Olimex Support Forum

ARM => JTAG => Topic started by: smite on January 04, 2021, 11:16:35 pm

Title: Olimex ARM-USB-OCD SRST pull-down resistance
Post by: smite on January 04, 2021, 11:16:35 pm
I'm trying to analyze why Olimex ARM-USB-OCD is having trouble resetting my board and it looks like resistance through SRST is kinda high.

I'm testing with "ftdi_set_signal nSRST 0" and "ftdi_set_signal nSRST z" with OpenOCD.
When "0" resistance is ~70 Ohms.
When "z" resistance is ~120 kOhms.

This is probably why SRST gets pulled from 4.4v to ~1.8v when trying to pull low.

Same happens when testing TRST on the ARM-USB-OCD.

Does this make sense?
Is my ARM-USB-OCD defect?
Could I test or replace something on the ARM-USB-OCD?

Title: Re: Olimex ARM-USB-OCD SRST pull-down resistance
Post by: smite on January 11, 2021, 03:58:14 pm
At ARM I read: "Active Low output from RVI to the target system reset, with a 4.7kΩ pull-up resistor for de-asserted state.".

When I test with a 70 ohm resistor, the signal goes to ~2V, so this means that my target probably has unusually strong pull-up.
Title: Re: Olimex ARM-USB-OCD SRST pull-down resistance
Post by: LubOlimex on January 12, 2021, 09:28:35 am
On the SRST line of ARM-UBS-OCD there is a 270 kOhm pull-down. This means that if there is unusually high pull-up on the reset line of the target there would be divider affect. Maybe don't use more than 10kOhm or 20kOhm pull up on the SRST line of the target.
Title: Re: Olimex ARM-USB-OCD SRST pull-down resistance
Post by: smite on January 14, 2021, 10:43:39 am
Thanks for the info. I don't have a lot of JTAG experience and I'm just experimenting with an old board to get some practice.

In case other people find this post:

It looks like my measurements of the ARM-USB-OCD are accurate and nomimal.
The ARM-USB-OCD is not broken.
The board I'm testing, Seyeon Flexwatch-CAM100, has an unusually strong pull-up on the combined SRST/TRST pin.

To get the reset pull-down strong enough I use a buffered inverter to invert the SRST signal from the ARM-USB-OCD and drive a NPN transistor connected to the SRST signal of the board and GND.