update README

Author: derekfountain

README.md

@@ -6,20 +6,24 @@ This device plugs into the back of a 48K ZX Spectrum and provides the
 functionality of the original 1984 "ZX Spectrum Expansion System". It
 emulates a ZX Interface One with 8 ZX Microdrives.
 
-The goal is total compatibility with all 1980s software with no ROM
-modifications or other hacks.
+The device is completely compatible with all 1980s software. It uses no
+ROM modifications or other hacks.
 
-The first version supports the ZX Microdrives; RS232 and ZX Net might follow.
+The first version is complete and supports the ZX Microdrives as
+designed; the RS232 and ZX Net features of the Interface One might follow.
 
-Open source, open hardware design.
+The project is open source, open hardware design, distributed under the
+GPL licence.
+
+Click through for a brief demonstration:
 
 [![IMAGE ALT TEXT](http://img.youtube.com/vi/kph2aS7qcVc/0.jpg)](http://www.youtube.com/watch?v=kph2aS7qcVc "ZX Spectrum Expansion System, Recreated")
 
-https://youtube.com/shorts/kph2aS7qcVc?feature=share
+https://youtube.com/kph2aS7qcVc?feature=share
 
 ## Status
 
-As of July 2023 version 1.0 is complet:
+As of July 2023, version 1.0 is complete:
 
 ![alt text](images/zses_2.jpg "v1.0")
 
@@ -28,10 +32,96 @@ It is functionally complete:
 * Spectrum can read and write all 8 microdrives
 * Uses "MDR" image format for "cartridges", fully compatible with emulators
 * Full ZX Interface One ROM support, all native BASIC commands and extensions work as designed
-* Bug for bug compatible
 * MDR files loaded from, and saved back to, SD card
 * User interface uses file selector to choose and "insert" a cartridge
-* Cartridge eject works, allowing cartridges to be changed as required
+* Cartridge eject, allowing cartridges to be changed as required
 * Configuration file allows auto-insertion of selected cartridges at start up
 
 Derek Fountain, July 2023
+
+
+# Operation
+
+It should be fairly obvious. Select the Microdrive with the rotary encoder, click the right
+button to bring up the file selector, use the rotary to select and click the right button
+again. The left button cancels.
+
+The file selector only brings up files named *.mdr, and it doesn't do subdirectories. This
+could be improved.
+
+If you create a file in the root of your SD card called zxes_config.txt (that's not a
+typo, I have an issue open to rename that file to match the reset of the project) and
+enter up to 8 lines, each with a filename giving the name of an MDR file on your SD card,
+those cartridge images will be loaded into the Microdrives on startup. (Startup meaning the
+Pico startup, not the Spectrum start up.)
+
+
+# Build Guidance
+
+If you want to have a go at building one, please ensure you check out the open issues first.
+There are a few, um, anomalies, which you might want to consider or work around. If I do
+another spin of the boards I should be able to clear these up.
+
+That said...
+
+All my surface mount component footprints are metric 3216 sized (1206i). Those are easy to hand solder.
+
+The OLED screen needs a SSD1306 chipset if you want to use my code. Be careful, some SSH1106
+based screens are labelled SSD1306. Those won't work without modifications to the code.
+
+I've used long legged header sockets (female) to raise the top board well up above the Picos. If you
+use the normal ones they probably won't be tall enough, unless you choose not to socket the Picos.
+
+## Bill of Materials
+
+You will need:
+
+2x [74LVC245 Bus transceiver](https://www.mouser.co.uk/ProductDetail/595-SN74LVC245ANSR)
+Bus transceiver, used as a level shifter
+<br>
+1x [3216 SMD Diode](https://www.mouser.co.uk/ProductDetail/581-SD1206S040S2R0)
+Schottky Diode for Pico input protection
+<br>
+1x [4075 OR gate](https://www.mouser.co.uk/ProductDetail/595-SN74HCS4075DR)
+3-way 3-input OR gate
+<br>
+4x [3216 SMD 0.1uF capacitor](https://www.mouser.co.uk/ProductDetail/581-12065C104KAT4A)
+Decoupling capacitor
+<br>
+1x [PMBT2222A SMD Transistor](https://www.mouser.co.uk/ProductDetail/771-PMBT2222A-T-R)
+NPN Transistor for /RESET
+<br>
+2x [3216 SMD 1K Resistor](https://www.mouser.co.uk/ProductDetail/603-RT1206FRE071KL)
+1K Resistor for /RESET and /ROMCS2 pulldown
+<br>
+8x [3216 SMD 150R Resistor](https://www.mouser.co.uk/ProductDetail/603-RT1206FRE07150RL)
+150R Resistor for data bus protection
+<br>
+1x [3216 SMD 330R Resistor](https://www.mouser.co.uk/ProductDetail/YAGEO/RT1206FRE07330RL)
+330R Resistor for LED current limitation
+<br>
+1x [APS6404L-3SQR-PSRAM64Mbit Serial DRAM](https://www.mouser.co.uk/ProductDetail/878-APS6404L-3SQR-SN)
+8MB SPI DRAM. A 1MB chip would probably be adequate.
+<br>
+
+Also, from your parts bin:
+
+3x Raspberry Pi Pico with pins and sockets if you want them
+<br>
+1x 3-pin header and socket
+<br>
+1x 6-pin header and socket
+<br>
+1x 10-pin header and socket
+<br>
+1x KY-040 rotary encoder. Mine came with a 90degree header pin which I replaced with a straight header
+<br>
+1x 90degree ZX Spectrum Edge Connector socket. I went for <a href="https://www.ebay.co.uk/itm/394156247495">this one</a>.
+<br>
+2x SPST switch. Any single pole, single throw switch should do, they are the reset buttons.
+<br>
+1x Push-push type SD card reader. I used <a href="https://www.ebay.co.uk/itm/133222265884">this one</a>.
+<br>
+1x 0.96" OLED display (SSD1306 based)
+<br>
+8x 3.0mm red LEDs. The faceplate holes are drilled for 3.0mm LEDs.