The following diagram represents the circuit we used for our project:
In the above diagram, R1 = 100k, R2 = 900k, R3 = 1000k, R4 = 48Ohms, C1 = .06uF, C2 = 22uF, and C3 = 1uF. The op-amps we used were LMC7111's, and the D/A converted was a DAC0808 (Look at the datasheet for circuit additions according to the DAC's specification). All the op-amps were driven with 5V and -5V, as well as the DAC. To do this we had to use an external power supply, which gave us no trouble at all when done correctly (connecting both grounds and Vdd's to the MCU).
This circuit takes the analog guitar signal, which varies between 0 and 500mV peak to peak centered at zero, amplifies it, converts it to digital for the MCU to process it, converts it back to analog, and attenuates it back to the original range for the speakers (guitar amp) to output it. The first capacitor combined with the voltage divider give the signal an offset of 2.5V, and afterwards, the first op-amp amplifies the signal to cover the range 0-5V. The signal is then converted to digital with the ATmega32's ADC, and is then processed by the MCU with the function chosen by the user through the buttons and the LCD. The signal is then output to the DAC to get an analog signal. Capacitor C2 gets rid of the 2.5V offset, and feeds the signal to the second op-amp. This op-amp was intended to be an inverting adder since initially we wanted to add the guitar to the processed signal for the effects of reverb and echo. Unfortunately, we couldn't get a switch to work so that it passed the guitar in the cases of echo and reverb but passing ground for the other effects (a transistor would not pass the negative voltages and the CMOS switch chip we could get was not working as it should, which could not be explained even by Professor Land). Therefore, at the end, to not change the circuit, we just added the guitar signal in the MCU whenever we wanted and hardwired ground to one of the inputs to our adder. The third op-amp is simply an inerter since the adder is an inverting adder. The signal should be in the original range at this point and is passed through an RC LPF to get rid of higher frequency noises. It is then ready to be fed to the speakers.
Initially, when we wanted to do simply an echo and didn't have enough ports for buttons or the LCD, the circuit would have done most of the project since it would be the one adding the echo to the signal. However, after we changed our mind about the 8515, as our project progressed and we added more effects, the project ended up depending mostly on the software, even though without the appropriate circuit (amplifying and biasing) it would have been impossible to process the signal digitally.