MC6850.cc

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#include "MC6850.hh"
#include "MidiInDevice.hh"
#include "MSXMotherBoard.hh"
#include "EmuTime.hh"
#include "serialize.hh"
#include <array>
 
namespace openmsx {
 
// control register bits
static constexpr unsigned CR_CDS1 = 0x01; // Counter Divide Select 1
static constexpr unsigned CR_CDS2 = 0x02; // Counter Divide Select 2
static constexpr unsigned CR_CDS  = CR_CDS1 | CR_CDS2;
static constexpr unsigned CR_MR   = CR_CDS1 | CR_CDS2; // Master Reset
// CDS2 CDS1
// 0    0     divide by 1
// 0    1     divide by 16
// 1    0     divide by 64
// 1    1     master reset (!)
 
static constexpr unsigned CR_WS1  = 0x04; // Word Select 1 (mostly parity)
static constexpr unsigned CR_WS2  = 0x08; // Word Select 2 (mostly nof stop bits)
static constexpr unsigned CR_WS3  = 0x10; // Word Select 3: 7/8 bits
static constexpr unsigned CR_WS   = CR_WS1 | CR_WS2 | CR_WS3; // Word Select
// WS3 WS2 WS1
// 0   0   0   7 bits - 2 stop bits - Even parity
// 0   0   1   7 bits - 2 stop bits - Odd  parity
// 0   1   0   7 bits - 1 stop bit  - Even parity
// 0   1   1   7 bits - 1 stop bit  - Odd  Parity
// 1   0   0   8 bits - 2 stop bits - No   Parity
// 1   0   1   8 bits - 1 stop bit  - No   Parity
// 1   1   0   8 bits - 1 stop bit  - Even parity
// 1   1   1   8 bits - 1 stop bit  - Odd  parity
 
static constexpr unsigned CR_TC1  = 0x20; // Transmit Control 1
static constexpr unsigned CR_TC2  = 0x40; // Transmit Control 2
static constexpr unsigned CR_TC   = CR_TC1 | CR_TC2; // Transmit Control
// TC2 TC1
// 0   0   /RTS low,  Transmitting Interrupt disabled
// 0   1   /RTS low,  Transmitting Interrupt enabled
// 1   0   /RTS high, Transmitting Interrupt disabled
// 1   1   /RTS low,  Transmits a Break level on the Transmit Data Output.
//                                 Interrupt disabled
 
static constexpr unsigned CR_RIE  = 0x80; // Receive Interrupt Enable: interrupt
// at Receive Data Register Full, Overrun, low-to-high transition on the Data
// Carrier Detect (/DCD) signal line
 
// status register bits
static constexpr unsigned STAT_RDRF = 0x01; // Receive Data Register Full
static constexpr unsigned STAT_TDRE = 0x02; // Transmit Data Register Empty
static constexpr unsigned STAT_DCD  = 0x04; // Data Carrier Detect (/DCD)
static constexpr unsigned STAT_CTS  = 0x08; // Clear-to-Send (/CTS)
static constexpr unsigned STAT_FE   = 0x10; // Framing Error
static constexpr unsigned STAT_OVRN = 0x20; // Receiver Overrun
static constexpr unsigned STAT_PE   = 0x40; // Parity Error
static constexpr unsigned STAT_IRQ  = 0x80; // Interrupt Request (/IRQ)
 
MC6850::MC6850(const std::string& name_, MSXMotherBoard& motherBoard, unsigned clockFreq_)
        : MidiInConnector(motherBoard.getPluggingController(), name_ + "-in")
        , syncRecv (motherBoard.getScheduler())
        , syncTrans(motherBoard.getScheduler())
        , clockFreq(clockFreq_)
        , rxIRQ(motherBoard, name_ + "-rx-IRQ")
        , txIRQ(motherBoard, name_ + "-tx-IRQ")
        , outConnector(motherBoard.getPluggingController(), name_ + "-out")
{
        reset(EmuTime::zero());
        setDataFormat();
}
 
// (Re-)initialize chip to default values (Tx and Rx disabled)
void MC6850::reset(EmuTime::param time)
{
        syncRecv .removeSyncPoint();
        syncTrans.removeSyncPoint();
        txClock.reset(time);
        txClock.setFreq(clockFreq);
        rxIRQ.reset();
        txIRQ.reset();
        rxReady = false;
        txShiftRegValid = false;
        pendingOVRN = false;
        controlReg = CR_MR;
        statusReg = 0;
        rxDataReg = 0;
        setDataFormat();
}
 
byte MC6850::readStatusReg() const
{
        return peekStatusReg();
}
 
byte MC6850::peekStatusReg() const
{
        byte result = statusReg;
        if (rxIRQ.getState() || txIRQ.getState()) result |= STAT_IRQ;
        return result;
}
 
byte MC6850::readDataReg()
{
        byte result = peekDataReg();
        statusReg &= byte(~(STAT_RDRF | STAT_OVRN));
        if (pendingOVRN) {
                pendingOVRN = false;
                statusReg |= STAT_OVRN;
        }
        rxIRQ.reset();
        return result;
}
 
byte MC6850::peekDataReg() const
{
        return rxDataReg;
}
 
void MC6850::writeControlReg(byte value, EmuTime::param time)
{
        byte diff = value ^ controlReg;
        if (diff & CR_CDS) {
                if ((value & CR_CDS) == CR_MR) {
                        reset(time);
                } else {
                        // we got out of MR state
                        rxReady = true;
                        statusReg |= STAT_TDRE;
 
                        txClock.reset(time);
                        switch (value & CR_CDS) {
                        case 0: txClock.setFreq(clockFreq,  1); break;
                        case 1: txClock.setFreq(clockFreq, 16); break;
                        case 2: txClock.setFreq(clockFreq, 64); break;
                        }
                }
        }
 
        controlReg = value;
        if (diff & CR_WS) setDataFormat();
 
        // update IRQ status
        rxIRQ.set(( value & CR_RIE) && (statusReg & STAT_RDRF));
        txIRQ.set(((value & CR_TC) == 0x20) && (statusReg & STAT_TDRE));
}
 
// Sync data-format related parameters with the current value of controlReg
void MC6850::setDataFormat()
{
        outConnector.setDataBits(controlReg & CR_WS3 ? DataBits::D8 : DataBits::D7);
 
        static constexpr std::array<StopBits, 8> stopBits = {
                StopBits::S2, StopBits::S2, StopBits::S1, StopBits::S1,
                StopBits::S2, StopBits::S1, StopBits::S1, StopBits::S1,
        };
        outConnector.setStopBits(stopBits[(controlReg & CR_WS) >> 2]);
 
        outConnector.setParityBit(
                (controlReg & (CR_WS3 | CR_WS2)) != 0x10, // enable
                (controlReg & CR_WS1) ? Parity::ODD : Parity::EVEN);
 
        // start-bits, data-bits, parity-bits, stop-bits
        static constexpr std::array<byte, 8> len = {
                1 + 7 + 1 + 2,
                1 + 7 + 1 + 2,
                1 + 7 + 1 + 1,
                1 + 7 + 1 + 1,
                1 + 8 + 0 + 2,
                1 + 8 + 0 + 1,
                1 + 8 + 1 + 1,
                1 + 8 + 1 + 1,
        };
        charLen = len[(controlReg & CR_WS) >> 2];
}
 
void MC6850::writeDataReg(byte value, EmuTime::param time)
{
        if ((controlReg & CR_CDS) == CR_MR) return;
 
        txDataReg = value;
        statusReg &= byte(~STAT_TDRE);
        txIRQ.reset();
 
        if (syncTrans.pendingSyncPoint()) {
                // We're still sending the previous character, only
                // buffer this one. Don't accept any further characters
        } else {
                // We were not yet sending. Start sending at the next txClock.
                // Important: till that time TDRE should go low
                //  (MC6850 detection routine in Synthesix depends on this)
                txClock.advance(time); // clock edge before or at 'time'
                txClock += 1; // clock edge strictly after 'time'
                syncTrans.setSyncPoint(txClock.getTime());
        }
}
 
// Triggered between transmitted characters, including before the first and
// after the last character.
void MC6850::execTrans(EmuTime::param time)
{
        assert(txClock.getTime() == time);
        assert((controlReg & CR_CDS) != CR_MR);
 
        if (txShiftRegValid) {
                txShiftRegValid = false;
                outConnector.recvByte(txShiftReg, time);
        }
 
        if (statusReg & STAT_TDRE) {
                // No next character to send, we're done.
        } else {
                // There already is a next character, start sending that now
                // and accept a next one.
                statusReg |= STAT_TDRE;
                if ((controlReg & CR_TC) == 0x20) txIRQ.set();
 
                txShiftReg = txDataReg;
                txShiftRegValid = true;
 
                txClock += charLen;
                syncTrans.setSyncPoint(txClock.getTime());
        }
}
 
// MidiInConnector sends a new character.
void MC6850::recvByte(byte value, EmuTime::param time)
{
        assert(acceptsData() && ready());
 
        if (statusReg & STAT_RDRF) {
                // So, there is a byte that has to be read by the MSX still!
                // This happens when the MSX program doesn't
                // respond fast enough to an earlier received byte.
                // The STAT_OVRN flag only becomes active once the prior valid
                // character has been read from the data register.
                pendingOVRN = true;
        } else {
                rxDataReg = value;
                statusReg |= STAT_RDRF;
        }
        // both for OVRN and RDRF an IRQ is raised
        if (controlReg & CR_RIE) rxIRQ.set();
 
        // Not ready now, but we will be in a while
        rxReady = false;
 
        // The MC6850 has separate TxCLK and RxCLK inputs, but both share a
        // common divider. This implementation hard-codes an input frequency
        // for both. Below we want the receive clock period, but it's OK
        // to calculate that as 'txClock.getPeriod()'.
        syncRecv.setSyncPoint(time + txClock.getPeriod() * charLen);
}
 
// Triggered when we're ready to receive the next character.
void MC6850::execRecv(EmuTime::param time)
{
        assert(acceptsData());
        assert(!rxReady);
        rxReady = true;
        getPluggedMidiInDev().signal(time); // trigger (possible) send of next char
}
 
// MidiInDevice queries whether it can send a new character 'now'.
bool MC6850::ready()
{
        return rxReady;
}
 
// MidiInDevice queries whether it can send characters at all.
bool MC6850::acceptsData()
{
        return (controlReg & CR_CDS) != CR_MR;
}
 
// MidiInDevice informs us about the format of the data it will send
// (MIDI is always 1 start-bit, 8 data-bits, 1 stop-bit, no parity-bits).
void  MC6850::setDataBits(DataBits /*bits*/)
{
        // ignore
}
void MC6850::setStopBits(StopBits /*bits*/)
{
        // ignore
}
void MC6850::setParityBit(bool /*enable*/, Parity /*parity*/)
{
        // ignore
}
 
// version 1: initial version
// version 2: added control
// version 3: actually working MC6850 with many more member variables
template<typename Archive>
void MC6850::serialize(Archive& ar, unsigned version)
{
        if (ar.versionAtLeast(version, 3)) {
                ar.template serializeBase<MidiInConnector>(*this);
                ar.serialize("outConnector", outConnector,
 
                             "syncRecv",  syncRecv,
                             "syncTrans", syncTrans,
 
                             "txClock", txClock,
                             "rxIRQ", rxIRQ,
                             "txIRQ", txIRQ,
 
                             "rxReady",         rxReady,
                             "txShiftRegValid", txShiftRegValid,
                             "pendingOVRN",     pendingOVRN,
 
                             "rxDataReg",  rxDataReg,
                             "txDataReg",  txDataReg,
                             "txShiftReg", txShiftReg,
                             "controlReg", controlReg,
                             "statusReg",  statusReg);
        } else if (ar.versionAtLeast(version, 2)) {
                ar.serialize("control", controlReg);
        } else {
                controlReg = 3;
        }
 
        if constexpr (Archive::IS_LOADER) {
                setDataFormat();
        }
}
INSTANTIATE_SERIALIZE_METHODS(MC6850);
 
} // namespace openmsx