## 3 Proposed Design Scheme
### Firmware Design  
The QSFP28 optical transceiver firmware design scheme proposed in this study was designed to meet the requirements of standard documents. The DS4830 of Maxim Integrated was used as the MCU for the QSFP28 optical transceiver [19]. The DS4830 supports optimized control for information monitoring of optical transceivers that meet the SFF-8472 standard. It provides ADC and DAC with 12-bit resolution and I2C slave interface that can operate reliably at a 400 kHz clock speed. Figure 6 shows the QSFP28 firmware design block diagram. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Layer**} & **Lay-Up** & \begin{tabular}{c} **Designed** \\ **Structure** \\ \end{tabular} & \begin{tabular}{c} **Dielectric** \\ **thickness** \\ **(um)** \\ \end{tabular} & \begin{tabular}{c} **Constant** \\ **(Dk)** \\ \end{tabular} & \
\begin{tabular}{c} **Dissipation** \\ **Factor (Df)** \\ \end{tabular} & **Function** \\ \hline \(1\)(copper &
\begin{tabular}{c} Copper, \\ plate) \\ \end{tabular} & 45 & & & & Transmission line \\ (dielectric & Low loss \\ layer) & material & 73 & 3.4 & 0.004 & \\
2 & Copper & 15 & & & Analog GND \\  & FR4 & 76 & 4.13 & 0.0158 & \\
3 & Copper & 15 & & & & \\  & FR4 & 101 & 4.13 & 0.0158 & \\
4 & Copper & 15 & & & Power \\  & FR4 & 43 & 4.13 & 0.0158 & \\
5 & Copper & 15 & & & Low speed \\  & FR4 & 101 & 4.13 & 0.0158 & \\
6 & Copper & 15 & & & & \\  & FR4 & 76 & 4.13 & 0.0158 & \\
7 & Copper & 15 & & & Power \\  & FR4 & 101 & 4.13 & 0.0158 & \\
8 & Copper & 15 & & & Low speed \\  & FR4 & 76 & 4.13 & 0.0158 & \\
9 & Copper & 15 & & & Digital GND \\  & FR4 & 78 & 3.4 & 0.004 & \\
10 & Copper, & 45 & & & Transmission line, low \\  & ENIG & & & & & spped signal \\ \hline \hline \end{tabular}
\end{table} 
Table 3: QSFP28 PAM4 printed circuit board (PCB) layer stack-up. The QSFP28 optical transceiver defines the internal registers for control and monitoring in the standard and uses I2C as the host access interface. In addition, the firmware implements digital diagnostic monitoring (DDM) and alarm functions to indicate the optical transceiver status and implements external pins functions connected by the connectors described in Section 2. Moreover, the firmware implements the PAM4 DSP IC drive function for PAM4 processing, the DAC control function to drive transmission IC, the ADC control function to update DDM information, and the debugging interface to help debug and set up the optical transceivers. The debugging interface is not directly implemented but is indirectly designed so that the manufacturer's area of the QSFP28 register is defined and used for debugging purposes. Figure 7 shows the firmware sequencing of the QSFP28 PAM4 optical transceiver. The operation is initiated when the optical transceiver is energized or the reset signal is released. First, the MCU initializes the peripherals. The peripherals to be used in the proposed scheme are ADC for temperature, drive voltage, and alarm monitoring; DAC to drive the transmitting optical components such as EML-TOSA; I2C slave functions to be used as host interfaces; and I2C master functions to communicate with the external ADC IC and PAM4 DSP IC. Additionally, the Watchdog timer is initialized for firmware stability and for setting up the General-Purpose Input/Output(GPIO) pin. When the MCU peripheral initialization is complete, the PAM4 DSP IC initialization process is followed. To operate the PAM4 DSP IC, the firmware for DSP IC drive must be downloaded and initialized. The ADC IC is added in this scheme for channel scalability and precise measurement. Thus, the operating program undergoes an initialization process to utilize the external ADC IC. In addition, the process of initializing the global variables required for QSFP28 optical transceiver operation is followed by an infinite polling loop. The operating program checks the status of the LPMode/TxDIS pin and, if enabled, switches the PAM4 DSP IC to a non-operational state and shuts off the transmitting optic components power.