CEM DT-8852
 | |
| Status | in progress |
|---|---|
| Connectivity | USB |
| Frequency range | 31.5Hz - 8kHz |
| Measurement range (A) | 30dB - 130dB |
| Resolution | 0.1dB |
| Accuracy (94dB@1kHz) | 1.4dB |
| Frequency weighting | A, C |
| Time weighting | F, S |
| Standards | IEC 61672-1 Class 2 |
| Website | cem-instruments.com |
The CEM DT-8852 is an IEC 61672-1 class 2 compliant sound level meter with USB connectivity.
See CEM DT-8852/Info for more details (such as lsusb -vvv output) about the device.
Some random facts:
- The device only starts logging to USB when the SETUP key is pressed on the keypad. This also disables auto power-off mode.
- Consumes 9.5mA, regardless of USB or recording status; 15mA when the backlight is on.
- The battery low warning starts when the battery delivers less than 7V.
- Sound pressure level measurements are sent to the host at a rate of 20Hz.
Hardware
- Holtek HT49R70A microcontroller
- Silicon Labs CP2102 USB-serial interface
Photos
Device, front
Device, back
Device, side
PCB
PCB: keypad and LCD
PCB detail
PCB detail
Microcontroller
CP2102 USB interface chip
Protocol
The device communicates at 9600 baud, no parity, 8 data bits, and 1 stop bit (9600/8n1).
Measurements and settings
Current settings and measurements are continually streamed over the serial bus, without any prompting from the PC. Data is encapsulated in packets of between two and five bytes. The packets are structured as follows:
| 1 | 2 | 3 | 4 | 5 |
| 0xa5 | Token | Data | ||
The number of data bytes depend on the token. These are the tokens:
| Token | Data | Description |
|---|---|---|
| 0x02 | 0 | Time weighting Fast |
| 0x03 | 0 | Time weighting Slow |
| 0x04 | 0 | Max hold mode |
| 0x05 | 0 | Min hold mode |
| 0x06 | 3 | Current time in BCD; first nibble unknown |
| 0x07 | 0 | Current measurement is over measurement range high threshold |
| 0x08 | 0 | Current measurement is under measurement range low threshold |
| 0x0a | 0 | |
| 0x0b | 1 | |
| 0x0c | 0 | |
| 0x0d | 2 | Current measurement as shown on the display, multiplied by 10, in BCD |
| 0x0e | 0 | Live measurements mode (not max/min hold) |
| 0x0f | 0 | Battery is low |
| 0x11 | 0 | |
| 0x19 | 0 | |
| 0x1a | 0 | |
| 0x1b | 1 | Frequency weighting dBA |
| 0x1c | 1 | Frequency weighting dBC |
| 0x1f | 0 | |
| 0x30 | 0 | Measurement range 30-80 dB |
| 0x40 | 0 | Measurement range 30-130 dB (auto) |
| 0x4b | 0 | Measurement range 50-100 dB |
| 0x4c | 0 | Measurement range 80-130 dB |
Recorded measurements
After the host has requested the recorded measurement log, a packet will be inserted into the regular output stream. Instead of 0xa5, this packet begins with 0xbb:
| 1 | 2 | 3 |
| 0xbb | Length | |
The length field is big-endian encoded, with an offset of 100. A length field of 0x0064 thus indicates no stored samples.
This is followed by a series of records:
| Token | Description |
|---|---|
| 0xaa | Measurements in this record use dBA frequency weighting. Metadata follows. |
| 0xcc | Measurements in this record use dBC frequency weighting. Metadata follows. |
| 0xac | Measurements follow, until the next token. Encoding is BCD time 10, as in the live stream. |
| 0xdd | End of recorded measurement dump. |
The metadata following 0xaa and 0xcc denotes the date and time this record was started, and the sampling rate. It is encoded in 7 bytes:
| Byte | Description |
|---|---|
| 0 | Last two digits of year (BCD) |
| 1 | Month (BCD) |
| 2 | Day of month (BCD) |
| 3 | Hour (BCD) |
| 4 | Minutes (BCD) |
| 5 | Seconds (BCD) |
| 6 | Samplerate |
Resources
TODO.