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PSG Stands for '''Programmable Sound Generator'''

The Amstrad CPC uses the ,CPC+ and KC Compact use a AY-3-8912 Programmable sound generator (referred to as PSGin this document), operating at 1Mhz. This chip features document will describe it's role in these systems. To understand the full functions please read the datasheet.  In these systems it is accessed through and connected to the [[8255]] PPI. (You should understand the operation of the 8255 if you wish to access the PSG)  As well as generating sound, the PSG is also used to scan the keyboard matrix to read the state of the keyboard and joysticks. The selected keyboard matrix line data is available at PSG I/O Port A. (The selected matrix line is programmed to PPI Port C bits 3 square-waves channels, 0. See the document about scanning the keyboard matrix for a noise generator full description of the algorithm involved).  The databus of the PSG is connected to PPI Port A. Data is read from/written to the PSG through this port.  The PSG function, defined by the BC1,BC2 and an enveloppe generator often BDIR signals, is controlled by bit 7 and bit 6 of PPI Port C.  PSG signals:  PPI Port C Bit PSG Signal 7 BDIR 6 BC1   PSG function selection:  PPI Port C Bit PSG Function 7 6 0 0 Inactive 0 1 Read from selected PSG register. When function is set, the PSG will make the register data available to PPI Port A 1 0 Write to selected PSG register When set, the PSG will take the data at PPI Port A and write it into the selected PSG register 1 1 Select PSG register When set, the PSG will take the data at PPI Port A and select a register   Note:  The "inactive" mode must be used between function changes. If it is not used then there will be problems with the program on the CPC+. See the document about CPC+ extra information for a description of this. The "inactive" mode can be used to prevent register corruption between changing modes.  Programming the PSGThe PSG has 16 internal registers and these control the operation. Before a register can be read from/written to it must be selected.  Register selectionTo write data to the PSG, PPI Port A must be operating as output. (See the document on the 8255 to see how to do this)  To select a register, write the register number into PPI Port A, then set bit 7 and bit 6 of Port C to "1".  The register will now be selected and the user can read from or write to it.  The register will remain selected until another is chosen.  Writing to the selected PSG registerTo write data to the PSG, PPI Port A must be operating as output. (See the document on the 8255 to see how to do this)  To write data into the selected PSG register:  write data to PPI Port A, set bit 7 to "1" and bit 6 to "0" of PPI Port C The data will then be written into the register.  Reading from the selected PSG registerTo read data from the PSG, PPI Port A must be operating as input. (See the document on the 8255 to see how to do this)  To read data from the selected register:  set bit 7 to "0" and bit 6 to "1" of PPI Port C read data from PPI Port A, The register data is available at high frequencies PPI Port A.  NotesIf the tone period is in the range 0-4 I can not hear any tone output. Programming ExamplesReading a value from a PSG register. This routine can be used to make different soundsread data from a PSG register. <pre>;; entry conditions:;; c = register index;; exit conditions:;; a = register data;; b,c,f corrupt;; PPI port A is returned to output;; assumptions:;; PPI port A is set to output, PPI port C is set to output. .read_from_psgld b,&f4 ;} Setup register index on PPI port Aout (c),c ;} ld bc,&f6c0 ;} Tell PSG to select register using data on PPI port Aout (c),c ;} ld bc,&f600 ;} Put PSG into inactive stateout (c),c  ;** Set PPI port A to input mode. **ld b,&f7 ; 8255 PPI Controlld c,%10010010 ; mode and port configurationout (c),c ; Port A input, Port B input, Port C output ; All operating in mode 0. (see Programming ; 8255 PPI) ld bc,&f640 ;} Tell PSG to put the data of the selected register to PPI port A toout (c),c ;} ld b,&f4 ;} Read data from PPI port Ain a,(c) ;}  ;** Set PPI port A to output mode. *ld b,&f7 ;8255 PPI controlld c,%10000010out (c),c ;Port A output, Port B input, Port C output ld bc,&f600 ;} Return PSG to inactive mode.out (c),c ;}ret Writing to a PSG register. This routine can be used to write to registers on the PSG. C contains the index of the PSG register to write. A contains the data to write to the PSG register.  ;; entry conditions:;; C = register number;; A = register data;; exit conditions:;; b,C,F corrupt;; assumptions:;; PPI port A and PPI port C are setup in output mode. .write_to_psgld b,&f4 ;} setup PSG register number on PPI port Aout (c),c ;} ld bc,&f6c0 ;} Tell PSG to select register from data on PPI port Aout (c),c ;} ld bc,&f600 ;} Put PSG into inactive state.out (c),c ;} ld b,&f4 ;} setup register data on PPI port Aout (c),a ;} ld bc,&f680 ;} Tell PSG to write data on PPI port A into selected registerout (c),c ;} ld bc,&f600 ;} Put PSG into inactive stateout (c),c ;}ret</pre>