TCA9548AInterface.h

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// SPDX-License-Identifier: AGPL-3.0-only
// Copyright (c) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// https://github.com/dstroy0/XPoint
 
#ifndef TCA9548A_INTERFACE_H
#define TCA9548A_INTERFACE_H
 
#include "I2CInterface.h"
 
class TCA9548AInterface : public I2CInterface
{
    I2CInterface *_bus;    
    uint8_t _muxAddr;      
    uint8_t _channelMask;  
 
    /* Per-mux-address cache of the last channel-select byte written.
     * Indexed by (_muxAddr & 0x7): slot 0 = 0x70, …, slot 7 = 0x77.
     * Sentinel 0xFFu means "unknown / needs re-select".
     *
     * Defined inside the class body → implicitly inline in C++11, so the
     * function-local static `s[]` is shared across all translation units. */
    static uint8_t &_lastSel(uint8_t slot)
    {
        static uint8_t s[8] = {0xFFu, 0xFFu, 0xFFu, 0xFFu, 0xFFu, 0xFFu, 0xFFu, 0xFFu};
        return s[slot];
    }
 
    /* Select this channel on the mux only if it differs from the cached value.
     * On a cache hit: zero extra I2C transactions. */
    inline void _selectIfNeeded()
    {
        uint8_t &sel = _lastSel(_muxAddr & 0x7u);
        if (sel != _channelMask)
        {
            _bus->writeByte(_muxAddr, _channelMask);
            sel = _channelMask;
        }
    }
 
  public:
    TCA9548AInterface(I2CInterface *bus, uint8_t muxAddr, uint8_t channel)
        : _bus(bus), _muxAddr(muxAddr), _channelMask((uint8_t)(1u << (channel & 0x7u)))
    {
    }
 
    void begin() override
    {
        _bus->begin();
        _lastSel(_muxAddr & 0x7u) = 0xFFu;
    }
 
    void writeRegister(uint8_t devAddr, uint8_t reg, uint8_t val) override
    {
        _selectIfNeeded();
        _bus->writeRegister(devAddr, reg, val);
    }
};
 
#endif // TCA9548A_INTERFACE_H