waveform energy) such that it may be safely clamped by the ESD protection diodes attached to the reset pin inside the microchip.