To connect a debugger to an 8-bit part (C8051 or EFM8) on a custom board, you will need either an EFM8 STK or a USB Debug Adapter (UDA). You will need to connect the signals C2CK, C2D, and ground to the target board. Note that the debugger does not provide power for the target board. A schematic of the EFM8 STK can be found in Simplicity Studio. Here is a schematic and pinout of the UDA.
When designing the target board, you should place bypass capacitors (0.1 µF and 4.7 µF) between VDD and ground. C2CK should have a pull up resistor (no more than 10kOhm, 1kOhm recommended) to VDD. Do not connect any capacitors to C2CK.
For layout, the bypass capacitors should be close to the part, the C2CK and C2D lines shouldn't be routed too close to high frequency signals (such as UART or SPI), and they should go through as few vias as possible. It is also a good idea to have a ground plane on the other side of the part and these signals.
What is the endurance of the EEPROM emulation using AN568?
The endurance will depend on the sector size and flash page size. If there are N sectors in the flash page, each page will be erased every 2*N writes. From this, you can expect the endurance to be 2*N more than the flash endurance, which is listed in the device's datasheet. For the BB1 family, minimum endurance is 20,000 cycles and typical is 100,000 cycles.
Does the DTMF generator in AN218 provide "twist" (different amplitude of the two tones)?
The example does not provide twist and outputs the two tones at the same amplitude. Twist can be implemented by creating two sinusoidal look up tables in the PCA0 ISR in place of the one array (sine_table) in the existing example. Then, adjust the amplitude of each table relative to each other to create the twist.