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pia6821: roll back port A read change.

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This commit is contained in:
Mike 2019-06-10 13:31:37 -07:00
parent c5be2b3fe8
commit 0074e4df45
1 changed files with 6 additions and 5 deletions
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11
src/devices/machine/6821pia.cpp
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@ -237,11 +237,12 @@ uint8_t pia6821_device::get_in_a_value()
}
}
// For port A, when the port is in output mode, other devices can drive the pins too.
// If they load the lines so the voltage changes from what the PIA is outputting, that
// value will be read back.
// TODO: Figure out if any boards do this, and compensate.
ret = (m_out_a & m_ddr_a) | (port_a_data & ~m_ddr_a);
// - connected pins are always read
// - disconnected pins read the output buffer in output mode
// - disconnected pins are HI in input mode
ret = (~m_port_a_z_mask & port_a_data) |
( m_port_a_z_mask & m_ddr_a & m_out_a) |
( m_port_a_z_mask & ~m_ddr_a);
return ret;
}