NEO-ZMC

Package	SOIC24
Manufacturer
First use	1995 ?
Used on	cartridges

Z80 Memory Controller. Provides a hardwired 32KiB bank and switchable 16, 8, 4, and 2KiB banks arranged as a register file. To save pins, the Z80's upper address lines (A15~A8) are used for data input. The chip's write strobe is port address decoded inside the system.

Pinout

File:Neo-zmc pinout.png

OpenOffice Draw file: File:Neo-zmc.odg

Pins 10, 11, 12 are surely not connected since the addressing scheme doesn't allow mapping over 512KiB.

• SDA0, SDA1, SDA8~15: Z80 address bus
• M1 A11~M1 A18: M1 ROM address lines
• SDRD0: Decoded write signal from NEO-D0 (latch on rising edge)

Operation

SDA8~15 is used for the bank number (data), SDA0 and SDA1 for selecting the bank zone.

SDA1	SDA0	Z80 port	Bank zone	Address range	Size	Latch size
0	0	$08	0	$F000~$F7FF	2KiB	8 bits
0	1	$09	1	$E000~$EFFF	4KiB	7 bits
1	0	$0A	2	$C000~$DFFF	8KiB	6 bits
1	1	$0B	3	$8000~$BFFF	16KiB	5 bits

Details

SDRD0 must be high before configuring banks.

To configure a bank to be accessed (e.g. bank 0 in the $8000~$BFFF range):

• Set SDRD0 low (prepare for new bank configuration, outputs are tri-stated)
• Set SDA0~15 = $8003 (select bank 0 and 16k range size) Why $8003 and not just $0003 ?
• Set SDRD0 high (latch bank, ready to convert inputs to proper output signals)
• Now, when the Z80 reads the $8000~BFFF range, NEO-ZMC will map this to the M1 ROM zone $00000~$03FFF;

To configure a bank to be accessed (e.g. bank 1 in the $8000~$BFFF range):

• Set SDRD0 low (prepare for new bank configuration, outputs are tri-stated)
• Set SDA0~15 = $8103 (select bank 1 and 16k range size) Why $8103 and not just $0103 ?
• Set SDRD0 high (latch bank, ready to convert inputs to proper output signals)
• Now, when the Z80 reads the $8000~BFFF range, NEO-ZMC will map this to the M1 ROM zone $04000~$07FFF;

Logic definition

Not tested.

reg [7:0] WINDOW_0;
reg [6:0] WINDOW_1;
reg [5:0] WINDOW_2;
reg [4:0] WINDOW_3;

assign MA = (!SDA[15]) ? {4'b0000, SDA[14:11]} :     // Pass-through
		(SDA[15:12] == 4'b1111) ? WINDOW_0 :
		(SDA[15:12] == 4'b1110) ? {WINDOW_1, SDA[11]} :
		(SDA[15:13] == 3'b110) ? {WINDOW_2, SDA[12:11]} :
		{WINDOW_3, SDA[13:11]};

always @(posedge nSDRD0)
begin
  case (SDA[1:0])
    0: WINDOW_0 <= SDA[15:8];
    1: WINDOW_1 <= SDA[14:8];
    2: WINDOW_2 <= SDA[13:8];
    3: WINDOW_3 <= SDA[12:8];
  encase
end