the flash loader. Since the host device is the master, the SSI on the flash loader device does not
need to determine the clock as it is provided directly by the host.
A.3 Packet Handling
All communications, with the exception of the UART auto-baud, are done via defined packets that
are acknowledged (ACK) or not acknowledged (NAK) by the devices. The packets use the same
format for receiving and sending packets, including the method used to acknowledge successful or
unsuccessful reception of a packet.
A.3.1 Packet Format
All packets sent and received from the device use the following byte-packed format.
struct
{
unsigned char ucSize;
unsigned char ucCheckSum;
unsigned char Data[];
};
ucSize The first byte received holds the total size of the transfer including
the size and checksum bytes.
ucChecksum This holds a simple checksum of the bytes in the data buffer only.
The algorithm is Data[0]+Data[1]+…+ Data[ucSize-3].
Data This is the raw data intended for the device, which is formatted in
some form of command interface. There should be ucSize–2
bytes of data provided in this buffer to or from the device.
A.3.2 Sending Packets
The actual bytes of the packet can be sent individually or all at once; the only limitation is that
commands that cause flash memory access should limit the download sizes to prevent losing bytes
during flash programming. This limitation is discussed further in the section that describes the serial
flash loader command, COMMAND_SEND_DATA (see “COMMAND_SEND_DATA
(0x24)” on page 502).
Once the packet has been formatted correctly by the host, it should be sent out over the UART or
SSI interface. Then the host should poll the UART or SSI interface for the first non-zero data returned
from the device. The first non-zero byte will either be an ACK (0xCC) or a NAK (0x33) byte from
the device indicating the packet was received successfully (ACK) or unsuccessfully (NAK). This
does not indicate that the actual contents of the command issued in the data portion of the packet
were valid, just that the packet was received correctly.
A.3.3 Receiving Packets
The flash loader sends a packet of data in the same format that it receives a packet. The flash loader
may transfer leading zero data before the first actual byte of data is sent out. The first non-zero byte
is the size of the packet followed by a checksum byte, and finally followed by the data itself. There
is no break in the data after the first non-zero byte is sent from the flash loader. Once the device
communicating with the flash loader receives all the bytes, it must either ACK or NAK the packet to
indicate that the transmission was successful. The appropriate response after sending a NAK to
the flash loader is to resend the command that failed and request the data again. If needed, the
host may send leading zeros before sending down the ACK/NAK signal to the flash loader, as the
June 22, 2010500
Texas Instruments-Production Data
Serial Flash Loader
