atmegaatmega328pavrreset
Attached is the schematic of a reset circuit. When I press the reset button, the voltage on the reset pin is 2.5 V. Rmoving R13 naturally solves the issue, but I'm unclear as to why the configuration doesn't work.
I assumed that with a 10 kΩ pull-up and 300 Ω pull-down, the reset pin would be 0.2 V.
Is it perhaps the zener diode that's causing my circuit to fail?
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
My bet is on boot0; Check that you've got it pulled low.
The boot0 pin selects where the MCU will start executing code from when it boots up, ie. comes out of reset. If boot0 is low, it'll execute your code from flash, but if it's high, it'll execute the internal bootloader from the system memory.
My guess is that you have left boot0 floating, and when you're doing a cold boot (ie. disconnecting and reconnecting power), it'll be close to zero, so the MCU will boot your code from flash, but when you do a warm boot with the reset signal, the pin will have floated to a higher value, is read as high, and the MCU will boot to the internal bootloader from system memory instead of your code.
This explanation assumes you haven't touched the boot-related option bytes, which would change the behaviour. Also, in case you'll be working with different STM32 MCUs at some point, please note that they can have slightly different behaviour. Some have different boot-related option bytes. All I've seen so far have had a boot0 pin which behaves identically, but some also have a boot1 pin (which might be shared with a gpio pin) that you might also have to set correctly. In the MCUs I remember seeing, though, boot1 is only relevant if you want to be able to boot from system memory or SRAM.
Best Answer
It seems like you're trying to follow this reset pin connection recommendation:
The recommendation according to the diagram above is to add an ESD protection diode.
But instead you're using a Zener diode. When doing so, you should connect to cathode of the Zener diode to the RESET pin, and the anode to GND.
Microchip Online Docs: