NEO-ZMC

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~A18) are used for data input. The chip's write strobe is port address decoded inside the system.

=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 (latch on rising edge)

=Operation=

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

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