GameBoyã

 

General Purpose Interface

 

Designers Manual

 

By S.G.Willis Rev 1.1

 

1.0 Introduction

 

The GameBoy general purpose interface 'GBGPI' converts a GBpocket or GBcolour into a multipurpose programmable development platform. The software defined input and output ports interface enables connection to a wide variety of sensor and control devices. Making GBGPI ideal for robotics, home control or portable development projects.

 

The dual boot internal flash ROM allows you to develop and simulate programs on your PC, then download them via the game link port to the GBGPI cartridge, eliminating the need for a dedicated programmer. Once programmed simply flip the boot switch and your ready to go!

 

A library of interface modules are provided, these can be added to the 'C' compiler - GBDK allowing connection to analogue, logic, thermistor, RTC and many more sensor devices.  The use of a high level development language makes programming easier to understand and readable to others.

 

 

1.1 Key Features:

 

     12 Channel 8bit High Speed ADC

     8 Channel 8bit Buffered DAC

     32KB of Internal Flash Program ROM

     4KB of Internal Flash Data Store

     Dual Boot Mode for Reprogramming

     No Programmer Required

     Rail-to-Rail Inputs and Outputs (0 to 5V)

     Standard D25 Output Connector

     6hrs Operation from NiMH Batteries

     'C' Development Library for GBDK

     Fits into standard games cartridge

2.0 Hardware Interface

 

Pin connections to 25 way socket (D25F) :-


 

 


Figure 1, D25 interface connector

 

 

Inputs

The 12 ADC input channels have an input voltage range of 0 to +5V and a resolution of  8bits  (0 to 255), hence 1LSB represents a voltage change of 19.6mV (5V/255). The inputs should be driven by a source impedance of less than 10KW to prevent errors due to input currents. Unused inputs should be connected to 0V.

The lower 4 channels connect directly to the GPI ADC while the upper 8bits are fed via multiplexer. For this reason inputs 0 to 3 are preferred for high speed/high impedance measurements.

 

Outputs

The 8 DAC output channels have an output voltage range of 0 to +5V and a resolution of 8bits  (0 to 255) hence 1LSB represents a voltage change of 19.6mV (5V/255). The output load current should be no greater than ±1mA (analogue mode) or ±5mA (digital mode) per channel.

Digital and analogue modes use the same hardware but the software emulates a digital output by producing either 0V (logic 0) or +5V (logic 1).

 

REF (pin7)

The reference output is connected to the ADC/DAC reference supply pins and represents the true full-scale input/output levels (~5V).  Use this pin to power low current devices (<5mA) eg: potentiometers, thermistors etc. This pin should not be used as a power supply pin.

 

5V (pin6)

The 5V supply pin is connected to the GameBoy power converter and can be used to power external circuits such as switches, Infrared sensors and real time clocks.  The current drawn from this pin should be no greater than 20mA.

 

CLK (pin19)

The clock output pin produces a ~1MHz (976uS) logic signal that can be used to with external circuits such as counters.  The clock is actually derived from the GameBoy clock and is buffered to drive loads of less than 5mA.

3.0 Standard Software Commands

The standard GBGPI software commands are located in \lib\GPI.H and \include\GPI.S, more advanced (specific) commands can be added to \lib\GPIA.H and \include\GPIA.S (see section 6.0).

To add the standard command set to your C applications simply add the following statement to the include list: #include <gpi.h>. An example program can be found in section 5.0.

 

read_adc(channel)

Read input channel and return analogue value.  Channel is 0 to 11, value is 0 to 255.

 

write_dac(channel, data);

Write to output channel with analogue value.  Channel is 0 to 11, value is 0 to 255.

 

reset_gpi();

Resets GPI internal hardware and sets all outputs to zero.

 

set_bit(channel);

Set output channel to logic 1 (analogue 255).  Channel is 0 to 11.

 

clear_bit(channel);

Clear output channel to logic 0 (analogue 0).  Channel is 0 to 11.

 

test_bit(channel, threshold);

Read input channel and return logic state. Threshold defines the input voltage level required to produce a logic 1.  Channel is 0 to 11, threshold is 0 to 255, state is 0 or 1.  The threshold value should be set to 40 (0.8V) for TTL/HCT logic or 128 (2.5V) for CMOS/HC logic.

 

 

 

All settings

ADC channel 0-3

ADC channel 4-11

read_adc()

 

100uS

200uS

write_dac()

325uS

 

 

reset_gpi()

2.5mS

 

 

set_bit()

350uS

 

 

clear_bit()

350uS

 

 

test_bit()

 

120uS

220uS

 

Figure 2, Standard Command Execution Times

4.0 Flash Memory

 

Selection of the internal flash memory bank is controlled by a combination of hardware logic and a mechanical slide switch. Physically the flash memory is contained within a single device but the functions of the three memory banks are different,

 

 

Bank

Size

Switch position

GB memory map

Function

0

32KB

N

0000 to 7FFF

32K User Program Store

1

1KB

N

B000 to BFFF

4K User Data Store

2

32KB

P

0000 to 7FFF

Boot loader for reprogramming

 

Figure 3, Flash Memory Banks

 

Bank 0, User Program Store

The 32KB program store contains the compiled ROM image of your application program. Memory locations 0000 to 014F are reserved (by the compiler) to tell the Gameboy what type of cartridge has been installed etc.  This reserved memory also holds the scrolling NINTENDO logo which appears on startup, this cannot be changed! The rest of bank 0 is free for development and should accommodate approximately 700 to 1000 lines of ‘C’ code.

 

Bank 1, User Data Store

The 4KB non-volatile programmable data store allows user-defined data parameters to be stored and retrieved even after the power has been switched off.  This is ideal for storing configuration or calibration data that cannot be defined when the program is written.  A set of special programming commands are provided to enable the user program to write and read from the data store,

 

Command

Execution Time

Function

in_byte(aaaa)

35uS  (RAM)

20mS (video)

35uS  (data store)

Read a single byte from any memory location

 

r=out_byte(aaaa)

35uS  (RAM)

20mS (video)

1mS   (data store)

Write a single byte to any memory location (not ROM)

r=read_store(*b,s)

s*3mS

Read (s*256) bytes from data store into array

r=write_store(*b,s)

s*256mS typ.

Write (s*256) bytes from array into data store

r=erase_store()

1sec typ.

Erase all of the data store (set to 0xFF)

 

  aaaa = Absolute memory address (0x000 to 0xFFFF).

     *b = Pointer to array of (s*256) bytes.

       s = Size of data store used * 256 bytes (1 to 16).

       r  = return result, 0=OK, 1=error.

 

Figure 4, Bank1 programming commands

The flash data store works like an EPROM in that you must first erase store setting all of the bytes to 0xFF. It is then possible to program individual bytes by setting the data bits from 1 to 0, it is not possible to set a bit back to 1 without erasing all of the store.

 

From figure 4 it is seen that there are two ways of writing to the data store, with the out_byte or the write_store commands the, choice of these commands will depend on your final application. The following examples illustrate this.

 

1.         If the application was a data logger then the program would start by erasing the data store. As each byte was logged it would be programmed into the data store using the out_byte command.  At the end of the run all the bytes would be read from the store before the program could restart.

 

2.         If the application was a temperature controller then the program would have to randomly update and save set-point and calibration values.  The only way of achieving this is to have a mirror of the data store in RAM (using an array), if a value is updated in the array a copy of the entire array would be made in the data store using the write_store command.  At the start of the program the array would be initialised from the data store using the read_store command.

The write_store command actually performs an erase, 256*s byte write, then checks the data was written correctly, this can be performed over 100,000 times.  If you do not wish to use all of the data store (s=16) then s can be reduced, this has the advantage of using less RAM memory which is limited to 8K.

 

Bank 2, Boot Loader Program

Set the manual switch to ‘P’ before switching on the GB to enable the boot loader program.  The programming boot loader menu will appear which allows programming of Banks 0/1 and reading of Bank 1 via the Game Link port to a PC.  The PC software is windows 95/8 compatible and will download a 32KB image in about 30seconds.  Note: software protection will prevent games ROM’s from being loaded so development ROM’s must have the licensee code  (0x014B) set to zero before compiling.

5.0 Example Program

 

 

/* GPI demo to read input channel 0 and output 0 to 255 to channel 0 */

 

#include <gb.h>

#include <stdio.h>

#include <gpi.h>                    /* standard GPI library */

 

void main()

{

  UBYTE din, dout;

 

  reset_gpi();                      /* reset GPI hardware */

  cls();

  printf("GPI Demo\n\n");

  printf("GPI IP0=\n");

  printf("GPI OP0=");

 

  do

  {

 

    gotoxy(8,2);

    din=read_adc(0);                /* read input channel 0 */

    printn(din, 10, UNSIGNED);      /* display in decimal */

    puts("  ");

 

    write_dac(0, dout);             /* output value */

    gotoxy(8,3);

    printn(dout, 10, UNSIGNED);

    puts("  ");

    dout++;                         /* dout=dout+1 */

 

    delay(100);

  }while(!0);                       /* do forever */

}

 

 

To try this program out simply cut and past it into a new document within your text editor/compiler (e.g. GBDS).  Save it to a suitable directory e.g. C:\SDK\GBZ80-GB\2-1-0\DEMO\DEMO.C. Then compile and run it.  If you get a message like ‘undefined GLOBAL reset_gpi()’ you have probably not installed the standard GBGPI library properly, see section 6.0.

 

6.0 Software installation

Before adding the GBGPI library’s you must have completed the following software installation step,

 

1)      Unzip GBDK onto your hard drive, which should look something like this,


 

 


Note: subdirectories have been added for \gb97 (emulator), \gbdev (development studio), \gbtd (tile designer), gbmb (map builder), gbgpi (programmer).

 

2)      Make all the GBDK libraries by running \lib\MAKE.BAT and \examples\MAKE.BAT.


 

 

 


3)      If you are using the GB development studio then set the ‘code editor’ ‘options’ like this,

6.1 Adding GPI Library Files

To add the standard GBGPI library commands to GBDK the following files must loaded.

 

\include\GPI.H  - parameter definition for commands

\lib\GPI.S                     - source code for commands

\lib\GPI.O                    - output file from GPI.S

\lib\GPIA.S                  - source code for advanced commands

\lib\GPIA.O                  - output file from GPIA.S

\lib\MAKEGPI.BAT    - make file for GPI.S and GPIA.S

 

It is also necessary to add a reference to GPI.O/GPIA.O in \lib\GB.LIB (add two lines to the end of the file GPI.O and GPIA.O).

 

To simplify this process just unzip GBGPI.ZIP into a within your GBDK path e.g. C:\SDK\GBGPI\ and run SETUP.BAT.  If you get and error during this process then you have not put the directory in the correct place or you have not installed GBDK correctly.

 

 

6.2 Advanced Library Files - GPIA

This section is under development.