Telechips TCC76x USB boot

In my previous post I opened up my RCA RC3000A and enabled USB boot on the Telechips TCC760 SoC. I'm now able to run code on it. The USB boot mode is a bit different than on later Telechips devices which are supported by the Rockbox tcctool utility. The last parameter is not the SDCFG register value, but the start address. Here is a bit of information about the USB boot mode.

All communication is in 64 byte packets. The first packet contains parameters for the USB loader routine. It must be 64 bytes, but all that matters is the first four 32-bit words:

1. Must be 0xF0000000. If it does not match, the loader will try to use the next packet as the parameter packet, and so on, repeating. 
2. The number of data packets, or in other words, total data size divided by 64. The loader will receive this number of 64 byte data packets after the parameter packet, and store them sequentially in memory.
3. The destination address. The loader will store the first data packet starting at this address and store other data packets after it. The loader has no capability for performing the special actions needed to write to flash, so this must be RAM.
4. The start address. Once the loader has received the specified number of data packets, it will jump to this address.

Note that since SDCFG is not configured, the SDRAM cannot be used and the upload should be to SRAM. The SRAM is 64 kilobytes from 0x30000000 to 0x3000FFFF, and it's also mapped to appear at 0x00000000 to 0x0000FFFF. The boot ROM is 4 kilobytes and copied to the start of SRAM, and it must not be overwritten while it is running there. You can start code at 0x00001000 or 0x30001000. The following tcctool entry can be used:

{"rc3000a",  "RCA RC3000A",                     0xb001, 0x00001000, 0x00001000 },