< 2/29>SCSP User's Manual/1. Sound System Overview

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SCSP User's Manual/Chapter 1 Sound System Overview

1.2 Starting the sound system

■1. Starting up from reset

The sound system is reset by "SMPC" immediately after power is turned on or after pressing the reset button.
In this state, the sound CPU "MC68EC000" and "SCSP" cannot operate.

●Reset cancellation

The sound system starts when "SMPC" releases the reset of "SCSP".
When the reset of SCSP is released, “SCSP” initializes its internal control registers over a period of approximately 30 μs. Access is prohibited during this 30 microsecond period.

By the way, it will not be reset by the "RESET" command of the sound CPU "MC68EC000".
Additionally, the use of the "RESET" command is prohibited as operation cannot be guaranteed in this system.

After 30μ seconds or more have passed after the reset is released, the sound block can be accessed from the main block (SH2, etc.) side.

At this point, it is finally possible to download the programs and data for the sound CPU, but before doing so, be sure to set "1b" to the "MEM4MB" bit of the control register shown in "Table 2.1" and "DAC18B". Please write “0b” to the “bit”.

Note
After resetting, write “0200H” to 25B00400H.

Note	After resetting, write “0200H” to 25B00400H.

Be sure to perform this task after resetting. Otherwise, operation cannot be guaranteed.

**Table 1.2 Initialization setting data after reset**
Main side address 25B00400/1h (Sound CPU side address 100400/1h)
address	25B00400h/100400h								25B00401h/100401h
bit	15	14	13	12	11	10	９	８	７	６	５	４	３	２	１	０
register	−						M4	D18	VER[7:4]				MVOL[3:0]
initial value	0	0	0	0	0	0	1	0	0	0	0	0	0	0	0	0
&H (hexadecimal)	0				2				0				0

If you write the data in the table above, you will be able to access the sound RAM, so please download the program, data, waveform data, etc. for the sound CPU.

When the download is complete, cancel the sound CPU reset (sound reset) by SMPC.
Once released, the sound CPU will start operating according to the downloaded program. (The reset is controlled by SMPC.)

Note
The following commands are prohibited when programming with MC68EC000.

RESET command

TAS command

reference:

Note	The following commands are prohibited when programming with MC68EC000.
RESET command
TAS command

"SEGASATURN Developer's Information STN-51" Instruction to prohibit use on 68000

■2.About reset vector

Regarding the files to be downloaded, please remember that the reset vector is always required to start up the sound CPU.
The reset vector is 8 bytes of data starting from address 000000 on the sound RAM. See Table 2.2.

**Table 1.3 Reset vector**
	MAIN/SOUND	D15　　　　　D８	D７　　　　　D０	MAIN/SOUND
RESET VECTOR-0	0(2)5A00000H/000000H	Stack address/upper word		0(2)5A00001H/000001H
	0(2)5A00002H/000002H	Stack address/lower word		0(2)5A00003H/000003H
RESET VECTOR-1	0(2)5A00004H/000004H	Program counter/upper word		0(2)5A00005H/000005H
	0(2)5A00006H/000006H	Program counter/lower word		0(2)5A00007H/000007H

After the reset is released, the sound CPU reads the reset vector and jumps to the same address as the program counter value.

The following figure 2.3 explains the reset procedure.

Figure 2.3 Reset sequence (operation order diagram)

■3. Regarding communication between main block and sound block

Regarding communication between the main block and the sound block, data is exchanged via the "SCU" in hardware.

Regarding access, it is possible to read and write data from the main block (SH2, SCU) to the sound block, but please note that it is not possible to access the main block from the sound block.

Note
Direction of access
It is possible to access the sound block from the main block (SH2, SCU).
It is impossible to access the main block from the sound block.

Note	Direction of access It is possible to access the sound block from the main block (SH2, SCU). It is impossible to access the main block from the sound block.

The memory mapping is the same as above , so please refer to it.

■4.About MIDI communication function

Saturn allows you to use MIDI communication functions.
The serial signal line necessary for the MIDI interface is output to the "communication connector" on the back of the Saturn.
All MIDI transmission/reception data and related controls can be performed in the SCSP control registers, so if you are using MIDI, please refer to the MIDI register section below.

reference:

"SCSP User's Manual" 4.2 Sound source register/ ■MIDI register

The signal line output to the communication connector cannot be used as is for MIDI.
If you want to use it, you will need an optional MIDI-I/F device, but such a product is not currently available.

■5.About interrupts

Interrupts in the sound block are generated from the SCSP and can be used not only for the sound CPU but also for the main block (SH2).

Figure 2.4 Interrupt relationship
The sound CPU interrupt uses the MC68EC000's autovector method.
For vectors, please refer to the vector table in Table 2.2.
A total of 7 interrupt levels from 1 to 7 can be used, but since the development tool uses level 7 interrupts, there are 6 types of interrupt levels from 1 to 6 that the user can use. Please note that this will occur.

Note
Interrupt levels available to the user: 6 levels from 1 to 6

Note	Interrupt levels available to the user: 6 levels from 1 to 6

It is also possible to set the interrupt level for each interrupt source. See Interrupt Control Register for details.

reference:

"SCSP User's Manual" 4.2 Sound source register/ ■Interrupt control register

Note
Interrupt level priority 7: Highest ← Priority → Lowest: 1

Table 1.4 Interrupt vector table for sound CPU
vector number address Interrupt vector contents

SH2 MC68EC000

0 0(2)5A00000H 000000H Reset vector initial SSP value

0(2)5A00004H 000004H Reset vector initial PC value

: : : :

25 0(2)5A00064H 000064H Autovector level 1 interrupt

26 0(2)5A00068H 000068H Autovector level 2 interrupt

27 0(2)5A0006CH 00006CH Autovector level 3 interrupt

28 0(2)5A00070H 000070H Autovector level 4 interrupt

29 0(2)5A00074H 000074H Autovector level 5 interrupt

30 0(2)5A00078H 000078H Autovector level 6 interrupt

31 0(2)5A0007CH 00007CH Autovector level 7 interrupt

Reference:

Note	Interrupt level priority 7: Highest ← Priority → Lowest: 1

**Table 1.4 Interrupt vector table for sound CPU**
vector number	address	Interrupt vector contents
SH2	MC68EC000
0	0(2)5A00000H	000000H	Reset vector initial SSP value
0(2)5A00004H	000004H	Reset vector initial PC value
:	:	:	:
25	0(2)5A00064H	000064H	Autovector level 1 interrupt
26	0(2)5A00068H	000068H	Autovector level 2 interrupt
27	0(2)5A0006CH	00006CH	Autovector level 3 interrupt
28	0(2)5A00070H	000070H	Autovector level 4 interrupt
29	0(2)5A00074H	000074H	Autovector level 5 interrupt
30	0(2)5A00078H	000078H	Autovector level 6 interrupt
31	0(2)5A0007CH	00007CH	Autovector level 7 interrupt

SSP in the table is the supervisor stack pointer value, which is the same as the stack pointer described above .
Also, PC is the program counter value, which is also the same as above .

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