[sdiy] Digi-Mod schematic

[Seb Francis]

I'm happy to say that I've now finished the schematic for the generic 
digital module on which the 2xD delay/sampler/etc. will be based.  The 
generic digital module is called "Digi-Mod" (powerful imagination to 
come up with that name, huh!)

So now I'm onto the PCB layout stage, but I thought I'd post up the 
schematic.  If anyone would care to look over it for any glaring errors 
it would be most appreciated.

I will be producing a prototype PCB before doing the larger run, but if 
anyone really can't wait you can get in at the prototype stage - in this 
case it would be in your interest to check over the schematic for errors!

http://burnit.co.uk/sdiy/stuff/Digi-Mod.pdf


The specs and options are as follows:


I/O SPECIFICATIONS
==================

Power In
--------
Pin 1: +15V
Pin 2: Analog Ground
Pin 3: -15V
Pin 4: Digital Ground
Pin 5: +5V

+5V supply can also be connected to +15V if +5V is unavailable
In this case a clip-on heatsink will be necessary on U4
Note the current demand this will then place on the +15V supply

Current requirements (theoretical, not measured):
+15V 80mA
-15V 40mA
+5V 270mA



Audio I/O
---------
Pin 1: Input 1
Pin 2: Feedback Loop Return 1
Pin 3: Feedback CV
Pin 4: Output 1 / Feedback Loop Send 1
Pin 5: Input 2
Pin 6: Feedback Loop Return 2
Pin 7: Feedback CV
Pin 8: Output 2 / Feedback Loop Send 2

Input signal levels nominally 10V ptp (3.536V rms)
Will accept 30V ptp without damage

Feedback CV range is 0V to 6V, approx 20dB/V, with unity gain at 4V
Will accept voltages from -15V to +15V without damage

Total of input + feedback return (after VCA) can be up to 17.3V ptp 
without distortion

Output level is nominally 10V ptp, but will depend on input level
Can be up to 17.3V ptp



Sample Rate Select
------------------
These inputs are not protected and so must not be driven from external 
sources.
Suggested configuration is to use a 2 pole, 4 position rotary switch 
wired as follows:

COM1  -> Pin 1 (MDIV1)
COM2  -> Pin 2 (MDIV2)
SW-A1 -> Pin 3 (+3.3V)
SW-A2 -> Pin 3 (+3.3V)
SW-B1 -> Pin 4 (DGND)
SW-B2 -> Pin 3 (+3.3V)
SW-C1 -> Pin 3 (+3.3V)
SW-C2 -> Pin 4 (DGND)
SW-D1 -> Pin 4 (DGND)
SW-D2 -> Pin 4 (DGND)

This will give:
Position A -> Sample rate / 4   (11.71875 kHz)
Position B -> Sample rate / 2   (23.4375 kHz)
Position C -> Sample rate / 1.5 (31.25 kHz)
Position D -> Full sample rate  (46.875 kHz)

Using a different crystal oscillator X1 will give different sample rates.
For example a 11.2896 MHz oscillator will give exactly 44.1kHz at full 
sample rate.
All sample rates must be within the range 4-54kHz that the CODEC supports.



CV In
-----
The full control range is from -5V to +5V
Will accept voltages from -15V to +15V



Gate In
-------
Off = <0.66V
On = >2.64V
Will accept voltages from -15V to +15V



MIDI In/Thru/Out
----------------
5mA current loop, with isolated input
MIDI Thru and MIDI Out will not work with devices that draw power from 
the MIDI bus itself.  Any devices that follow the MIDI specification 
will work fine.



Direct I/O
----------
Direct access to 10 pins on the dsPIC
Can be used for general I/O, SPI or I2C
SPI1 = Pins 1,2,3 (not available if internal EEPROM option installed)
SPI2 = Pins 4,5,6
I2C1 = 7,8
I2C2 = 9,10

Electrical specs -

INPUT:
Low = <0.66V  (Abs Min = 0V)
High = >2.64V (Abs Max = 5.5V)

OUTPUT:
Low = <0.4V
High = >2.4V

The max current in or out of each pin is 4mA

While these pins are unbuffered and unprotected they are fine for inputs 
for panel switches and buttons.
+3.3V and DGND voltages are provided on the PCB header.



ICSP/ICD
--------
Pin 1: !MCLR
Pin 2: VDD
Pin 3: VSS
Pin 4: PGD
Pin 5: PGC



OPTIONAL COMPONENTS
===================

Audio I/O:
The VCA circuitry for the feedback return is optional and requires the 
following components:
U10, U15, R87, R88, R89, R90, R91, R98, R99, R100, R101, R102, C49, C57, 
D12, D13, D15, D16
The VCA circuitry is needed when both voltage control of the feedback 
and external access to the feedback loop is required (for example to 
insert processing of the feedback signal).
If voltage control of feedback is not required then feedback control can 
be provided with a potential divider, with resistors R114 and R115 being 
included to sum the feedback return with the input.  If the VCA 
circuitry is included then resistors R114 and R115 should be omitted.
If external access to the feedback loop is not required then feedback 
can be implemented in software with voltage control as required via the 
CV inputs.

CV inputs:
If any CV inputs are unused then the associated buffer components can be 
omitted.

Gate inputs:
If any gate inputs are unused then the associated protection resistor 
and diodes may be omitted.
Any pull-up resistors should however be left in place to avoid floating 
inputs on the dsPIC inputs that do not have built in pull-ups.

Optional EEPROM to save samples, etc:
Components: U13, C44
Uses Direct I/O pins 1,2,3

MIDI In/Thru/Out:
Components: U11, U12, R57, R58, R59, R61, R62, R63, R64
If MIDI Thru not required, omit R57, R58
If MIDI Out not required, omit U12, R61, R62