The EverDrive-64 X7 is a flash cart made by Krikzz.

Serial Communication

The EverDrive-64 X7 supports serial over USB communication to a host PC. This can be used to load ROMs and allows running ROMs to send an receive data from the host PC. Krikzz provides a reference implementation here.

Serial Registers

The EverDrive-64 X7 provides various registers starting at the address 0x1F800000 in the cartridge address space. The following registers are used for USB communcation.

List of Registers
Name	Description	Address relative to 0x1F800000	Size (bytes)
REG_USB_CFG	Reads USB status and sends read and write commands	0x0004	4
REG_USB_DATA	The temporary data buffer used to read and write to USB	0x0400	512
REG_KEY	Set to 0xAA55 on initialization	0x8004	4
REG_SYS_CFG	Set to 0 on initialization	0x8000	4

The REG_USB_CFG register contains various bits as described below

REG_USB_CFG bits
Name	Description	Bits (15-0)	Hex Mask
USB_LE_CFG	Set to a 1 when reading and writing data	15	0x8000
USB_LE_CTR	Set to a 1 when reading and writing data	14	0x4000
USB_STA_BSY	Behavior not known	13	0x2000
USB_STA_PWR	Set to when 1 when data can be read or written	12	0x1000
USB_STA_TXE	Set to 0 when data can be written	11	0x0800
USB_CFG_RD/USB_STA_RXF	Writing a 1 switches the controller to read mode. Writing a 0 switches the controller to write mode. Reads as a 0 when data is available to be read.	10	0x0400
USB_CFG_ACT/USB_STA_ACT	Writing a 1 starts reading or writing to REG_USB_DATA. The direction of data is specified by the value of USB_CFG_RD. The amount of data written is specified by baddr. When reading this bit, a 1 indicates the USB port is busy reading or writing data.	9	0x0200
baddr	These 5 bits give the address into REG_USB_DAT where USB data is read from and written to. Data is read starting at `baddr` offset until the end of the REG_USB_DAT buffer. This means the amount of data read is `512 - baddr`. Because of this, when copying data to and from REG_USB_DAT, you don't always start copying at the beginning of the buffer.	8..0	0x01FF

Initializing Serial Data

Below is the code used to initialize USB IO from krikzz's reference code with comments

void bi_init() {
    // sets the PI handler latency
    IO_WRITE(PI_BSD_DOM1_LAT_REG, 0x04);
    // sets the pulse for the PI manager
    IO_WRITE(PI_BSD_DOM1_PWD_REG, 0x0C);
    
    // the specific value of 0xAA55 needs to be written here to enable USB IO
    bi_reg_wr(REG_KEY, 0xAA55);
    bi_reg_wr(REG_SYS_CFG, 0);
    // this flushes serial buffer of any pending data
    bi_usb_init();

    // sets the save type. Not needed for USB IO
    bi_set_save_type(SAVE_OFF);
}