INES Mapper 168

iNES Mapper 168 represents the board used for Racermate Challenge 2. It could be thought of as a hybrid of UNROM and CPROM.

Overview

• PRG ROM size: 64 KiB
• PRG ROM bank size: 16 KiB
• PRG RAM: No
• CHR capacity: 64 KiB RAM, half (or all) battery-backed.
• CHR bank size: 4 KiB
• Nametable mirroring: Hardwired vertical mirroring
• Subject to bus conflicts: no

Banks

• CPU $8000-$BFFF: 16 KiB switchable PRG ROM bank
• CPU $C000-$FFFF: 16 KiB PRG ROM bank, fixed to the last bank
• PPU $0000-$0FFF: 4 KiB CHR RAM bank, fixed to the last bank, but see notes in Hardware.
• PPU $1000-$1FFF: 4 KiB switchable CHR RAM bank, two's complement of written value (write 7 = bank 8; write 0 = last bank)

Jumpers

• J1: force deselect SRAM U1 when +5V fails
• J2: allow SRAM U1 to remain selected after +5V fails
• J3: Power SRAM U1 from +5V line
• J4: Power SRAM U1 from battery-backed supply

SRAM U2 is always protected and battery-backed.

J1+J4 vs J2+J3 should be selected as a pair. This modification allows 64kB of battery-backed memory.

To the best of our knowledge (5 samples), all delivered boards had J2+J3 selected.

Registers

Bank Select ($8000-$BFFF)

7  bit  0
---- ----
PPxx CCCC
||   ||||
||   ++++- Select 4 KB CHR RAM bank for PPU $1000-$1FFF
++-------- Select 16 KB PRG ROM bank for CPU $8000-$BFFF

RAM Protection and IRQ acknowledge ($C000-$FFFF)

15   11 address   0  7 data  0
---- ---- ---- ----  ---- ----
11.. .... Y... ....  XXXX XZXX
          |          |||| ||||
          +----------++++-++++- Write 1 then 0 to deprotect SRAM

Only one of the 9 above bits is used, but the software has no a priori reason to know which. The firmware thus writes $FF to $F080 and $00 to $F000. The PCBs were originally laid out to use the bit above marked "Y" (A7).

It is believed that this is also the interrupt acknowledge/counter reset.

To the best of our knowledge (5 samples), all delivered boards were modified to instead use the bit labeled "Z" (D2)

Interrupt

The /IRQ line will be driven low with a 50% duty cycle for a period of 2048 M2 cycles. (874 Hz) If the IRQ is not acknowledged, the /IRQ line will continue to be asserted for 1024 cycles every 2048 cycles.

A board could have the resistor placed in the adjacent position, which would choose the 437 Hz signal instead, but to the best of our knowledge (5 samples), none were.

Hardware

This cartridge uses two 74LS00s, two 74HCT32s, a 74HCT74, a 74LS174, a 74HCT4040, and a PNP transistor as a low-voltage detection circuit.

Unlike CPROM or iNES Mapper 180, which use AND logic to fix the first bank to 0 and switch the upper bank, the CHR-RAM here instead uses NAND logic. Because nothing else can read or write the RAMs except through this interface, the only question is whether the fixed bank is battery backed. It is NOT. For emulation purposes this could be replaced with AND logic where the fixed bank was 0 and there was no two's complement; this change would be invisible to the NES.

It is of casual interest, but also completely irrelevant, that the lower bits of the CHR bank register are out of order: 3012. Once again, because nothing else can read or write the RAMs except through this bank register, this only could matter if one were to desolder the RAMs to read them.

There is a space on the PCB to accept a donor CIC, labelled U11. In editions released before the top-loading NES-101, this space was populated with a Tengen CIC.