Frame format, logical channels and RPC protocol

Three version identifiers

firmware.version identifies the Git/release build; evMuxVersion defines the required session protocol; EVMUX/1 identifies wire framing. Determine compatibility from evMuxVersion/capabilities, then display, record or verify firmware.version. Do not infer protocol from date strings.

Byte layout of a frame

0x1e + ASCII "EVMUX/1 h=<H> p=<P> c=<CRC32>\r\n"
       + H bytes of UTF-8 metadata JSON
       + P bytes of payload
       + 0x1f

Figure 12: H/P define body boundaries; EOF is not a payload delimiter. JPEG, binary file data and text can contain 0x1e, 0x1f and CRLF. Never truncate the body on those bytes.

H/P count UTF-8 or binary bytes. A character such as U+4F60 typically occupies three UTF-8 bytes; JavaScript string length is not payload_bytes.

CRC32 covers only payload, not metadata/header. Use standard CRC32, e.g. host Python zlib.crc32(payload) & 0xffffffff, encoded as eight hexadecimal digits. Device frames include payload CRC; incoming c=00000000 skips verification. CRC detects corruption, not identity or final operation completion.

For JPEG responses and previews, a valid EV-MUX frame alone does not establish a valid image. Firmware validates encoder output before queueing it; Core additionally checks the JPEG structure and dimensions before delivery. Invalid capture responses return INVALID_IMAGE locally and invalid previews are dropped; see Preview, debug capture, backpressure and statistics for the supported image checks and their limits.

Hosts must handle arbitrary fragmentation and coalescing

read #1: [SOF][header first half]
read #2: [header tail][metadata first half]
read #3: [metadata tail][payload first half]
read #4: [payload tail][EOF][whole next frame][next SOF]

Figure 13: A streaming parser retains state across reads and delivers only fully validated frames. Record format/length/CRC/deadline errors and resynchronize in a controlled manner; never pass partial frames into REPL/file writes.

Current device RX limits are 384 metadata bytes and 2048 payload bytes. RX parser deadlines are approximately 3000 ms idle and 10000 ms per complete frame. These limit host commands received by the device, not preview image size or host receive timeouts for large images.

Metadata fields

Common fields
Field	Type	Purpose
`channel`	String	Select channel, e.g. `user.rpc` or `repl.stdout`.
`type`	String	RPC request `req`, response `rsp`, notification `event` or data `data`.
`seq`	uint32	Request/response correlation number; identical numbers across channels/types do not complete each other.
`method`	String	RPC method or stdio source descriptor; `c-stdio` is not an RPC method.
`encoding`	Optional string	Semantic hint such as `json`, `text` or `binary`.
`sessionId`	32-character lowercase hexadecimal string	Current owner authorization/output scope; discovery requests omit tokens.
`vmEpoch`	uint32	VM operation/output generation scope.
`sid`	Optional string	Logical source/domain such as `stdio`; not sessionId.
`ts_ms`	Optional integer	Device timestamp, not automatically synchronized with host time.
`contentType`、`width`、`height`	Channel-defined	Describe image payload; JPEG bytes remain in payload, not JSON.

Metadata and RPC request payload must be complete top-level JSON objects with correctly typed known fields and no duplicates. Unknown extensions may be ignored. Escaped strings require a real JSON parser; trailing garbage is not a valid complete object.

Channels and streams

Wire channels
channel	Direction	stream	Payload / purpose
`user.rpc`	Bidirectional	user	JSON discovery/session/script/device/network control.
`debug.rpc`	Bidirectional	debug	Debug RPC; capture success responds directly with binary JPEG.
`repl.stdin`	Host to device	user	Raw REPL input bytes, authorized by session/epoch with a separate size limit.
`repl.signal`	Host to device	user	Currently accepts only one byte `0x03` for Ctrl-C.
`repl.stdout`	Device to host	user	Text blocks from Python/REPL/HAL and C stdout.
`repl.stderr`	Device to host	user	Explicit stderr paths such as C stderr; Python exceptions may use stdout.
`log.idf`	Device to host	debug	IDF/platform log text.
`preview.frame`	Device to host	user	JPEG data frames produced by `img.flush()`.

Channel-to-stream assignment is fixed. Hosts do not bind separate serial ports for preview/stdout/RPC. The owner holds both streams; RPC replies return to the exact request link.

RPC methods and execution layers

EV-ATP control plane
channel	method	Execution layer	Description
user.rpc	`hello` / `capabilities`	transport	Discovery; hello can explicitly acquire ownership.
user.rpc	`session.keepalive` / `session.release`	transport	Renew lease, query ready/epoch, release owner.
user.rpc	`transport.uart.prepare` / `transport.uart.probe` / `transport.uart.commit`	transport	UART owner baud negotiation; ordinary UART output pauses until commit or rollback.
user.rpc	`file.write`	transport / native VFS	Verified binary upload; session/VM-scoped without interpreter execution.
user.rpc	`script.write` / `script.run`	VM	Legacy Base64 writer / queue script execution.
user.rpc	`device.control`	Primarily transport	Dispatch stop/reset/status, network., storage.writer., launcher status and other actions.
debug.rpc	`transport.state` / `route.get`	transport	Observe state/routes without changing public stream bindings.
debug.rpc	`debug.info`	VM, except stats	device/memory/sensor/fs.list/fs.read; transport.stats does not require VM.
debug.rpc	`debug.capture_frame`	VM	Encode the last independently published application acquisition in this RPC rsp; no sensor initialization or physical acquisition. Requires `debug.capture_frame.observed.v1`.
debug.rpc	`debug.capture_screen`	VM	Encode the last LCD RGB565 frame as JPEG in this RPC rsp, without camera calls.
debug.rpc	`route.bind` / `route.auto`	Rejected	v4 returns `UNSUPPORTED_OPERATION`.

Device Core APIs files.read/remove and python.execute do not imply identically named wire RPCs. Their file/Python composition is described in Interactive Python, scripts and file operations.

debug.capture_screen requests use debug.rpc with the current session and VM epoch; optional quality is 1–100. Successful responses retain request seq/method, set encoding:binary and contentType:image/jpeg, include width / height and carry JPEG payload. An absent LCD returns JSON error NOT_SUPPORTED; no valid displayed frame returns DISPLAY_NOT_READY. Never substitute camera output. Core’s capture({ source: 'screen', format: 'jpeg' }) uses this RPC; see Preview, debug capture, backpressure and statistics.

Requests, responses and asynchronous events can interleave

Figure 14: While waiting for seq=20, the reader must continue handling other complete frames. Match at least type, seq, channel and method, then session/epoch as appropriate. Even an identically numbered event/hello cannot serve as rsp/hello.

Acquisition replies may allocate a new session; discovery replies grant no owner authorization. Successful VM replies must match the requested epoch; control replies may report a later current epoch. Some authorization errors omit session fields: handle their codes without accepting returned values as new authorization.

Reject owner repl.stdout, repl.stderr and preview.frame output from other sessions or old VMs. Never attribute images/control replies to whichever request was most recent.

A verifiable host encoder

This host CPython example explains framing. Use the shared client in Host integration, Wi-Fi configuration and recovery for actual communication.

import json
import zlib

def frame(metadata, payload):
    meta = json.dumps(metadata, ensure_ascii=False,
                      separators=(",", ":")).encode("utf-8")
    data = payload if isinstance(payload, bytes) else payload.encode("utf-8")
    crc = zlib.crc32(data) & 0xffffffff
    header = ("\x1eEVMUX/1 h=%d p=%d c=%08x\r\n" %
              (len(meta), len(data), crc)).encode("ascii")
    return header + meta + data + b"\x1f"

request = frame(
    {"channel": "user.rpc", "type": "req", "method": "hello",
     "seq": 1, "encoding": "json"},
    json.dumps({"evMuxVersion": 4, "acquire": True,
                "clientNonce": "0123456789abcdef0123456789abcdef"}),
)

This function only encodes frames. It implements no session, concurrent reader, deadlines, RPC correlation or recovery, and cannot replace a complete client. Do not assume host zlib/thread/socket combinations are available unchanged in device MicroPython.

Authorized file.write requests are the bounded exception: binary payloads can carry 16384 file bytes plus a 20-byte ID/offset prefix, with CRC32. All other control payloads retain the 2048-byte limit. See Interactive Python, scripts and file operations for begin/data/commit/abort.

Runtime UART baud negotiation

A UART-enabled firmware advertises capabilities.uartBaud with baseline:115200, current, candidates:[2000000,1500000,921600,460800,115200], probeBytes:512, timeoutMs:5000 and switchDelayMs:100. The field is null without a UART backend. These values describe application communication; ROM/stub flashing has its own protocol and lifecycle.

The authenticated UART owner sends the following transport-scoped methods on user.rpc with its current session ID; they remain available during a VM soft reset. USB-OTG, USB-Serial-JTAG and TCP owners cannot change the UART baud rate.

transport.uart.prepare takes {baudRate,token}, where the rate is a listed candidate and token is 1–32 ASCII letters, digits, underscores or hyphens. Its response includes {ok:true,baudRate,token,timeoutMs,switchDelayMs} at the old rate. Firmware pauses ordinary UART output, finishes any already-started frame, drains the hardware FIFO and shift register, then switches after the advertised delay. Host transports reopen/update their serial port and wait at least the advertised delay plus a guard before probing.
transport.uart.probe takes {token,data}, where data is 1–512 printable ASCII bytes. The response returns identical token and data plus baudRate. Hosts compare several different lengths and patterns in both directions; the first usable rate wins in the advertised candidate order. Probes use normal EV-MUX CRC32.
transport.uart.commit takes {token} after at least one probe and returns {ok:true,token,baudRate,...}. Ordinary output resumes after the commit response is queued to hardware. Probe/commit with the same committed token are idempotent, allowing a host to recover a lost commit acknowledgement.

An unfinished negotiation expires after 5000 ms, releases its owner, restores 115200 and clears UART RX/parser state. After a successful negotiation, session release or lease expiry also restores 115200; a lost final commit response may therefore require recovery through the 15000 ms session lease. Hard reset starts at 115200. VM soft reset preserves the physical rate and session lifecycle. Firmware without this capability stays at the initial rate; no host should send speculative baud commands to it.