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

Edit this pinout

Pins 10, 11, 12 are certainly 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];
  endcase
end