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+// 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
+}