Mooshimeter

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Mooshimeter DMM-BLE-2X01A
Mooshimeter-2X01A.jpg
Status in progress
Source code ble-dmm
Counts 24 bit
IEC 61010-1 CAT III (600V)
Connectivity BLE
Measurements voltage, resistance, diode, continuity, frequency, current, temperature
Features autorange, logging to SD card, current / voltage simultaneously
Website moosh.im

The Mooshimeter is a 24-bit, two channel, CAT III (600V) remote access digital multimeter with Bluetooth Low Energy (BLE) connectivity.

Hardware

Multimeter:

Photos

Multimeter:

Protocol

Though the BLE SoC powering the Mooshimeter appears to be a TI CC2540, the firmware running on it is not the standard "UART to BLE" that is commonly found on them. Instead, it implements the entire meter control and its own custom protocol.

The Mooshimeter protocol is broken down into several layers in a communication stack.

The lowest layer is the BLE GATT stack, which provides two characteristics: one to write packets to the meter and one to receive them from it. The MTU for a packet in either direction is 20 bytes. This is implemented in the GATT abstraction, so we can talk to it via simple write commands and a read callback.

The next layer is the serial stream: each BLE packet in either direction has a 1-byte header of a sequence number. Despite what the documentation says, this is present in both directions (not just meter output) and is NOT reset on the meter output on BLE connection. So the implementation here needs to provide an output sequence number and incoming reassembly for out of order packets (I haven't actually observed this, but supposedly it happens, which is why the sequence number is present). So the structure of packets received looks like:

1 byte 1-19 bytes
SeqNum Serial Data

On top of the serial layer is the "config tree" layer. This is how the meter actually exposes data and configuration. The tree itself is composed of nodes, each with a string name, data type, and a list of children (zero or more). For value containing (non-informational) nodes, they also contain a 7-bit unique identifier.

Access to the config tree is provided by packets on the serial stream, each packet has a 1-byte header, where the uppermost bit (0x80) is set when writing (i.e. never by the meter) and the remaining 7 bits are the node identifier.

The length of the packets varies based on the datatype of the tree node. This means that any lost/dropped packets can make the stream unrecoverable (i.e. there's no defined sync method other that reconnection).

Packets are emitted by the meter in response to a read or write command (write commands simply back the value) and at unsolicited times by the meter (e.g. continuous sampling and periodic battery voltage). A read packet send to the meter looks like:

1 bit 7 bits
0 NodeID

In response to the read, the meter will send:

1 bit 7 bits 1-N bytes
0 NodeID NodeValue

A write packet sent to the meter:

1 bit 7 bits
1 NodeID NodeValue

In response to the write, the meter will send a read response:

1 bit 7 bits 1-N bytes
0 NodeID NodeValue

For the data in the tree, all values are little endian (least significant bytes first). The supported type codes are:

Code Description Wire Format
0 Plain
1 Link
2 Chooser uint8_t
3 U8 uint8_t
4 U16 uint16_t
5 U32 uint32_t
6 S8 int8_t
7 S16 int16_t
8 S32 int32_t
9 String uint16_t length; char value[length]
10 Binary uint16_t length; uint8_t value[length]
11 Float float

Plain and Link nodes are present to provide information and/or choices but do not provide commands codes for direct access (see serialization below). Chooser nodes are written with indices described by their Plain type children (e.g. to select a choice identified by the second child of a chooser, write 1 to the chooser node itself).

On initial connection only three nodes at fixed identifiers are available:

Node ID Type
ADMIN:CRC32 0 U32
ADMIN:TREE 1 Binary
ADMIN:DIAGNOSTIC 2 String

The handshake sequence is to read the contents of ADMIN:TREE, which contains the zlib compressed tree serialization, then write the CRC of the compressed data back to ADMIN:CRC32 (which the meter will echo back). Only after that is done will the meter accept access to the rest of the tree.

After zlib decompression the tree serialization is as follows:

Type Description
uint8_t The node data type code from above
uint8_t Name length
char[length] Node name (e.g. "ADMIN" or "CRC32")
uint8_t Number of children
Node[count] Child serialization (length varies)

Once the tree has been deserialized, each node needs it identifier assigned. This is a depth first tree walk, assigning sequential identifiers first the the current node (if it needs one), then repeating recursively for each of its children. Plain and Link nodes are skipped in assignment but not the walk (so the recursion still happens, but the identifier is not incremented).

So, for example a write to the ADMIN:CRC32 as part of the handshake would be a write by us (the host):

SerSeq NodeID U32 (CRC)
1 byte 1 byte 4 bytes
0x01 0x80 0xDEADBEEF

The meter will respond with a packet like:

SerSeq NodeID U32 (CRC)
1 byte 1 byte 4 bytes
0x42 0x00 0xDEADBEEF

A spontaneous error from the meter (e.g. in response to a bad packet) can be emitted like:

SerSeq NodeID U16 (len) String
1 byte 1 byte 2 bytes len (=8) bytes
0xAB 0x20 0x0008 BAD\x20DATA

The config tree at the time of writing looks like:

   <ROOT> (PLAIN)
       ADMIN (PLAIN)
               CRC32 (U32) = 0
               TREE (BIN) = 1
               DIAGNOSTIC (STR) = 2
       PCB_VERSION (U8) = 3
       NAME (STR) = 4
       TIME_UTC (U32) = 5
       TIME_UTC_MS (U16) = 6
       BAT_V (FLT) = 7
       REBOOT (CHOOSER) = 8
               NORMAL (PLAIN)
               SHIPMODE (PLAIN)
       SAMPLING (PLAIN)
               RATE (CHOOSER) = 9
                       125 (PLAIN)
                       250 (PLAIN)
                       500 (PLAIN)
                       1000 (PLAIN)
                       2000 (PLAIN)
                       4000 (PLAIN)
                       8000 (PLAIN)
               DEPTH (CHOOSER) = 10
                       32 (PLAIN)
                       64 (PLAIN)
                       128 (PLAIN)
                       256 (PLAIN)
               TRIGGER (CHOOSER) = 11
                       OFF (PLAIN)
                       SINGLE (PLAIN)
                       CONTINUOUS (PLAIN)
       LOG (PLAIN)
               ON (U8) = 12
               INTERVAL (U16) = 13
               STATUS (U8) = 14
               POLLDIR (U8) = 15
               INFO (PLAIN)
                       INDEX (U16) = 16
                       END_TIME (U32) = 17
                       N_BYTES (U32) = 18
               STREAM (PLAIN)
                       INDEX (U16) = 19
                       OFFSET (U32) = 20
                       DATA (BIN) = 21
       CH1 (PLAIN)
               MAPPING (CHOOSER) = 22
                       CURRENT (PLAIN)
                               10 (PLAIN)
                       TEMP (PLAIN)
                               350 (PLAIN)
                       SHARED (LINK)
               RANGE_I (U8) = 23
               ANALYSIS (CHOOSER) = 24
                       MEAN (PLAIN)
                       RMS (PLAIN)
                       BUFFER (PLAIN)
               VALUE (FLT) = 25
               OFFSET (FLT) = 26
               BUF (BIN) = 27
               BUF_BPS (U8) = 28
               BUF_LSB2NATIVE (FLT) = 29
       CH2 (PLAIN)
               MAPPING (CHOOSER) = 30
                       VOLTAGE (PLAIN)
                               60 (PLAIN)
                               600 (PLAIN)
                       TEMP (PLAIN)
                               350 (PLAIN)
                       SHARED (LINK)
               RANGE_I (U8) = 31
               ANALYSIS (CHOOSER) = 32
                       MEAN (PLAIN)
                       RMS (PLAIN)
                       BUFFER (PLAIN)
               VALUE (FLT) = 33
               OFFSET (FLT) = 34
               BUF (BIN) = 35
               BUF_BPS (U8) = 36
               BUF_LSB2NATIVE (FLT) = 37
       SHARED (CHOOSER) = 38
               AUX_V (PLAIN)
                       0.1 (PLAIN)
                       0.3 (PLAIN)
                       1.2 (PLAIN)
               RESISTANCE (PLAIN)
                       1000.0 (PLAIN)
                       10000.0 (PLAIN)
                       100000.0 (PLAIN)
                       1000000.0 (PLAIN)
                       10000000.0 (PLAIN)
               DIODE (PLAIN)
                       1.2 (PLAIN)
       REAL_PWR (FLT) = 39

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