ARM编程中的ARM Boot 示例

程序主要作用是通过串口下载VxWorks到ARM Flash ROM,主要过程是初始化ARM硬件,初始化串口UART1,从串口接收VxWorks image到DRAM,初始化Flash ROM.最后将DRAM中的VxWorks写入Flash ROM. arm 串口与PC机串口连接,由PC机下载(download)VxWorks 到主板Flash ROM.

该程序对学习和理解arm编程很有帮助.为了便于理解,这里我全部采用实际地址,不用宏定义.

程序语言为 arm 汇编,具体过程参考arm编程,具体寄存器接口定义参见arm硬件手册

VramBaseAddress EQU 0xC0000000 ;DRAM起始地址,在arm硬件地址映射定义,见arm编程
Length1 EQU 0x100


FlashBase EQU 0x70000000 ;Flash的基地址,在Boot模式下,为0x70000000
MMUCP CP 15 ;协处理器15

AREA |C$$code|, CODE, READONLY

ENTRY

EXPORT main

main
MOV r14, #0x70 ;设置MMU无效, 32位模式, Little endian
MCR MMUCP, 0, r14, c1, c0, 0

MRS r14, CPSR
BIC r14, r14, #0x1f ;Mask
ORR r14, r14, #0xc0 + 0x13 ;关闭所有 IRQ FIQ, 用SVC32 模式
MSR CPSR, r14

LDR r11, =0x80000000 ;内部寄存器基地址 0x80000000
LDR r12, =0x80001000

MOV r0, #0

STR r0, [r11, #0x280] ;disable 所有中断
STR r0, [r12, #0x280]

LDR r0, =0x840100 ;在系统控制寄存器SYSCON1中
STR r0, [r11, #0x100] ;使能外部时钟 EXCKEN 和串口 UART1

　
LDR r0, =0x06 ;在系统控制寄存器SYSCON2中
STR r0, [r12, #0x100] ;使能16-bit DRAM, 键盘KBD6

LDR r0, =0x03010100 ;在存储控制寄存器MEMCFG1中
STR r0, [r11, #0x180] ;设置外设数据线宽度CS0:16位,CS1,2:32位,CS3:8位

;特别注意:

1. 如设置线宽为16位,由于CPU是32位,CPU会把32位数据分两次操作写入,每次写16位,这样对32位的外设来说是一种浪费

例如: LDR r0, =0x10000000
LDR r1, =0x12345678
STR r1,[r0]
实际情况是 第一次写入地址0x10000000的数据是 0x1234
第二次写入地址0x10000002的数据是 0x5678

示波器上观察的情况是一个片选信号/CS低电平范围内,有两个写入信号/WE低电平.

2. 如设置线宽为32位,CPU会把32位数据一次写入,如果外设是16位的话,如ISA网卡,就会造成高16位丢失.

例如: LDR r0, =0x10000000
LDR r1, =0x12345678
STR r1,[r0]
实际情况是 一次写入地址0x10000000的数据是 0x12345678

LDR r0, = 0xff ;DRAM刷新率refresh rate,设置要适当,否则数据丢失
STR r0, [r11, #0x200]

LDR r13, =0xc0020000 ;设置堆栈 Stack, r13 为arm定义的堆栈指针
;Init OK
LDR r0, =receiving_msg
BL printmsg

BL dram_test ;跳转指令,执行测试DRAM

LDR r0, =startloader_cmdmsg
BL printmsg

BL Beep

LDR r0, =0x60005 ;设置波特率 38400, 8位
LDR r11, =0x80000000
STR r0, [r11, #0x4c0] ;初始化串口 UART
;Receive data and save it to buffer ;开始接收数据
LDR r12, =SaveAddr ;First 8 bytes are start address and length
;Receive address
LDR r5, =0x4 ;Read first 4 bytes
MOV r0, #0
rx_loop1
LDR r1, [r11, #0x140] ;检查系统状态寄存器System Status Register 1
TST r1, #0x00400000 ;UART1 Rx 是否为空
BNE rx_loop1 ;等待数据 Wait data
LDR r1, [r11, #0x480] ;读数据 Read data
AND r1, r1, #0xff
ORR r0, r1, r0, ROR #8
SUBS r5, r5, #1
BNE rx_loop1
MOV r0, r0, ROR #8

STR r0, [r12]
ADD r12, r12, #4

;Receive count
LDR r12, =Count
LDR r5, =0x4 ;Read second 4 bytes
MOV r0, #0
rx_loop2
LDR r1, [r11, #0x140] ;检查系统状态寄存器System Status Register 1
TST r1, #0x00400000 ;UART1 Rx 是否为空
BNE rx_loop2 ;等待数据 Wait data
LDR r1, [r11, #0x480] ;读数据 Read data
AND r1, r1, #0xff
ORR r0, r1, r0, ROR #8
SUBS r5, r5, #1
BNE rx_loop2
MOV r0, r0, ROR #8

STR r0, [r12]
MOV r6, r0 ;Save the count to r6

;Receive data
MOV r5, r0
MOV r9, #0
LDR r10, =BufferBase
MOV r12, #0
rx_loop3
LDR r1, [r11, #0x140] ;System Status Register 1
TST r1, #0x00400000 ;UART1 Rx Empty
BNE rx_loop3 ;Wait data
LDR r1, [r11, #0x480] ;Read data
AND r1, r1, #0xff
STRB r1, [r10, r12]
ADD r12, r12, #1

nextbyte ;读下一个字节
SUBS r5, r5, #1
BNE rx_loop3
;Receive data OK ;数据接收完毕

MOV r1, #0x31
BL send_char
MOV r1, #0x0d
BL send_char
MOV r1, #0x0a
BL send_char

;Save it to Flash ROM 把数据写入Flash ROM