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Significant accuracy improvements, documentation and other updates to the ER-2055 EAROM emulation used for several Atari games, based on schematics and datasheets. It now uses a state machine to keep track of the current EAROM bus state. [Lord Nightmare]
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Lord-Nightmare
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@ -1,14 +1,38 @@
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// license:BSD-3-Clause
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// copyright-holders:Aaron Giles
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// copyright-holders:Aaron Giles, Jonathan Gevaryahu
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/***************************************************************************
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Atari vector hardware
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General Instruments ER-2055 EAROM
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64 word x 8 bit Electrically Alterable Read Only Memory
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Atari often called this part "137161-001" on their technical manuals,
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but their schematics usually called it by its proper ER-2055 name.
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Centipede, Millipede, Dig Dug CTRL port wiring:
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7 6 5 4 3 2 1 0
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x x x x | | | \-- EAROM CLK (directly connected)
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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)
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x x x x | \------ EAROM C2 (directly connected)
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x x x x \-------- EAROM CS1 (postive enable, directly connected)
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Gravitar, Red Baron, Black Widow, Tempest, Liberator, Asteroids Deluxe, Runaway CTRL port wiring:
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7 6 5 4 3 2 1 0
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x x x x | | | \-- EAROM CLK (directly connected)
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x x x x | | \---- EAROM C2 (directly connected)
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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)
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x x x x \-------- EAROM CS1 (postive enable, directly connected)
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***************************************************************************/
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#include "emu.h"
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#include "atari_vg.h"
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#define ER2055_IDLE (0)
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#define ER2055_READ_WAITING_FOR_CLOCK (1)
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#define ER2055_READ_DRIVING_BUS (2)
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#define ER2055_WRITING_BITS (3)
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#define ER2055_ERASING_BITS (4)
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// device type definition
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const device_type ATARIVGEAROM = &device_creator<atari_vg_earom_device>;
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@ -23,45 +47,71 @@ atari_vg_earom_device::atari_vg_earom_device(const machine_config &mconfig, cons
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{
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}
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// Activate the output buffer (LS244)'s /G1 and /G2 lines to read whatever is on the EAROM's output bus
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READ8_MEMBER( atari_vg_earom_device::read )
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{
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logerror("read earom: %02x(%02x):%02x\n", m_offset, offset, m_data);
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return (m_data);
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logerror("read from earom output: %02x(%02x):%02x\n", m_in_offset, offset, m_out_data);
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return (m_out_data);
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}
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// Write to the address input (LS174) and data input (LS374) latches respectively
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WRITE8_MEMBER( atari_vg_earom_device::write )
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{
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logerror("write earom: %02x:%02x\n", offset, data);
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m_offset = offset;
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m_data = data;
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logerror("write to earom buffers: offset:data of %02x:%02x\n", offset, data);
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m_in_offset = offset;
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m_in_data = data;
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}
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/* 0,8 and 14 get written to this location, too.
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* Don't know what they do exactly
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/* CTRL controls the CTRL latch (LS175):
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See top of file: there are two possible wirings!
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*/
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WRITE8_MEMBER( atari_vg_earom_device::ctrl_w )
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{
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logerror("earom ctrl: %02x:%02x\n",offset, data);
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/*
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0x01 = clock
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0x02 = set data latch? - writes only (not always)
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0x04 = write mode? - writes only
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0x08 = set addr latch?
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*/
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if (data & 0x01)
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static const char *const er2055_State[5] = { "IDLE", "RD_WAIT", "RD_OUTPUT", "WRITE", "ERASE" };
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logerror("EAROM_CTRL written with %02x: ", data);
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switch(data&0xe)
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{
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m_data = m_rom[m_offset];
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//printf("Read %02X = %02X\n", m_offset, m_data);
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case 0x0: case 0x2: case 0x4: case 0x6: // CS was low, chip is idle
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m_state = ER2055_IDLE;
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break;
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case 0x8: // C1 = 1, C2 = 0: Read, (wait for clock, if clock has arrived, idle).
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if ((m_state==ER2055_READ_WAITING_FOR_CLOCK)&&(((m_old_ctrl&1)==0)&&((data&1)==1))) // rising clock edge?
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m_state = ER2055_READ_DRIVING_BUS;
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else if (m_state!=ER2055_READ_DRIVING_BUS) // if we're already driving the bus, stay driving it. otherwise we're still waiting.
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m_state = ER2055_READ_WAITING_FOR_CLOCK;
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break;
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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)
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m_state = ER2055_WRITING_BITS;
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break;
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case 0xC: // C1 = 1, C2 = 1: Invalid
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logerror("INVALID CTRL STATE!");
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m_state = ER2055_WRITING_BITS;
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break;
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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!)
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m_state = ER2055_ERASING_BITS;
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break;
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}
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if ((data & 0x0c) == 0x0c)
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logerror("state is now %s\n", er2055_State[m_state]);
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switch(m_state)
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{
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m_rom[m_offset]=m_data;
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logerror(" written %02x:%02x\n", m_offset, m_data);
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//printf("Write %02X = %02X\n", m_offset, m_data);
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case ER2055_IDLE: // idle, do nothing.
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case ER2055_READ_WAITING_FOR_CLOCK: // waiting for a clock rising edge which we haven't yet seen, do nothing
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m_out_data = 0xFF;
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break;
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case ER2055_READ_DRIVING_BUS: // bus is being driven with data
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m_out_data = m_rom[m_in_offset];
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break;
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case ER2055_WRITING_BITS: // write contents of bus to rom
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m_rom[m_in_offset] = m_in_data;
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m_out_data = m_in_data;
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break;
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case ER2055_ERASING_BITS: // erase contents of rom
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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
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m_out_data = m_in_data;
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break;
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}
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m_old_ctrl = data;
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}
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//-------------------------------------------------
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@ -100,8 +150,12 @@ void atari_vg_earom_device::nvram_write(emu_file &file)
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void atari_vg_earom_device::device_start()
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{
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/* register for save states */
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save_item(NAME(m_offset));
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save_item(NAME(m_data));
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save_item(NAME(m_state));
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save_item(NAME(m_old_ctrl));
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save_item(NAME(m_in_offset));
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save_item(NAME(m_in_data));
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save_item(NAME(m_out_data));
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save_item(NAME(m_rom));
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}
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//-------------------------------------------------
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@ -110,6 +164,9 @@ void atari_vg_earom_device::device_start()
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void atari_vg_earom_device::device_reset()
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{
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m_data = 0;
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m_offset = 0;
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m_state = ER2055_READ_WAITING_FOR_CLOCK; // start in read mode
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m_old_ctrl = 0;
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m_in_offset = 0;
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m_in_data = 0;
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m_out_data = 0xFF; // the value driven to the bus by the resistors if EAROM is open bus
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}
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@ -41,8 +41,11 @@ public:
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DECLARE_WRITE8_MEMBER( write );
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DECLARE_WRITE8_MEMBER( ctrl_w );
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private:
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int m_offset;
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int m_data;
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int m_old_ctrl;
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int m_state;
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int m_in_offset;
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int m_in_data;
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int m_out_data;
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char m_rom[EAROM_SIZE];
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};
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