Javier Valcarce's Homepage

Summary
This VHDL macro is intended for interface (say) PicoBlaze with the 8-bit ADC AD7823 from Analog Devices or compatible ones. Such device is included in Digilent AIO1 peripheral board sold by Digilent

## Chip Description

The AD7823 is an 1-channel 8-bit DAC with a synchronous serial interface. It seems that the most appropriated solution to interface this device is to use a FSM and a shift register. The maximum ADC sample rate is 200 kSPS, enough for audio applications.

## Ports and Usage

The macro has the following ports:

Port DIR Type Description
reset Input signal Asynchronous reset
clk Input signal Clock signal (> 30MHz)
clk10MHz Input signal Slow clk for internal macro use (measure time)
start Input signal Start ADC conversion
eoc Output signal End Of Conversion, it's a "ready" signal that notifies that the conversion has ended and it is possible to order a new conversion
di Input signal Serial data input from AD7823
sclk Output signal Control signal for AD7823 (clock signal)

The circuit waits the start signal to initiate the conversion and puts the acquired 8-bit sample after 5 us (250 system clock cycles @50MHz).

After power up the circuits, wait at least 6 us. Then assert start and wait for eoc to go 1 (only for one clk cycle, use it as a interrupt signal if you want), the acquired 8-bit sample will be present at do port. Repeat as many times as you want.

## Block Diagram And Control State Machine

The figure depicted below shows the RTL block diagram of the circuit. All sequential blocks share the same asynchronous reset and clock signals. The control finite state machine (FSM) has 11 states, there are 7 control signals for datapath.

### Control

State ctlrs ctr_ce sh_ce tmr_ini eoc sclk convst
e00 1 0 0 0 0 0 1
e01 0 0 0 0 0 0 0
e02 1 0 0 0 0 0 0
e03 0 0 0 0 0 0 1
e04 0 0 0 0 0 0 0
e05 1 0 0 0 0 0 1
e06 0 0 0 0 0 1 1
e07 0 1 1 0 0 1 1
e08 1 0 0 0 1 0 1
e09 0 0 0 0 0 0 1
e10 1 0 0 0 1 0 1

The FSM has the following graph, state after reset is e0, the control signals in each state are shown in the table. The signals printed in green are registered because random effects on this signals could cause a malfunctioning of entire system (AD7823 expects inputs free of spurious transitions)

## Synthesis Report

    Device xc3s400-4ft256

slices=22
ff=28
lut=41
ob=16
gclk=2
maxfreq=200.942MHz