SN76489 Sound Generator Chip

Pinout

Warning

NB: Texas Instruments at one time decided to name the most significant bit as bit zero, and increment into the less significant bits. Thus, a byte would be labelled as D0 to D7, instead of D7 to D0 like everyone else. This chip and its data sheet are from that time but this on-line document uses the standard convention to avoid confusion. The figures have been edited too.

VHDL description

This chip has a synthesisable model, written by Arnim Läuger, for his VHDL implementation of the Colecovision. It can be extracted for implementing other machines such as the BBC micro.

Data Sheet

Features

• 3 Programmable Tone Generators
• Programmable White Noise Generator
• Programmable Attenuation
• Simultaneous Sounds
• TTL Compatible
• Up to 4 MHz Clock input*

* Part SN76489N is identical to the SN76489A except that the maximum clock input frequency is 500 kHz. A "divide by eight" stage is deleted from the input circuitry and only 4 clock pulses are required to load the data, compared to 32 pulses from the SN76489AN.

TEXAS INSTRUMENTS RESERVES THE RIGHT TO MAKE CHANGES AT ANY TIME IN ORDER TO IMPROVE DESIGN AND TO SUPPLY THE BEST PRODUCT POSSIBLE.

Description

The SN76489AN digital complex sound generator is an I2L/Bipolar IC designed to provide low-cost tone/noise generation capability in microprocessor systems. The SN76489AN is a data bus based I/O peripheral.

Recommended Operating Conditions

1. TONE GENERATORS

Each tone generator consists of a frequency synthesis section and an attenuation section. The frequency synthesis section requires 10 bits of information (F9-F0) to define half the period of the desired frequency (n). F9 is the most significant bit and F0 is the least significant bit. This information is loaded into a 10 stage tone counter, which is decremented at a N/16 rate where N is the input clock frequency. When the tone counter decrements to zero, a borrow signal is produced. This borrow signal toggles the frequency flip-flop and also reloads the tone counter. Thus, the period of the desired frequency is twice the value of the period register.

The frequency can be calculated by the following:

f = N / 32n

where N = ref clock in Hz
n = 10 bit binary number

The output of the frequency flip-flop feeds into a four stage attenuator. The attenuator values, along with their bit position in the data word, are shown in Table 1. Multiple attenuation control bits may be true simultaneously. Thus, the maximum attenuation is 28 dB.

Table 1: ATTENUATION CONTROL BIT POSITION

2. NOISE GENERATOR

The Noise Generator consists of a noise source and an attenuator. The noise source is a shift register with an exclusive OR feedback network. The feedback network has provisions to protect the shift register from being locked in the zero state.

Table 2: NOISE FEEDBACK CONTROL

Whenever the noise control register is changed, the shift register is cleared. The shift register will shift at one of four rates as determined by the two NF bits. The fixed shift rates are derived from the input clock.

Table 3: NOISE GENERATOR FREQUENCY CONTROL

The output of the noise source is connected to a programmable attenuator as shown in Figure 4.

3. OUTPUT BUFFER/AMPLIFIER

The output buffer is a conventional operational amplifier summing circuit. It sums the three tone generator outputs, and the noise generator output. The output buffer will generate up to 10 mA.

4. CPU to SN76489AN INTERFACE

The microprocessor interfaces with the SN76489AN by means of the 8 data lines and 3 control lines (!WE, !CE and READY). Each tone generator requires 10 bits of information to select the frequency and 4 bits of information to select the attenuation. A frequency update requires a double byte transfer, while an attenuator update requires a single byte transfer.

If no other control registers on the chip are accessed, a tone generator may be rapidly updated by initially sending both bytes of frequency and register data, followed by just the second byte of data for succeeding values. The register address is latched on the chip, so the data will continue going into the same register. This allows the 6 most significant bits to be quickly modified for frequency sweeps.

5. CONTROL REGISTERS

The SN76489AN has 8 internal registers which are used to control the 3 tone generators and the noise source. During all data transfers to the SN76489AN, the first byte contains a three bit field which determines the destination control register. The register address codes are shown in Table 4.

Table 4: REGISTER ADDRESS FIELD

6. DATA FORMATS

The formats required to transfer data are shown below.

7. DATA TRANSFER

The microprocessor selects the SN76489AN by asserting !CE (active low). Unless !CE is asserted, no data transfer can occur. When !CE is asserted, the !WE signal strobes the contents of the data bus to the appropriate control register. The data bus contents must be valid at this time.

The SN76489AN requires approximately 32 clock cycles to load the data into the control register. The open-collector READY output is used to synchronize the microprocessor to this transfer and is pulled low immediately following the leading edge of !CE. It is released to be pulled high (by an external pullup resistor) when the data transfer is complete.

The data transfer timing is shown below.

Figure 1. DATA TRANSFER TIMING

Table 5: FUNCTION TABLE*

* This table is valid when the device is:

1. not being clocked, and
2. is initialized by pulling !WE and !CE high.

PIN ASSIGNMENT

The table below defines the SN76489AN pin assignment and describes the function of each pin.

ELECTRICAL CHARACTERISTICS OVER RECOMMENDED OPERATING CONDITIONS (UNLESS OTHERWISE NOTED)

Figure 2. EXTERNAL AUDIO INPUT INTERFACE

** These capacitance values are determined by the frequency response desired and the audio amplifier used.

SWITCHING CHARACTERISTICS, Vcc = 5V, TA = 25ºC

*!CE Pulse: 0-3V, t_rise <= 7ns, t_fall <= 7ns

Figure3. t_PLL TEST CIRCUIT

BLOCK DIAGRAM

BLOCK DIAGRAM DESCRIPTION

This device consists of three programmable tone generators, a programmable noise generator, a clock scaler, individual generator attenuators and an audio summer output buffer. The SN76489AN has a parallel 8 bit interface through which the microprocessor transfers the data which controls the audio output.