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Diffstat (limited to 'src/mame/machine/atari_vg.cpp')
-rw-r--r-- | src/mame/machine/atari_vg.cpp | 172 |
1 files changed, 172 insertions, 0 deletions
diff --git a/src/mame/machine/atari_vg.cpp b/src/mame/machine/atari_vg.cpp new file mode 100644 index 00000000000..c7d8d514817 --- /dev/null +++ b/src/mame/machine/atari_vg.cpp @@ -0,0 +1,172 @@ +// license:BSD-3-Clause +// copyright-holders:Aaron Giles, Jonathan Gevaryahu +/*************************************************************************** + + General Instruments ER-2055 EAROM + 64 word x 8 bit Electrically Alterable Read Only Memory + + Atari often called this part "137161-001" on their technical manuals, + but their schematics usually called it by its proper ER-2055 name. + + Centipede, Millipede, Dig Dug CTRL port wiring: + 7 6 5 4 3 2 1 0 + x x x x | | | \-- EAROM CLK (directly connected) + x x x x | | \---- /EAROM C1 (note this is inverted! the value written here is inverted, then connected to EAROM C1 AND to the /OE of the 'input latch' which drive the EAROM bus when writing) + x x x x | \------ EAROM C2 (directly connected) + x x x x \-------- EAROM CS1 (positive enable, directly connected) + + Gravitar, Red Baron, Black Widow, Tempest, Liberator, Asteroids Deluxe, Runaway CTRL port wiring: + 7 6 5 4 3 2 1 0 + x x x x | | | \-- EAROM CLK (directly connected) + x x x x | | \---- EAROM C2 (directly connected) + x x x x | \------ /EAROM C1 (note this is inverted! the value written here is inverted, then connected to EAROM C1 AND to the /OE of the 'input latch' which drive the EAROM bus when writing) + x x x x \-------- EAROM CS1 (positive enable, directly connected) + +***************************************************************************/ + +#include "emu.h" +#include "atari_vg.h" + +#define ER2055_IDLE (0) +#define ER2055_READ_WAITING_FOR_CLOCK (1) +#define ER2055_READ_DRIVING_BUS (2) +#define ER2055_WRITING_BITS (3) +#define ER2055_ERASING_BITS (4) + + +// device type definition +const device_type ATARIVGEAROM = &device_creator<atari_vg_earom_device>; + +//------------------------------------------------- +// atari_vg_earom_device - constructor +//------------------------------------------------- + +atari_vg_earom_device::atari_vg_earom_device(const machine_config &mconfig, const char *tag, device_t *owner, UINT32 clock) + : device_t(mconfig, ATARIVGEAROM, "Atari VG EAROM", tag, owner, clock, "atari_vg_earom", __FILE__), + device_nvram_interface(mconfig, *this) +{ +} + +// Activate the output buffer (LS244)'s /G1 and /G2 lines to read whatever is on the EAROM's output bus +READ8_MEMBER( atari_vg_earom_device::read ) +{ + logerror("read from earom output: %02x(%02x):%02x\n", m_in_offset, offset, m_out_data); + return (m_out_data); +} + +// Write to the address input (LS174) and data input (LS374) latches respectively +WRITE8_MEMBER( atari_vg_earom_device::write ) +{ + logerror("write to earom buffers: offset:data of %02x:%02x\n", offset, data); + m_in_offset = offset; + m_in_data = data; +} + +/* CTRL controls the CTRL latch (LS175): +See top of file: there are two possible wirings! + */ +WRITE8_MEMBER( atari_vg_earom_device::ctrl_w ) +{ + static const char *const er2055_State[5] = { "IDLE", "RD_WAIT", "RD_OUTPUT", "WRITE", "ERASE" }; + logerror("EAROM_CTRL written with %02x: ", data); + switch(data&0xe) + { + case 0x0: case 0x2: case 0x4: case 0x6: // CS was low, chip is idle + m_state = ER2055_IDLE; + break; + case 0x8: // C1 = 1, C2 = 0: Read, (wait for clock, if clock has arrived, idle). + if ((m_state==ER2055_READ_WAITING_FOR_CLOCK)&&(((m_old_ctrl&1)==0)&&((data&1)==1))) // rising clock edge? + m_state = ER2055_READ_DRIVING_BUS; + else if (m_state!=ER2055_READ_DRIVING_BUS) // if we're already driving the bus, stay driving it. otherwise we're still waiting. + m_state = ER2055_READ_WAITING_FOR_CLOCK; + break; + case 0xA: // C1 = 0, C2 = 0: Write (set gate 0 if the bit is a 0, set gate 1 if the bit is a 1) HACK: we treat this as WRITE always to handle both wiring variants + m_state = ER2055_WRITING_BITS; + break; + case 0xC: // C1 = 1, C2 = 1: Invalid // HACK: we treat this as WRITE always to handle both wiring variants + logerror("INVALID CTRL STATE!"); + m_state = ER2055_WRITING_BITS; + break; + case 0xE: // C1 = 0, C2 = 1: Erase (zero both gates for all bits of the current byte; byte will read as random garbage after this, different each time!) + m_state = ER2055_ERASING_BITS; + break; + } + logerror("state is now %s\n", er2055_State[m_state]); + + switch(m_state) + { + case ER2055_IDLE: // idle, do nothing. + case ER2055_READ_WAITING_FOR_CLOCK: // waiting for a clock rising edge which we haven't yet seen, do nothing + m_out_data = 0xFF; + break; + case ER2055_READ_DRIVING_BUS: // bus is being driven with data + m_out_data = m_rom[m_in_offset]; + break; + case ER2055_WRITING_BITS: // write contents of bus to rom + m_rom[m_in_offset] = m_in_data; + m_out_data = m_in_data; + break; + case ER2055_ERASING_BITS: // erase contents of rom + m_rom[m_in_offset] = 0; //TODO: this should actually set the rom contents to an invalid state which reads inconsistently until it is written to by a WRITE command + m_out_data = m_in_data; + break; + } + m_old_ctrl = data; +} + +//------------------------------------------------- +// nvram_default - called to initialize NVRAM to +// its default state +//------------------------------------------------- + +void atari_vg_earom_device::nvram_default() +{ + memset(m_rom,0,EAROM_SIZE); +} + +//------------------------------------------------- +// nvram_read - called to read NVRAM from the +// .nv file +//------------------------------------------------- + +void atari_vg_earom_device::nvram_read(emu_file &file) +{ + file.read(m_rom,EAROM_SIZE); +} + +//------------------------------------------------- +// nvram_write - called to write NVRAM to the +// .nv file +//------------------------------------------------- + +void atari_vg_earom_device::nvram_write(emu_file &file) +{ + file.write(m_rom,EAROM_SIZE); +} + +//------------------------------------------------- +// device_start - device-specific startup +//------------------------------------------------- +void atari_vg_earom_device::device_start() +{ + /* register for save states */ + save_item(NAME(m_state)); + save_item(NAME(m_old_ctrl)); + save_item(NAME(m_in_offset)); + save_item(NAME(m_in_data)); + save_item(NAME(m_out_data)); + save_item(NAME(m_rom)); +} + +//------------------------------------------------- +// device_reset - device-specific reset +//------------------------------------------------- + +void atari_vg_earom_device::device_reset() +{ + m_state = ER2055_READ_WAITING_FOR_CLOCK; // start in read mode + m_old_ctrl = 0; + m_in_offset = 0; + m_in_data = 0; + m_out_data = 0xFF; // the value driven to the bus by the resistors if EAROM is open bus +} |