Arduino and SP0256A-AL2

Having had a bit of a play with the SP0256A-AL2 Speech Synthesis chip, this post looks at how to drive it directly from an Arduino.

• Part 1 – Basic introduction and getting started
• Part 2 – Arduino programmable clock
• Part 3 – Using a Raspberry Pi Pico as a programmable clock
• Part 4 – Using a HC4046 PLL as the clock
• Part 5 – Using an I2C SI5351 programmable clock
• Part 6 – Adding MIDI

Warning! I strongly recommend using old or second hand equipment for your experiments.  I am not responsible for any damage to expensive instruments!

If you are new to Arduino, see the Getting Started pages.

Parts list

• Arduino Uno
• SP0256A-AL2 speech synthesizer chip
• 3.579545 crystal oscillator
• 2x 1KΩ resistors
• 2x 100nF capacitors
• 1x 1uF capacitor
• 1x 3.5mm jack socket
• Breadboard and jumper wires

The Circuit

This is based on several other circuits I’ve found, and reading through the datasheet:

• Arduino + Vintage Speech Chip Instructable: https://www.instructables.com/Arduino-Vintage-Speech-Chip/
• MG005 Speech Synthesizer for RC2014: https://jerryfrost1.wixsite.com/tech/blank-page-4

Here are the required connections for the SP0256A-AL2 to the crystal, output circuit and the Arduino:

GND	1: VSS	OSC 2: 28	GND
RESET	2: /RESET	OSC 1: 27	Crystal OUT
N/C	3: ROM DISABLE	ROM CLOCK: 26	N/A
N/C	4: C1	/SBY RESET: 25	5V
N/C	5: C2	DIG OUT: 24	Audio Filter
N/C	6: C3	VDI: 23	5V
5V	7: VDD	TEST: 22	GND
D9	8: SBY	SER IN: 21	N/C
N/C	9: /LRQ	/ALD: 20	D8
GND	10: A8	SE: 19	5V
GND	11: A7	A1: 18	D2
N/C	12: SER OUT	A2: 17	D3
D7	13: A6	A3: 16	D4
D6	12: A5	A4: 15	D5

The datasheet suggests a PWM filter comprising 33K resistors and 22nF capacitors. This creates a significant amount of filtering but results in a very low output signal. The datasheet pushes the output of the filter into an amplifier as shown below.

But for my messing around, I was happy to compromise a little on the filter in order to have a stronger output signal. So I used two 1K resistors and two 100nF capacitors in my filter stage and then connected it directly to a 3.5mm jack socket.

I’m also using a square, 3.579545 crystal oscillator connected to OSC 2, with OSC 1 connected to GND. The pinout of the oscillator is often listed as if it was the four corner pins of an 8-pin DIP package, so pins 1, 4, 5 and 8, with a “dot” next to pin 1, when oriented with the dot to the top left. The pin usage is as follows:

• Pin 1: squared corner next to the dot: not connected
• Pin 4: GND
• Pin 5: Oscillator output
• Pin 8: VCC

As I’ve fixed A7 and A8 to GND, A1 to A6 can map directly onto D2 to D7 which are the 6 higher bits of PORTD on an ATMega328P. Recall that D0 and D1 are the UART, so I’ll be leaving those free.

I originally had /RESET tied permanently to 5V but I was finding the chip wouldn’t always start properly and sometimes got stuck, so I tied it directly to RESET on the Arduino and it seemed to work a lot more reliably at that point.

The Code

The code is relatively straight forward. I need a list of the allophones, which I provide via a series of #define statements, and a list of the corresponding delays between them, which I provide in an array of 16-bit values, using the allophone number as the index.

I’m using direct PORT IO to access the data/A lines of the SP0256A-AL2. This involves shifting the allophone number left by 2 bits to avoid clashing with the UART on D0/D1, whilst preserving the values in D0/D1.

I’ve written two macros, one to handle a write via PORTD and one to check for the STBY signal via D9 on PORTB.

#define pinALD 8
int pinA[6] = {2,3,4,5,6,7}; // Use direct PORT IO
#define SP_OUT(addr) {PORTD = (PIND & 0x03) | ((addr)<<2);}
#define pinSTBY 9
#define SP_CHK() ((PINB & 0x02)!=0x02)

The algorithm required to set an allophone is determined from the datasheet as follows:

WAIT for STBY signal to go HIGH
Set the data on A1-A6 (A7 and A8 are left at GND)
Toggle the /ALD line LOW for between 0.2 and 1.1mS
Wait for the allophones allotted delay

Once again, a sequence of allophones can be determined using the methods described previous in SP0256A-AL2 Speech Synthesis.

Find it on GitHub here.

Closing Thoughts

It is great to get this working with an Arduino. One thing I’d like to try, is to see if I can clock the chip using the PWM output of the Arduino, in the same way as I did with the AY-3-8910. If so, then I might be able to experiment with varying the clock, which might have some interesting results.

I’d also like to get all this onto a shield PCB at some point too and then I get on to those musical ideas.

Kevin