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An ATmega328P UART driver written entirely in Assembler.
; ===================================================================
; Project: ATmega328P UART Driver
; ===================================================================
; Author: Kevin Thomas
; E-Mail: [email protected]
; Version: 1.0
; Date: 12/29/24
; Target Device: ATmega328P (Arduino Nano)
; Clock Frequency: 16 MHz
; Toolchain: AVR-AS, AVRDUDE
; License: Apache License 2.0
; Description: This program is a simple I/O to which when a button
; connected to PD2 is pressed, an external LED connected
; to PD4 will illuminate, otherwise it will be off and
; utilizes UART at 9600 baud and echos every char
; pressed.
; ===================================================================
; ===================================================================
; SYMBOLIC DEFINITIONS
; ===================================================================
.equ DDRD, 0x0A ; Data Direction Register for PORTD
.equ PORTD, 0x0B ; PORTD Data Register
.equ PIND, 0x09 ; Input Pins Address for PORTD
.equ PD2, 2 ; Pin 2 of PORTD (D2 on Nano)
.equ PD4, 4 ; Pin 4 of PORTD (D4 on Nano)
.equ UCSR0A, 0xC0 ; USART0 Control & Status Reg A
.equ UCSR0B, 0xC1 ; USART0 Control & Status Reg B
.equ UCSR0C, 0xC2 ; USART0 Control & Status Reg C
.equ UBRR0L, 0xC4 ; USART0 Baud Rate Register Low
.equ UBRR0H, 0xC5 ; USART0 Baud Rate Register High
.equ UDR0, 0xC6 ; USART0 I/O Data Register
.equ RXEN0, 4 ; USART0 Receiver Enable
.equ TXEN0, 3 ; USART0 Transmitter Enable
.equ UDRE0, 5 ; USART0 Data Register Empty
.equ RXC0, 7 ; USART0 Receive Complete
.equ UCSZ00, 1 ; character size
.equ UCSZ01, 2 ; character size
; ===================================================================
; PROGRAM ENTRY POINT
; ===================================================================
.global program ; global label; make avail external
.section .text ; start of the .text (code) section
; ===================================================================
; PROGRAM LOOP
; ===================================================================
; Description: Main program loop which executes all subroutines and
; then repeads indefinately.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.87 RCALL – Relative Call to Subroutine
; 6.90 RJMP – Relative Jump
; ===================================================================
program:
RCALL Config_Pins ; config pins
RCALL UART_Init ; UART init
program_loop:
RCALL Check_Button ; check button state; control LED
RCALL Poll_UART ; non-blocking check; incoming char
RJMP program_loop ; infinite loop
; ===================================================================
; SUBROUTINE: Config_Pins
; ===================================================================
; Description: Main configuration of pins on the ATmega128P Arduino
; Nano.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.33 CBI – Clear Bit in I/O Register
; 6.95 SBI – Set Bit in I/O Register
; 6.88 RET – Return from Subroutine
; ===================================================================
config_pins:
CBI DDRD, PD2 ; set PD2 as input (button pin)
SBI PORTD, PD2 ; enable pull-up resistor on PD2
SBI DDRD, PD4 ; set PD4 as output (LED pin)
RET ; return from subroutine
; ===================================================================
; SUBROUTINE: UART_Init
; ===================================================================
; Description: Initializes UART0 for serial communication at 9600 baud,
; 8 data bits, no parity, 1 stop bit.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.63 LDI – Load Immediate
; 6.117 STS – Store Direct to Data Space
; 6.83 ORI – Logical OR with Immediate
; 6.88 RET – Return from Subroutine
; ===================================================================
UART_Init:
LDI R16, 103 ; set 9600 baud rate low byte
STS UBRR0L, R16 ; store in UBRR0L
LDI R16, 0 ; set 9600 baud rate high byte
STS UBRR0H, R16 ; store in UBRR0H
LDI R16, (1 << RXEN0) ; enable RX (Receiver Enable)
ORI R16, (1 << TXEN0) ; enable TX (Transmitter Enable)
STS UCSR0B, R16 ; store in UCSR0B
LDI R16, (1 << UCSZ00) ; set UCSZ00 (Bit 2) for 8 data bits
ORI R16, (1 << UCSZ01) ; set UCSZ01 (Bit 1) for 8 data bits
STS UCSR0C, R16 ; store in UCSR0C
RET ; return from subroutine
; ===================================================================
; SUBROUTINE: Check_Button
; ===================================================================
; Description: Checks if PD2 is pressed and if so, drive LED high,
; otherwise drive LED low.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.97 SBIS – Skip if Bit in I/O Register is Set
; 6.96 SBIC – Skip if Bit in I/O Register is Cleared
; 6.88 RET – Return from Subroutine
; ===================================================================
Check_Button:
SBIS PIND, PD2 ; skip next inst if PD2 is high
RCALL LED_On ; if PD2 is low; BTN pressed
SBIS PIND, PD2 ; skip next inst if PD2 is low
RCALL LED_Off ; if PD2 is high; BTN not pressed
RET ; return from subroutine
; ===================================================================
; SUBROUTINE: LED_On
; ===================================================================
; Description: Sets PB5 high to turn on the LED.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.95 SBI – Set Bit in I/O Register
; 6.88 RET – Return from Subroutine
; ===================================================================
led_on:
SBI PORTD, PD4 ; set PD4 high
RET ; return from subroutine
; ===================================================================
; SUBROUTINE: LED_Off
; ===================================================================
; Description: Clears PB5 to turn off the LED.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.33 CBI – Clear Bit in I/O Register
; 6.88 RET – Return from Subroutine
; ===================================================================
LED_Off:
CBI PORTD, PD4 ; set PD4 low
RET ; return from subroutine
; ===================================================================
; SUBROUTINE: Poll_UART
; ===================================================================
; Description: Non-blocking UART polling. If a byte arrives, it is
; echoed back via UART.
; -------------------------------------------------------------------
; Instructions: AVR Instruction Set Manual
; 6.114 LDS – Load Direct from Data Space
; 6.101 SBRS – Skip if Bit in Register is Set
; 6.88 RET – Return from Subroutine
; 6.115 STS – Store Direct to Data Space
; ===================================================================
Poll_UART:
LDS R16, UCSR0A ; load UCSR0A
SBRS R16, RXC0 ; skip next if RXC0 is set; has data
RET ; return from subroutine
LDS R16, UDR0 ; read received byte from UDR0
.Poll_TX_Wait:
LDS R17, UCSR0A ; load UCSR0A
SBRS R17, UDRE0 ; skip next if UDRE0 is set; buf rdy
RJMP .Poll_TX_Wait ; wait until buffer is ready
STS UDR0, R16 ; write byte to UDR0 (transmit)
RET ; return from subroutine