微處理器 - MediaTek MT7688 (LinkIt Smart 7688) - Assembly - CPU Clock


MT7688支援25MHz、40MHz兩種XTAL


目前MT7688板子使用40MHz


CPU設定表格如下：

CPU最高頻率XTALRegister設定
580MHz40MHz XTAL_FREQ_SEL = 1
DIS_BBP_SLEEP = 0
EN_BBP_CLK = 0
CPU_FRM_BBP = 0
CPU_FRM_XTAL = 0
575MHz25MHz XTAL_FREQ_SEL = 0
DIS_BBP_SLEEP = 0
EN_BBP_CLK = 0
CPU_FRM_BBP = 0
CPU_FRM_XTAL = 0
480MHz40MHz XTAL_FREQ_SEL = 1
DIS_BBP_SLEEP = 1
EN_BBP_CLK = 1
CPU_FRM_BBP = 1
CPU_FRM_XTAL = 0
480MHz25MHz XTAL_FREQ_SEL = 0
DIS_BBP_SLEEP = 1
EN_BBP_CLK = 1
CPU_FRM_BBP = 1
CPU_FRM_XTAL = 0
40MHz40MHz XTAL_FREQ_SEL = 1
DIS_BBP_SLEEP = 0
EN_BBP_CLK = 0
CPU_FRM_BBP = 0
CPU_FRM_XTAL = 1
25MHz25MHz XTAL_FREQ_SEL = 0
DIS_BBP_SLEEP = 0
EN_BBP_CLK = 0
CPU_FRM_BBP = 0
CPU_FRM_XTAL = 1

XTAL_FREQ_SEL設定外接的晶振頻率


CPU_FRM_XTAL、CPU_FRM_BBP設定CPU頻率來源



CPU_FDIV、CPU_FFRAC用來設定除頻係數


公式如下： 

CPU frequency = PLL_FREQ * (CPU_FFRAC / CPU_FDIV)

範例：
XTAL_FREQ_SEL = 1
DIS_BBP_SLEEP = 0
EN_BBP_CLK = 0
CPU_FRM_BBP = 0
CPU_FRM_XTAL = 0
PLL_FREQ = 580MHz
CPU_FFRAC = 1
CPU_FDIV = 10
CPU頻率 = 580MHz * (1 / 10) = 58MHz

main.s 

    .extern _start
    .set noreorder

    .equiv SYSCFG0,     0xb0000010
    .equiv DYN_CFG0,    0xb0000440
    .equiv GPIO_CTRL_1, 0xb0000604
    .equiv GPIO_DATA_1, 0xb0000624
    .equiv LED,         (44 - 32)

    .text
_start:
    b reset
 
    .org 0x400
reset:
    mfc0 $10, $16
    and $10, 0xfffffff8
    ori $10, 0x00000003
    mtc0 $10, $16

    li $8, SYSCFG0
    lw $9, 0($8)
    or $9, (1 << 6)
    sw $9, 0($8)

    li $8, DYN_CFG0
    li $9, 0x00030101
    sw $9, 0($8)

    li $8, GPIO_CTRL_1
    li $9, (1 << LED)
    sw $9, 0($8)
loop:
    li $8, GPIO_DATA_1
    xori $9, $10, (1 << LED)
    move $10, $9
    sw $9, 0($8)

    li $8, 500000
0:
    addi $8, $8, -1
    bnez $8, 0b
    nop

    b loop
    nop

P.S. CPU頻率 = 580MHz * (1 / 1) = 580MHz，目前不知為何MT7688需要開啟CP0 KSeg0的Cacheable，才可以讓CPU速度上來，這部份是參考U-Boot程式碼

完成