diff options
Diffstat (limited to 'openpicc/os')
| -rw-r--r-- | openpicc/os/boot/boot.s | 31 |
1 files changed, 30 insertions, 1 deletions
diff --git a/openpicc/os/boot/boot.s b/openpicc/os/boot/boot.s index 0000479..c065a6d 100644 --- a/openpicc/os/boot/boot.s +++ b/openpicc/os/boot/boot.s @@ -250,10 +250,39 @@ my_fiq_handler: ldr r11, =ssc_tx_fiq_fdt_cdiv ldr r11, [r11] /* r11 == ssc_tx_fiq_fdt_cdiv */ +/* Problem: LDR from the timer still takes too long and causes us to miss the exact time. + * Strategy: Spin on TC2 till we are very near the actual time. Then load the timer value, calculate + * the difference to the target, then do a countdown spin without reading the timer. + * + * At 47.923200 MHz 7 processor cycles are 2 carrier cycles of the 13.56MHz carrier + */ + .equ SUB_TIME, 20 /* subtract 20 carrier cycles == 70 processor cycles */ + .equ ADD_TIME, (70-20) /* Add x processor cycles */ + + mov r8, #SUB_TIME + sub r11, r11, r8 + .wait_for_fdt_cdiv: ldr r8, [r12, #TC2_CV] cmp r8, r11 - bmi .wait_for_fdt_cdiv /* spin while TC2.CV is less fdt_cdiv */ + bmi .wait_for_fdt_cdiv /* spin while TC2.CV is less fdt_cdiv-SUB_TIM */ + + ldr r8, [r12, #TC2_CV] + sub r11, r11, r8 /* r11 == fdt_cdiv-SUB_TIME - TC2.CV */ + + mov r8, #0x07 + mul r11, r8, r11 /* r11 = r11 * 7 */ + mov r11, r11, ASR #1 /* r11 = r11 / 2 */ + + mov r8, #ADD_TIME + add r11, r11, r8 /* r11 = r11 + ADD_TIME */ + + mov r11, r11, ASR #2 /* r11 = r11 / 4 (4 is the number of cycles per loop run below) */ + + mov r8, #1 +.wait_for_my_time: + subs r11, r11, r8 + bge .wait_for_my_time str r9, [r12, #TC_CCR] /* SWTRG on TC0 */ |