Decompiling 8051 binary, read from EEPROM - binary

Question

I'm trying to decompile the firmware of a Logitech Freedom 2.4 Cordless Joystick. I've managed to get something of the EEPROM. (here)
The EEPROM that is used is the Microchip 25AA320, which is a 32Kbit SPI-EEPROM. The MCU is a nRF24E1G , that contains a 8051 MCU.
The ROM should be 4096 bytes, so I think that my reading program looped over it self 4 times.
I managed to extract a 4kB ROM (here), but the start of the file doesn't look clean.
I loaded both files into IDA Pro and Ghidra and selected the 8051 processor. They don't generate anything useful.
Could anyone help me decompiling this ROM?
I used this Arduino Sketch to dump the rom.
Together with this python script
## Author: Arpan Das
## Date: Fri Jan 11 12:16:59 2019 +0530
## URL: https://github.com/Cyberster/SPI-Based-EEPROM-Reader-Writer
## It listens to serial port and writes contents into a file
## requires pySerial to be installed
import sys
import serial
import time
start = time.time()
MEMORY_SIZE = 4096 # In bytes
serial_port = 'COM5'
baud_rate = 115200 # In arduino, Serial.begin(baud_rate)
write_to_file_path = "dump.rom"
output_file = open(write_to_file_path, "wb")
ser = serial.Serial(serial_port, baud_rate)
print("Press d for dump ROM else CTRL+C to exit.")
ch = sys.stdin.read(1)
if ch == 'd':
ser.write('d')
for i in range(MEMORY_SIZE/32): # i.e. MEMORY_SIZE / 32
# wait until arduino response with 'W' i.e. 1 byte of data write request
while (ser.read() != 'W'): continue
ser.write('G') # sends back write request granted signal
for j in range(32):
byte = ser.read(1);
output_file.write(byte);
print(str(MEMORY_SIZE - (i * 32)) + " bytes remaining.")
print '\nIt took', time.time()-start, ' seconds.'

Answer 1

This is what I did, the next part left is for you. My machine is a Win10 notebook, however I used unix tools because they are so capable.
First of all, I divided the 16KB dump into four 4KB parts. The first one was different from the other three. And the provided 4KB dump is different to all of these parts. I did not investigate this further, and simply took one of the other three parts that are all equal.
$ split -b 4K LogitechFreedom2.4CordlessJoystick.rom part
$ cmp partaa partab
partaa partab differ: byte 1, line 1
$ cmp partab partac
$ cmp partac partad
$ cmp dump.rom partaa
dump.rom partaa differ: byte 9, line 1
$ cmp dump.rom partab
dump.rom partab differ: byte 1, line 1
From the microcontroller's data sheet I learned that the EEPROM contents has a header of at least 3 bytes (chapter 10.2 at page 61).
These bytes are:
0b Version = 00, Reserved = 00, SPEED = 0.5MHz, XO_FREQ = 16MHz
03 Offset to start of user program = 3
0f Number of 256 bytes block = 15
The last entry seems to be off by one, because there seems to be code in the 16th block, too.
Anyway, these bytes look decent, so I cut the first 3 bytes.
$ dd if=partad of=rom.bin bs=1 skip=3
4093+0 records in
4093+0 records out
4093 bytes (4,1 kB, 4,0 KiB) copied, 0,0270132 s, 152 kB/s
$ dd if=partad of=head.bin bs=1 count=3
3+0 records in
3+0 records out
3 bytes copied, 0,0043809 s, 0,7 kB/s
$ od -Ax -t x1 rom.bin > rom.hex
$ od -Ax -t x1 head.bin > head.hex
The hex files are nice for loading them into an editor and look around.
I loaded the remaining 4093 bytes into a disassembler I once wrote and peeked around a bit. It looks promising, so I think you can go on without me now:
C0000: ljmp C0F54
C0003: setb 021H.2
reti
C000B: xch a,r5
inc r6
xrl a,r6
mov a,#0B2H
movc a,#a+pc
movx #r1,a
mov r7,a
setb 021H.2
reti
C0F54: mov psw,#000H
mov sp,#07BH
mov r0,#0FFH
mov #r0,#000H
djnz r0,C0F5C
ljmp C0C09