doc: Add doc for in-system programming

The page on wiki is here:
https://wiki.flashrom.org/ISP

Change-Id: If4752f0f02ae973b3d832f42166de643d95c9f97
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/83451
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Alexander Goncharov <chat@joursoir.net>

doc/user_docs/in_system.rst

@@ -0,0 +1,45 @@
+=====================
+In-System Programming
+=====================
+
+**In-System Programming** (ISP) sometimes also called **in situ programming** is used to describe
+the procedure of writing a flash chip while it is (already/still) attached to the circuit
+it is to be used with. Of course any normal "BIOS flash" procedure is a kind of ISP
+but when we refer to ISP we usually mean something different: programming a flash chip by external means
+while it is mounted on a motherboard.
+
+This is usually done with SPI chips only. Some mainboards have a special header for this
+(often named "ISP", "ISP1", or "SPI") and there should be no problem with accessing the chip
+then as long as the wires are not too long.
+
+If there is no special header then using a special SO(IC) clip is an easy and reliable way
+to attach an external programmer. They are produced by different vendors (e.g. Pomona, 3M)
+and are available from many distributors (e.g. Distrelec) for 20-50$/€.
+
+Problems
+========
+
+* Check the other potential problems (:doc:`misc_notes`) with other types of programming setups first.
+* The SPI bus is not isolated enough. Often parts of the chipset are powered on partially
+  (by the voltage supplied via the Vcc pin of the flash chip). In that case
+  disconnect Vcc from the programmer and power it with its normal PSU and:
+
+  * Try powering up the board normally and holding it in reset (e.g. use a jumper instead of the reset push button).
+  * Some chipsets (e.g. Intel ICHs/PCHs) have edge triggered resets. In this case holding them in reset will not work.
+    This is especially a problem with Intel chipsets because they contain an EC (named ME by Intel, see :doc:`management_engine`),
+    which uses the flash (r/w!). In this case you can trigger the reset line in short intervals.
+    For example by connecting it to the chip select (CS) line of the SPI bus or a dedicated clock signal from the programmer.
+    This should not be too fast though! Reset lines usually require pulses with a minimum duration.
+  * On some boards, you can try disconnecting the ATX12V header (yellow/black wires only) from the motherboard,
+    or even remove the CPU or RAM - if the programmer supports SPI sniffing, you may be able to verify that the there is no SPI traffic.
+
+Images
+========
+
+Pomona 8-pin SOIC clip with attached jumper wires.
+
+.. image:: Pomona_5250_soic8.jpg
+
+A cheap, but very fragile alternative: DIP socket as clip
+
+.. image:: 1200px-DIP_socket_as_SOIC_clip.jpg

doc/user_docs/index.rst

@@ -9,4 +9,5 @@ Users documentation
     chromebooks
     management_engine
     misc_intel
+    in_system
     misc_notes