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List:       openocd-development
Subject:    Re: [OpenOCD-devel] openocd newbie flashing questions
From:       Michael Schwingen <michael () schwingen ! org>
Date:       2018-03-29 6:57:57
Message-ID: 20180329085757.7dcb253c () a-tuin ! ms ! intern
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On Wed, 28 Mar 2018 22:38:20 -0500
John Griessen <john@ecosensory.com> wrote:

> On 03/28/2018 08:16 PM, John Griessen wrote:
> > What is the meaning of Vref in the SWD config
> > (interface/ftdi/swd-resistor-hack.cfg)
> > 
> > 
> > " Vref of the adapter should be supplied with a voltage equal
> > to the target's (preferrably connect it to Vcc).
> > 
> > # FTDI                          Target
> > # ----                          ------
> > # 1  - Vref   ----------------- Vcc
> > # 3  - nTRST  -
> > # 4  - GND    ----------------- GND
> > # 5  - TDI    ---/\470 Ohm/\--- SWDIO
> > # 7  - TMS    -
> > # 9  - TCK    ----------------- SWCLK
> > # 11 - RTCK   -
> > # 13 - TDO    ----------------- SWDIO
> > # 15 - nSRST  - - - - - - - - - nRESET
> > "  
> 
> There's VCCIO , type power input, but that's a VCC supply voltage
> going into the FTDI JTAG adapter, not an ADC input like I think Vref
> should be.

VREF is typically used to power the buffers/level translators on the
JTAG adapter board that are used to drive the cable. Depending on the
design of the JTAG adapter, VREF *may* be unused, but in that case, the
JTAG adapter will only support a limited range of target voltages (eg.
the chinese ST-Link clones).

If the FTDI chip has a VCCIO power pin, that would be exactly what VREF
is used for when not using external buffer chips.

Having an ADC input that can measure and report back VREF voltage is a
nice-to-have feature present on only a few JTAG adapters.

cu
Mcihael



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