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acia6850: Prevent transmission of a spurious character following master reset

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This commit is contained in:
AJR 2020-03-25 22:08:42 -04:00
parent f69dd8c736
commit e26ad29829
1 changed files with 7 additions and 5 deletions
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12
src/devices/machine/6850acia.cpp
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@ -219,7 +219,10 @@ void acia6850_device::control_w(uint8_t data)
m_tx_state = STATE_START; m_tx_state = STATE_START;
output_txd(1); output_txd(1);
m_status &= SR_CTS; /// TODO: find out whether this has any immediate effect on TDRE
/// TDRE probably shouldn't be cleared here if TDR wasn't loaded, despite what the datasheet implies
/// If TDRE is cleared, then a separate internal flag is needed to avoid transmitting a spurious first character
m_status &= SR_CTS | SR_TDRE;
if (m_dcd) if (m_dcd)
{ {
@ -252,13 +255,13 @@ void acia6850_device::data_w(uint8_t data)
{ {
LOG("MC6850 '%s' Data: %02x\n", tag(), data); LOG("MC6850 '%s' Data: %02x\n", tag(), data);
/// TODO: find out if data stored during master reset is sent after divider is set /// TODO: find out if data stored during master reset is sent after divider is set (and if TDRE can be cleared by such a write)
if (m_divide == 0) if (m_divide == 0)
{ {
logerror("%s:ACIA %p: Data write while in reset!\n", machine().describe_context(), (void *)this); logerror("%s: ACIA data write while in reset!\n", machine().describe_context());
} }
/// TODO: find out what happens if TDRE is already clear when you write /// TODO: find out whether this overwrites previous data or has no effect if TDRE is already clear when you write
m_tdr = data; m_tdr = data;
m_status &= ~SR_TDRE; m_status &= ~SR_TDRE;
@ -484,7 +487,6 @@ WRITE_LINE_MEMBER( acia6850_device::write_txc )
{ {
m_tx_counter++; m_tx_counter++;
/// TODO: check txd is correctly generated, check atarist mcu is reading data, start checking receive data.
switch (m_tx_state) switch (m_tx_state)
{ {
case STATE_START: case STATE_START: