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0X_ARMv8_指令集_基于汇编的UART驱动

0X_ARMv8_指令集_基于汇编的UART驱动

【摘要】:笨叔在完成ARMv8一些指令集操作之后,留了一个编写串口驱动的大作业,我觉得这个也算实操了,根据树莓派的datasheet,我们使用ARMv8的汇编完成UART驱动,也是对这一段时间学习的总结和运用。

【目标】:基于树莓派4B,编写基于UART的汇编驱动,实现UART的初始化、数据发送和数据接收的简单功能(不包含对于串口的FIFO等高级配置)

1. 初始化

树莓派串口的初始化操作和一般的bare-mental环境一致的,需要完成以下初始化操作:

  • 配置GPIO复用,选择UART0为功能引脚,而不是GPIO口。
  • 配置UART的RTL的时钟分频器,还要检查是否UART的时钟被使能。
  • 配置UART协议相关项目,115200波特率,8位,1位,无奇偶校验。
  • 配置UART中断,这里咱是配置为关闭。
  • 使能UART整个模块的开关。

1.1 配置GPIO

根据文档1,11.3. Primary UART Inputs and Outputs (Table 171. UARTAssignment on the GPIO Pin map),查询到UART0的TXD0对应GPIO14, RXD0对应GPIO15,物理属性皆为下拉,选择ALT功能为ALT0。

image-20220402162146459

配置寄存器的GPFSEL1 Register的FSEL15和FSEL14配置GPIO14,GPIO15为100'b(0x4)作为ALT0功能。GPFSEL1的基地址是:0x7e200000 + 0x04。我们可以使用汇编LDR指令,把(1<<17) | (1<< 14) 配置到32位寄存器里面。

#define U_BASE       (PBASE+0x00201000)
#define GPFSEL1         (PBASE+0x00200004)
#define GPSET0          (PBASE+0x0020001C)
#define GPCLR0          (PBASE+0x00200028)
#define GPPUD           (PBASE+0x00200094)
#define GPPUDCLK0       (PBASE+0x00200098)
#define GPIO_PUP_PDN_CNTRL_REG0 (PBASE+0x002000E4)
#define U_DATA_REG   (U_BASE)
#define U_FR_REG     (U_BASE+0x18)
#define U_IBRD_REG   (U_BASE+0x24)
#define U_FBRD_REG   (U_BASE+0x28)
#define U_LCRH_REG   (U_BASE+0x2C)
#define U_CR_REG     (U_BASE+0x30)
#define U_IMSC_REG   (U_BASE+0x38)

ldr w15, =GPIO_SEL1
ldr w15, [x15]
mov w16, #1, lsl #17
orr w16, w16, #1, lsl #14
orr w16, w15, w16
ldr w15, =GPIO_SEL1
str w16, [w15]

1.2 UART配置

这部分内容应该在UART的控制寄存器(CR register)

  • 31:16:0000_0000_0000_0000
  • 15-CTSEN: CTS硬件流控制,set 0
  • 14-RTSEN: RTS流控,set 0
  • 13:12 set 00
  • 11: RTS: 0
  • 10: DTR: 0
  • 9: RXE, 使能接受 1
  • 8: TXE,使能发送 1
  • 7: LBE,数字回环 0
  • 6:3 : 0000
  • 2: SIRLP: 0
  • 1: SIREN: 0
  • 0: UARTEN 1

配置如上,应该向CR reg(0x7e201000 + 0x30)写入: 1000_0000_1100_0000_0000_0000_0000_0000'b -> 0x80C00000

//diable uart
ldr w16, =U_CR_REG
str wzr, [w16]

mov w16, xzr
add w16, w16, #0x80C00000
ldr w15, =U_CR_REG
str w16, [w15]

相应的波特率,中断寄存器也是这样的操作

	/* set BRD */
	ldr x1, =U_IBRD_REG
	mov w2, #26
	str w2, [x1]

	ldr x1, =U_FBRD_REG
	mov w2, #3
	str w2, [x1]

	ldr x1, =U_LCRH_REG
	mov w2, #0x70  //(1<<4) | (3<<5)
	str w2, [x1]

	ldr x1, =U_IMSC_REG
	str wzr, [x1]

	ldr x1, =U_CR_REG
	mov w2, #0x301 //1 | (1<<8) | (1<<9)
	str w2, [x1]

2. 发送和接收

发送和接收使用数据寄存器和标志位寄存器。

image-20220402170210574

判断标志位寄存器:

image-20220402170254923

.align 2
.global asm_uart_recv
.global asm_uart_send

asm_uart_send:
    ldr x1, =U_FR_REG               // load FR register address.
    ldr x1, [x1]                    // load address content to x2
    mov x2, #0x01
    and x2, x1, x2, lsl #5          //  get TXFF bit flag.
    cmp x2, #0x0
    b.gt asm_uart_send
    ldr x1, =U_DATA_REG
    str x0, [x1]
    ret

asm_uart_recv:
    ldr x1, =U_FR_REG
    ldr x1, [x1]
    mov x2, #0x01
    and x2, x1, x2, lsl #4
    cmp x2, #0x0
    b.gt asm_uart_recv
    ldr x1, =U_DATA_REG
    ldr x0, [x1]
    and x0, x0, #0xFF

image-20220402170429616

Ref

Footnotes

  1. bcm2711/bcm2711-peripherals.pdf

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