[flashrom] Impedance match of input wires: by resistors or capacitors

Rudolf Dovičín rudolf.dovicin at gmail.com
Sun Aug 28 22:04:48 CEST 2016


I try to reprogram BIOS of ThinkPad X200 notebook "in situ" by
OrangePi PC (its SPI bus).

I cannot read it consistently even if on low frequency 32 kHz.
I use 100 mm short wires.

On the help page:
there is recommendation about matching the impedance of inputs & wires:
"The impedances of the wires/traces do not match the impedances of the
input pins (of either the circuit/chip on the mainboard or the
external programmer). Try using shorter wires, adding small (<100 Ohm)
series resistors or parallel capacitors (<20pF) as near as possible to
the input pins (this includes also the MISO line which ends near the
programmer) and/or ask someone who has experience with high frequency

I know basic of electrical engineering & a bit about electronic, but I
am very very far from expert.

I try to search this mailing list archives for words "capacitor" or
"capacitors", but with no demanded answer.

I try google it, e. g. :
(ROM|PROM|EPROM|EEPROM)(flashing|program|rewrite)("ISP"|"in situ"|"in
system")(capacitor|impedance)"input" -crystal -oscillator
but it found me only articles about
which I think that I quite understand, yet
or documentation from Actel, Advanced Micro Devices, Altera, Atmel,
CheckSum, CML Microcircuits, Cypress Semiconductor, Elnec, LAPIS
Semiconductor, Lattice Semiconductor, Microchip Technology, Microsemi,
Numonyx, Silicon Labs, STMicroelectronics, Texas Instruments, Xilinx
etc. sometimes about In-System Programming, but never about these

What is better, resistor or capacitor (or combination)?
How to connect them? (Resistor is clear - in series with the wire. But
what about capacitor? Parallel with which?)

On page (bottom "Notes about stability")
there is recommendation about foiled twisted pair with link to image
- should not be second wire of the pair respectively foil connected to
the ground?
Rudolf Dovičín
Partizánska 1506/92-34
957 01  Bánovce nad Bebravou 1
mailto:Rudolf.Dovicin at Gmail.com
xmpp:ruwolf at jabbim.sk
(cellular: +421 940 872 846, short messages only)

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