Arduino and SP0256A‑AL2

By Kevin on August 3, 2025

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 Arduio.

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