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bit90: Add notes describing tape interface (not implemented)

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This commit is contained in:
Frank Palazzolo 2021-05-08 22:03:14 -04:00
parent 32151b1f0c
commit 1d6cae52cb
1 changed files with 8 additions and 2 deletions
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10
src/mame/drivers/coleco.cpp
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@ -84,6 +84,9 @@ uint8_t coleco_state::paddle_1_r()
uint8_t coleco_state::paddle_2_r()
{
// Tape notes:
// Signal is averaged to set the threshold voltage for a comparator
// Output of the comparator goes to bit 7
return m_joy_d7_state[1] | coleco_paddle_read(1, m_joy_mode, m_joy_analog_state[1]);
}
@ -139,7 +142,10 @@ void bit90_state::u32_w(uint8_t data)
{
// Write to a 74LS174 at u32
// Bits 4-7 are connected for keyboard scanning (actually only 4-6 are used)
// Bits 0-1 unknown (probably tape?)
// Bits 0-1 Tape Write
// Notes: Tape waveform is generated using bits 0 and 1 connected as 2-bit DAC, with four output levels
// The analog output is put through an RC filter to approximate a sine wave
// Tape waveforms appear to be full cycles of one of 2 frequencies
m_keyselect = (data >> 4) & 0x0f;
uint8_t temp = data & 0x03;
if (m_unknown != temp) {
@ -188,7 +194,7 @@ void bit90_state::bit90_io_map(address_map &map)
map(0xc0, 0xc0).mirror(0x1f).w(FUNC(coleco_state::paddle_on_w));
map(0xe0, 0xe0).mirror(0x1d).r(FUNC(coleco_state::paddle_1_r));
map(0xe0, 0xe0).mirror(0x1b).w(FUNC(bit90_state::u32_w)); // bits7-4 for keyscan, (to bcd decoder) and bits1-0 tape out
map(0xe2, 0xe2).mirror(0x1d).r(FUNC(coleco_state::paddle_2_r)); // also, bit7 is tape read?
map(0xe2, 0xe2).mirror(0x1d).r(FUNC(coleco_state::paddle_2_r)); // also, bit7 is tape in
map(0xe4, 0xe4).mirror(0x1b).w("sn76489a", FUNC(sn76489a_device::write));
// IORQ goes to pin 55 of the Bit90 expansion port,