I'm trying to set up UART on my Raspberry Pi 4 and it isn't working. I'm sure that usb-uart converter isn't a problem as it supports 3.3V. According to documentation GPIO base address is 0x7E20 5000 but that is not true, it's 0x7E20 0000.
I used ```echo < /dev/ttyUSB0``` to read data.
Code:
Code: Select all
.global uart_prepare
.global uart_write
.section .text
// UART0 0x7E201000
// UART1 0x7E201200 mini UART
// UART2 0x7E201400
// UART3 0x7E201600
// UART4 0x7E201800
// UART5 0x&E201A00
// DR 0x00 0x7E201000
// RSRECR 0x04
// FR 0x18
// ILPR 0x20 NOT IN USE
// IBRD 0x24
// FBRD 0x28
// LCRH 0x2C
// CR 0x30
// IFLS 0x34
// IMSC 0x38
// RIS 0x3C
// MIS 0x40
// ICR 0x44
// DMACR 0x48
// ITCR 0x80
// ITIP 0x84
// ITOP 0x88
// TDR 0x8C
uart_prepare:
// LCRH Register
// RRRRRRRR RRRRRRRR RRRRRRRR RRRRRRRR
// RRRRRRRR RRRRRRRR RRRRRRRR SWWFSEPB
// 31:8 Reserved
// 7 SPS stick parity select
// 6:5 WLEN Word length
// 4 FEN Fifos enable
// 3 STP2 Two stop bits select
// 2 EPS Even parity select
// 1 PEN Parity enable
// 0 BRK Break
mov x0, 0x7E20 // we are using UART2
lsl x0, x0, #16
mov x1, 0x1400
add x0, x0, x1
mov w1, 0b01110000 // set options; see above
str w1, [x0, 0x2C] // write options to LCRH
// IFLS Register
// 31:12 Reserved
// 11:6 Unsupported
// 5:3 RXIFLSEL
// 2:0 TXIFLSEL
mov w1, 0b100000 // interrupt when retrieve fifo is 7/8 full
// and when transmit fifo is 1/8 full
str w1, [x0, 0x34] // write to IFLS
// IBRD Register
// 31:16 Reserved
// 15:0 IBRD
mov w1, #4 // 9600 baud
str w1, [x0, 0x24]
// FBRD Register
// 31:6 Reserved
// 5:0 FBRD
mov w1, #0
//lsl w1, w1, #12
str w1, [x0, 0x28]
// IMSC Register
// 10 OEIM
// 9 BEIM
// 8 PEIM
// 7 FEIM
// 6 RTIM
// 5 TXIM must set
// 4 RXIM must set
// 3 DSRMIM unsupported
// 2 DCDMIM unsupported
// 1 CTSMIM
// 0 RIMIM unsupported
mov w1, 0b11111110010 // enable all possible interrupts
str w1, [x0, 0x38]
// Base address of GPIO register is 0x7E21 5000; no it's 0x7E20 0000
mov x1, 0x7E20 //7e21 we set x1 to GPIO base address
lsl x1, x1, #16
//mov x2, 0x5000
//add x1, x1, x2
mov w2, 0b011011 // This makes GPIO 0:1 uart pins
str w2, [x1, 0x0]
// Choose pull down resistors for GPIO 0:1
ldr w2, [x1, 0xE4] // read register
and w2, w2, 0xfffffff0 // clear GPIO 0:1
add w2, w2, 0b1010 // set GPIO 0:1
str w2, [x1, 0xE4]
// CR Register
// 32:16 Reserved
// 15 CTSEN
// 14 RTSEN
// 13 OUT2
// 12 OUT1
// 11 RTS unsupported
// 10 DTR unsupported
// 9 RXE
// 8 TXE
// 7 LBE
// 6:3 Reserved
// 2 SIRLP unsupported
// 1 SIREN unsupported
// 0 UARTEN
mov w1, 0b110000001 // enable uart; both receive and transmit
str w1, [x0, 0x30] // write options to CR, uart is enabled
ret
uart_write:
// x0 points to string we want to write
//ldrb w1, [x0], #1 // read char and move pointer by one byte
//cmp w1, 0x0
//bne uart_write
//ret
mov x0, 0x7E20
lsl x0, x0, #16
mov x1, 0x14
lsl x1, x1, #8
add x0, x0, x1
mov w1, #57 // this is '9'
strb w1, [x0, 0x0]
