UART (Serial Communication)

The UART module is a serial communication module that allows the microcontroller to communicate with other devices using the Universal Asynchronous Receiver/Transmitter (UART) protocol. It is capable of sending up to 20 bits per second using vanilla Redstone wire.

This module is designed similarly to the ACIA 6551 chip, which is a serial communication chip used in the Commodore 64 computer.

Terms

• Baud Rate: The number of bits per second that the module can transmit or receive.
• Data Frame: A data frame is a sequence of bits that contains the data to be transmitted or received. It consists of a start bit, data bits, an optional parity bit, and 1 or 2 stop bits.
• Start Bit: The start bit is used to indicate the beginning of a data frame. The start bit is always 1.
• Data Bits: The data bits are the bits that contain the data to be transmitted or received. The number of data bits can be set to 1 or 8 bits.
• Parity Bit: The parity bit is used to check the integrity of the data. The parity bit can be set to EVEN or ODD parity.
• Stop Bit: The stop bit is used to indicate the end of a data frame. The stop bit is always 0.

Working Principle

This module has two data registers, UARTTXD and UARTRXD, which are used to transmit and receive data respectively. The module also has UARTCTL and UARTCMD registers to configure serial communication.

Upon writing data to the UARTTXD register, the module will start transmitting the data to the connected device. If it is recieving data, the module will store the data in the UARTRXD register.

If two microcontrollers are connected to each other, they must have the same configuration for the serial communication to work properly. The configuration includes the baud rate, data bits, parity, and stop bits.

Status Register (UARTSTS)

The UARTSTS register is used to check the status of the UART module. Here are the list of status flags (N/U means Not Used):

Reading this register will return the status of the UART module and clear the IRQ, OVERRUN_ERROR, FRAME_ERROR, and PARITY_ERROR flags.

Writing to this register will trigger a software reset, which will clear all flags and reset the module's configuration to default.

Status Flags

• IRQ - Interrupt Request: This flag is set when the UART module has finished transmitting or receiving data if their respective interrupt enable bits are set in the UARTCMD register. If this flag is set, the IRQ line of the processor will be held LOW until the flag is cleared.

• TX_BUFFER_EMPTY - TX Buffer Empty/Ready: This flag is set when the UARTTXD register is empty and the module is ready to transmit data. Writing data to the UARTTXD register while it is not empty will cause and overrun error.

• RX_BUFFER_FULL - RX Buffer Full: This flag is set when the UARTRXD register is full and it contains received data.

• OVERRUN_ERROR - Overrun Error: This flag is set when the UARTRXD register is full and new data is received or when the UARTTXD register is written to while it is not empty.

• FRAME_ERROR - Frame Error: This flag is set when the received data does not match the configured frame format. This is triggered specifically when the recieved stop bit is not a 0.

• PARITY_ERROR - Parity Error: This flag is set when the received data does not match the configured parity bit.

Control Register (UARTCTL)

The UARTCTL register is used to configure the module's baud rate, word length, and stop bit.

By default, it is set to 1 stop bit, 8 data bits, and a baud rate of 10. This is set every hard/software reset.

Stop Bits

Following the protocol, the stop bit is used to indicate the end of a data frame. The stop bit is always 0.

• 0 - 1 stop bit
• 1 - 2 stop bits

Word Length

The word length is the number of bits in a data frame. The word length can be set to 1 or 8 bits. WRDLEN is a 3-bit field, so the resulting number of data bits follows this formula: WRDLEN + 1.

Baud Rate

The baud rate is the number of bits per second that the module can transmit or receive. BAUD_RATE is a 4-bit field, so the resulting baud rate follows this table:

Obviously, since Minecraft only runs 20 ticks per second, the maximum baud rate is 20. Higher bit rates are planned to be supported using custom Redstone wire from the mod, and they may be added in the future.

Tip:

It is recommended to set the baud rate to 10 for signal lines with repeaters and 20 for direct connections with Redstone wire.

Command Register (UARTCMD)

The UARTCMD register is used to configure how the module deals with received and transmitted data. Here are the list of command flags (N/U means Not Used):

Command Flags

• TX_PIN_ENABLE - TX Pin Enable: This flag is used to enable the pin selected in register UARTTXP to transmit data. When enabled, the module will override the pin's output and transmit data. However, the pin must be an OUTPUT by setting the pin's direction using the GPIO module.

• RX_PIN_ENABLE - RX Pin Enable: This flag is used to enable the pin selected in register UARTRXP to receive data. When enabled, the module will read the pin's input to assemble incoming serial data. The pin must be an INPUT by setting the pin's direction using the GPIO module.

• PARITY_TYPE - Parity Type: This flag is used to select the type of parity bit to use. If it is set, the module will use EVEN parity, and If it is cleared, it will use ODD parity.

• PARITY_ENABLE - Parity Enable: This flag is used to enable the parity bit in the data frame. If this flag is set, the module will add a parity bit to the data frame. When receiving data, the module will check the parity bit to see if the data is correct (keep in mind that parity configuration must be the same on both devices).

• ECHO_ENABLE - Echo Enable: This flag is used to enable the echo feature. When enabled, the module will transmit the received data back through TX.

• TX_INTERRUPT_ENABLE - TX Interrupt Enable: This flag is used to enable the interrupt request when the UARTTXD register is empty and the module is ready to transmit data.

• RX_INTERRUPT_ENABLE - RX Interrupt Enable: This flag is used to enable the interrupt request when the UARTRXD register is full and contains received data.

Transmit Data Register (UARTTXD)

The UARTTXD register is used to transmit data to the connected device. When data is written to this register, the module will start transmitting the data to the connected device.

If the TX_BUFFER_EMPTY flag is cleared while this register is written to, the module may transmit garbled data and set the OVERRUN_ERROR flag.

Receive Data Register (UARTRXD)

The UARTRXD register is used to store received data from the connected device. The instant the entire data frame is read, the RX_BUFFER_FULL status is set.

Reading this register will clear the RX_BUFFER_FULL, OVERRUN_ERROR, FRAME_ERROR, and PARITY_ERROR flag. If data is not read from this register before new data is received, the module will set the OVERRUN_ERROR flag.

Reading this register while it the RX_BUFFER_FULL flag is cleared may return garbled data.

Transmit Pin Register (UARTTXP)

The UARTTXP register is used to select the pin that will be used to transmit data. The selected pin must be an OUTPUT by setting the pin's direction using the GPIO module.

By default, the UARTTXP register is set to 0, which corresponds to the first pin in the GPIO module.

Receive Pin Register (UARTRXP)

The UARTRXP register is used to select the pin that will be used to receive data. The selected pin must be an INPUT by setting the pin's direction using the GPIO module.

By default, the UARTRXP register is set to 1, which corresponds to the second pin in the GPIO module.

Register Locations

Register addresses depends on every microcontroller, so you have to refer to their respective summaries to know where the EL registers are located. Find the register summaries here.