Functional description

There are few basic functional blocks inside SMkII which it's user should understand:

• MCU (AT32UC3) - runs the USB Audio class and configuration endpoints and the I2S/Digital audio port
• Adaptive clock source - a PLL circuit which provides CLK to all parts of the audio subsystem. CLK frequency is controlled by the MCU, either set as fixed value configured by the user, or in adaptive mode based on the state of the audio buffers. See chapter Clock operation for details.
• Bi-Directional isolators - galvanically isolate the SMkII from the device under test (DUT) connected to the I2S Connector. For CLK signals (Bit-Clock BCLK, Master-Clock MCLK and Frame-sync FSYNC, the isolators can be set as inputs or outputs or put into high impedance (Hi-Z). Utility signals are also bi-directional or Hi-Z. For TX signal it can be only output or Hi-Z, for RX always input or Hi-Z.
• Audio codec & Headphone output - to allow direct listening of the streamed or recorded signal, and audio codec is included on the board. Codec is Clocked from the internal or external MCLK source. User can switch whether codec is playing back the TX or the RX stream.

Detailed explanation

1. USB Connector - USB Mini B type connector. SMkII is a USB HiSpeed device.
2. Codec TX/RX switch - this switch selects source (TX stream or RX stream) for the Headphone output
3. I2S Reset - the button drives the I2S Reset utility signal. LED indicates direction of the I2S signal (Red-In, Green-Out, Off-HiZ)
4. Error - error LED indicates when SMkII enters and error-state. Error message is usually displayed on the LCD. SMkII doesn't perform any actions when in error state and cannot be re-configured until reset.
5. Mute - the button drives the Mute utility signal. LED indicates direction of the I2S signal (Red-In, Green-Out, Off-HiZ)
6. TX - LED indicates data traffic on the TX channel. TX is always Output or HiZ, therefore LED is always green or off.
7. RX - LED indicates data traffic on the RX channel. RX is always Input or HiZ, therefore LED is always red or off.
8. MCLK Dir - LED indicated direction of the MCLK pin on the I2S connector ((Red-In, Green-Out, Off-HiZ/Disabled). Note that SMkII doesn't need MCLK for it's basic operation, it's only useful when DUT requires it. In I2S Slave mode, MCLK is only necessary to keep the headphone output codec running.
9. BCLK Dir - LED indicated direction of the BCLK pin on the I2S connector (Red-In, Green-Out, Off-HiZ/Disabled)
10. FSYNC Dir - LED indicated direction of the FSYNC pin on the I2S connector (Red-In, Green-Out, Off-HiZ/Disabled)
11. Reset - this button performs manual SMkII reset. It's equivalent to a power re-connect. All configured settings which were not saved into persistent memory (EEPROM) will be lost. Settings stored in the persistent memory are re-loaded.
12. I2S Connector - 14 pin connector providing I2S signals, utility signals and external power.
13. Connector power control - LED indicator and switch selecting internal or external power supply for circuits outside the isolated scope. As a rule of the thumb, the indicator must always be ON for proper operation. Detailed explanation is provided in section Isolated inputs/outputs
14. Volume control - allows adjustment of headphone output volume in range -79 to 0 dB
15. Headphone output - stereo output with headphone driver for quick check of the test signals. RX/TX switch (2.) can be used to select source of this signal.

Status LCD description

1. Logical number of the SMkII unit - this number is configurable via the configuration tool. See detailed description.
2. Firmware version - version of the FW loaded onto SMkII
3. PLL Mode - as explained above, SMkII can operate it's internal clock in two basic modes - Fixed and Auto (Adaptive). This line indicates which mode is used currently.
4. Sampling frequency [Hz] - sampling frequency of the currently played audio stream. This value is equal to the FSYNC frequency of the I2S interface. Sampling frequency cannot be set in configuration settings, it's dependent on the frequency used by the audio editor/playback tool or OS audio path setting.
5. BCLK multiplier - information on the multiplier used to produce BCLK frequency from the sampling (FSYNC) frequency. The multiplier value can be set in the configuration settings.
6. MCLK frequency [MHz] - information on the MCLK frequency used internally by the SMkII. This number can change slightly during audio playback if PLL operates in adaptive mode. This number also changes when sampling frequency is changed in the audio editor. In case SMkII operates as I2S slave and allows MCLK input from DUT, it's better to ignore this value as it DOES NOT REPRESENT the actual frequency of the MCLK signal obtained from the DUT.
7. Headphone volume [dB] - indicator of the headphone volume

I2S Connector description

I2S connector is a standard 14pin 2.54mm pitch connector with following pinout:

Usual ribbon cables can be used to connect SMkII to various DUT. Maximum allowed cable length depends greatly on the DUT's I2S I/O and used BCLK/MCLK frequencies, but generally speaking lengths of 15~20 cm worked for most measurements so far.

For connecting two SMkII against each other, a TX/RX wires must be crossed on the ribbon.

Isolated inputs/outputs

As SMkII input/output signals are galvanically isolated, it's necessary to provide 3.3V (min 2.7V, max 5,5V) power for the isolated part of the circuit. For fully isolated operation, this power needs to be drawn from the DUT.

In case DUT doesn't provide power, it's possible to power the isolated circuit from SMkII internal power source and the GND and 3.3V power line become galvanically connected to DUT. In this case the isolation is no longer achieved and it's possible to experience ground loops or USB-related GND noise, depending on the DUT's own circuitry and power source.