Bootloader

Overview

The purpose of DSerial bootloader is to:

• Boot DSerial firmware
• Return firmware version
• Upgrade firmware via SPI (other ports may be supported by the bootloader in the future)

The following sections will describe how the bootloader functions.

Detecting DSerial

Check whether DSerial is inserted using a SPI Flash compatible command:

1. DS sends the byte 0x9F (RDID)
2. DSerial responds with 0x9F 0x01 0xAB

This is command is non-destructive to DS game cards.

Interrupt Vector Table (Currently Used Method)

Interrupt vector table on the C8051F320 is always at offset 0 in FLASH. Unfortunately, we cannot re-target it from bootloader interrupt vector table into the program interrupt vector table with a magic register, since no such register exists in the architecture. Instead, we have to re-target each interrupt separately.

The User Bit in PSW register (also called F1) is used to select bootloader table or firmware table.

<cpp> // Code in bootloader, interrupt.h

/* UART0 interrupt */ void redirS0() __interrupt (4) _naked { _asm push psw jnb psw.1, 00001$ ; if user bit not set, jump to firmware irq

pop psw lcall _uartInterrupt ; otherwise jump to bootloader irq reti

00001$: pop psw ljmp #FIRMWARE_OFFSET + IRQ_S0 ; firmware irq _endasm; }

// repeat for the other irqs </cpp>

This code jumps to bootloader interrupt if PSW.1 is set to 1, otherwise it jumps to firmware.

<cpp> // Code in firmware

void main() { // ... F1 = 0; // relocate interrupts to FIRMWARE_OFFSET (0x0800) EA = 1; // global interrupt enable // ... } </cpp>

Interrupt Vector Table (Not Used Method)

The following code shows another way that could have been used. It's advantage is that the interrupt vector can be relocated to any address dynamically. The disadvantage is that there's bigger overhead.

<cpp> // Code in bootloader

// Redirects UART IRQ either into our (bootloader's) handler or into firmware handler. void R_uartInterrupt() __interrupt (4) _naked { _asm mov a, _IrqVector ; compare IrqVector to 0 jnz 00001$ mov a, (_IrqVector+1) jz 00002$ 00001$: ; if not 0, then we need to jump to it mov dpl, _IrqVector mov dph, (_IrqVector+1) mov a, #0x23 ; this is the offset for uart irq jmp @a+dptr ; call the interrupt handler in firmware 00002$: ; if it's 0, then we'll handle the irq lcall _uartInterrupt ; call our own interrupt handler reti _endasm; }

// repeat for the other irqs </cpp>

IrqVector is a global variable that is set to the location of interrupt vector before enabling interrupts. The bootloader should set IrqVector to 0 while the program should set it to it's location.

<cpp> // Code in firmware

__data __at (0x7e) unsigned int IrqVector;

void main() { // ... IrqVector = 0x0800; // we're relocated to 0x0800 EA = 1; // global interrupt enable // ... } </cpp>