Update README.md

Added arrows to CPU header diagram
Added suggestions for future expansion cards
Specified EEPROM addresses for each slot
Clarified types of regulator used
Noted that BMC runs from standby regulator
Note that internal line-in doesn't work if external line-in used (but line-output does work if headphone out is used)

Author: thejpster

README.md

@@ -18,8 +18,8 @@ Contains KiCad footprints and symbols shared across multiple Neotron projects. I
 * AC'97 Pin Header
   * Headphone Out
   * Microphone In
-* Extra line-level output pin header (e.g. for additional RCA audio jacks)
-* Extra line-level input pin header (e.g. CD-ROM audio)
+* Extra line-level output pin header (e.g. for additional RCA audio jacks - operates in addition to 3.5mm headphone jack output)
+* Internal line-level input pin header (e.g. for CD-ROM audio - disabled when line-in 3.5mm jack in-use)
 
 ### Super VGA output
 
@@ -28,7 +28,7 @@ Contains KiCad footprints and symbols shared across multiple Neotron projects. I
 * 3peak TPF133A or Texas Instruments THS7316 RGB video buffer
    * 36 MHz bandwidth - 1024x768@60Hz maximum
    * 6dB gain
-   * Drives 75 ohm output
+   * Drives 75 ohm standard VGA interface
 * I²C/DDC level shifter and EMC filter
 
 ### RS232 Interface
@@ -59,7 +59,8 @@ Contains KiCad footprints and symbols shared across multiple Neotron projects. I
   * TSSOP-20 package
 * Controls two PS/2 ports
 * Monitors 5V and 3.3V rails
-* Controls reset, soft-on and soft-off for main CPU
+* Controls system reset, soft-on and soft-off for main CPU
+* Can the main 5V regulator on and off
 * Runs from 3.3V stand-by regulator
 * I²C interface (with dedicated IRQ line) with main CPU
 
@@ -91,29 +92,26 @@ Contains KiCad footprints and symbols shared across multiple Neotron projects. I
 * Twin mini 6-pin DIN ports
 * Controlled via Board Management Controller
 
-### Joystick/Joypad
-
-* I²C interface to main CPU
-* Two 9-pin Atari/SEGA/Commodore digital joystick/joypad interfaces
-* Supports 3-button SEGA MegaDrive/Genesis joypads
-
 ### Power Supply
 
-* Unregulated 7V to 28V input fused at 5A
-* 3A 5.0V main regulator (DC-DC module)
-* 30mA 5.0V stand-by regulator (low-power linear regulator)
-* 1A 3.3V regulator (high-power linear regulator)
-* Power-on reset circuit support
-* Soft power-off support
-* Controlled by Board Management Controller
+* Unregulated 12V (7V to 28V) input fused with a PTC at 2A
+* 3A 5.0V main regulator (DC-DC switch-mode regulator module)
+* 30mA 3.3V stand-by regulator (a micropower linear regulator)
+* 1A 3.3V regulator (a high-power 1117 type linear regulator)
+* Controlled by the Board Management Controller
 
 ### CPU Socket
 
 The CPU can either be designed to solder direct to your particular motherboard design, or it can be fitted to a second PCB 'CPU card' which plugs into a socket on the motherboard. We have designed a special 80-pin socket pinout, which provides all the signals for required for the standard peripherals available in this design.
 
 The socket is basically two 2x20 pin headers, placed 130 mil apart. This should give enough space to fit even a TQFP-176 microcontroller and some TSOP-54 SDRAM.
 
+The pins are labelled in column.row fashion, i.e. 1.1 is top left, 1.20 is top right and 4.20 is bottom right.
+
 ```
+ +----- Pin 1.1
+ |
+ v
 +---------------------------------------+
 |o o o o o o o o o o o o o o o o o o o o|
 |o o o o o o o o o o o o o o o o o o o o|
@@ -132,10 +130,11 @@ The socket is basically two 2x20 pin headers, placed 130 mil apart. This should
 |o o o o o o o o o o o o o o o o o o o o|
 |o o o o o o o o o o o o o o o o o o o o|
 +---------------------------------------+
+                                       ^
+                                       |
+                         Pin 4.20 -----+
 ```
 
-The pins are labelled in column.row fashion, i.e. 1.1 is top left, 1.20 is top right and 4.20 is bottom right.
-
 | Pin  | Function             |
 |:-----|:---------------------|
 | 1.1  | VGA_RED5             |
@@ -215,11 +214,11 @@ The pins are labelled in column.row fashion, i.e. 1.1 is top left, 1.20 is top r
 | 4.19 | ~IRQ1                |
 | 4.20 | ~IRQ0                |
 
-### Expansion Socket
+## Expansion
 
-The expansion socket allows you to add on I²C or SPI based devices at a later date. It provides a single chip-select and a single IRQ line - the motherboard design should ensure each socket gets a unique signal for each of these. Each expansion device should also contain a AT24C256 or similar EEPROM device. To allow these EEPROM devices to be scanned, each slot also contains three `EEPROM_ADDRESS` pins, tied to Vcc or GND in a unique combination. These should be connected through to the EEPROM address lines on your AT24C256, thus ensuring that each expansion card has its EEPROM at a unique address.
+The (up-to) seven expansion sockets allow you to add on I²C or SPI based devices at a later date. Each provides a single chip-select and a single IRQ line - the motherboard design should ensure each socket gets a unique signal for each of these. Each expansion device should also contain a AT24C256 or similar EEPROM device. To allow these EEPROM devices to be scanned, each slot also contains three `EEPROM_ADDRESS` pins, tied to Vcc or GND in a unique combination. These should be connected through to the EEPROM address lines on your AT24C256, thus ensuring that each expansion card has its EEPROM at a unique address - 0x50 on Slot 0 through to a maximum possible 0x57 for Slot 7. Where your board has on-board devices, you should fit an AT24C256 EEPROM for each device so that the on-board devices can be discovered, exactly as if they were on an expansion card.
 
-The expansion slot is a simple 2x10 header. We suggest the use of a TE card-edge connector, but you could equally use a pin-header if desired.
+The expansion slot is a simple 2x10 header. We suggest the use of a TE card-edge connector, but you could equally use two 1x10 pin-headers if desired.
 
 The pin functions are:
 
@@ -236,6 +235,20 @@ The pin functions are:
           GND   19  20   GND
 ```
 
+## Expansion Ideas
+
+Why not design and build your own expansion card? You could try designing:
+
+* A dual Atari/SEGA 9-pin Joypad Interface
+* A Mikro Eletronika Click adaptor, allow many of the range of [Click board](https://www.mikroe.com/click) to be fitted
+* A Wi-Fi/Bluetooth card, using an Espressif ESP32
+* A second processor card - perhaps with a RISC-V microcontroller, or classic Zilog Z80
+* An OPL2 or OPL3 based FM synthesiser card
+* An ISA adaptor card (taking an ISA card at right-angles, i.e. parallel to the base board) - a simple microcontroller should be able to bit-bang the ISA bus at 8 MHz and offer an SPI peripheral interface to the Neotron Expansion Slot
+* An IDE interface card, allowing 40-pin IDE Hard Disk Drives and CD-ROM drives to be used - this will be quite similar to an ISA bus adaptor
+* A floppy drive controller card - either using an eSPI Super I/O chip, or connecting a legacy ISA bus floppy controller as per the ISA adaptor
+* A video card for a second monitor output, perhaps based on the CPLD used in the [VGAtonic](https://hackaday.io/project/6309-vga-graphics-over-spi-and-serial-vgatonic)
+
 ## Changelog
 
 See [CHANGELOG.md](./CHANGELOG.md) for a list of detailed changes.