• Home

Micro Uart

A UART built using very little logic.
  FPGA proven NO
  ASIC proven NO

Introduction

The Micro UART comes from a challenge I set myself: to write a fully functional UART using a minimum number of registers. The term 'fully functional' is rather vague so I set the following targets:

• Programmable baud rate
• One format: 1 start bit, 8 data bits, 1 stop bit, no parity.
• Double buffered input
• Receiver overrun detection

The UART has no standard processor interface. Instead it uses two 'select' signals in combination with a read and a write to give access to the registers. This also makes it easier to control the UART with a hardware FSM. The data bus is 16 bits wide to match the width of the programmable baud rate. (Eight bit processors are rather out of date anyway).
The code also contains a set of APB wrappers which will interface the UART to an ARM APB bus.
The UART has 4 registers:

• 16-bit baud rate (Read and write)
• Data transmit (Write only)
• Data receive (Read only)
• Status register (Read only)

I managed to built the UART using 76 registers. If you can live with a static baud rate you can save 16 registers (or more). I am using 8 times oversampling. This has shown to work reliable in the past. (For the second UART in the Raspberry-Pi chip I also used 8 times oversampling).

In the code you find a few more versions, each slightly bigger and more sophisticated then the previous.

• Micro UART 1
A slightly bigger (78 registers) version which also supports interrupts.
This UART has one more CPU accessible register: A control register (Write only) which holds the interrupt enable bits.
The status register has been expanded with two more interrupt pending status bits.
• Micro UART 2
This one has all the features of Micro UART 1 plus a separate clock for the baudrate.
It allows the baud rate to remain unchanged if the CPU frequency varies.
• Micro UART 3
This one has all the features of Micro UART 2 and also detects a write to the baud-rate register and changes to the new baud-rate immediately.
• Micro UARTx APB
For each of the above four UARTs there exists an APB wrapper .
This allows an APB interface to directly read and write each of the four variants.

Pointers:

• micro_uart.v.
  
• micro_uart1.v.
  
• micro_uart2.v.
  
• micro_uart3.v.
  
• micro_uart_apb.v.
  
• micro_uart1_apb.v.
  
• micro_uart2_apb.v.
  
• micro_uart3_apb.v.
  
• micro_uart_testbench.v.
  
• micro_uart1_testbench.v.
  
• micro_uart2_testbench.v.
  
• micro_uart3_testbench.v.
  

A document describing the UART(s) (initial release) is available here. The complete package can be found in this zip archive.