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LP-477 Describe iBus packet format and add packet type validation check

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Also add a compile time protection to make sure we don't compile with bad buffer lengths.
This commit is contained in:
Jan NIJS 2017-01-07 00:38:21 +01:00
parent 07dd0df102
commit 9ed4f644aa
1 changed files with 41 additions and 4 deletions
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45
flight/pios/common/pios_ibus.c
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@ -1,7 +1,7 @@
/**
******************************************************************************
* @file pios_ibus.c
* @author The LibrePilot Project, http://www.librepilot.org Copyright (C) 2016.
* @author The LibrePilot Project, http://www.librepilot.org Copyright (C) 2016-2017.
* dRonin, http://dRonin.org/, Copyright (C) 2016
* @addtogroup PIOS PIOS Core hardware abstraction layer
* @{
@ -33,11 +33,39 @@
#ifdef PIOS_INCLUDE_IBUS
// 1 sync byte, 1 unknown byte, 10x channels (uint16_t), 8 unknown bytes, 2 crc bytes
#define PIOS_IBUS_BUFLEN (1 + 1 + PIOS_IBUS_NUM_INPUTS * 2 + 8 + 2)
#define PIOS_IBUS_SYNCBYTE 0x20
/*
iBus packet format learned from: https://bitbucket.org/daveborthwick/ibus_pic12f1572
An iBus packet consists of:
- the total packet length (1 byte)
- command / type field (1 byte)
- sequence of 16-bit (little endian) variables (2 bytes each)
- checksum (2 bytes)
The most common packet length is 0x20 = 32 bytes
A packet of this type has enough room to transport data for 14 channels.
The command / type field tells you the meaning of the next variables.
This field is set to 0x40 for RC command channels.
We must verify this field to be 0x40 so we can be sure that the next fields
can be interpreted as RC command channels.
The checksum equals 0xFFFF minus the sum of all previous bytes in the packet.
*/
// 1 length byte, 1 type byte, 10x channels (uint16_t), 4 extra channels (unint16_t), 2 crc bytes
// We only decode the first 10 channels at this time.
#define PIOS_IBUS_BUFLEN (1 + 1 + PIOS_IBUS_NUM_INPUTS * 2 + (14 - PIOS_IBUS_NUM_INPUTS) * 2 + 2)
#define PIOS_IBUS_SYNCBYTE 0x20 // sync on the packet length byte
#define PIOS_IBUS_TYPEBYTE 0x40
#define PIOS_IBUS_MAGIC 0x84fd9a39
// make sure to always allocate sufficient bufferspace:
#if (PIOS_IBUS_SYNCBYTE != PIOS_IBUS_BUFLEN)
#error PIOS_IBUS_BUFLEN must be updated to match the expected packet length
#endif
/**
* @brief IBus receiver driver internal state data
*/
@ -182,9 +210,16 @@ static uint16_t PIOS_IBUS_Receive(uint32_t context, uint8_t *buf, uint16_t buf_l
}
for (int i = 0; i < buf_len; i++) {
// byte 0: sync byte (packet length) should match:
if (ibus_dev->buf_pos == 0 && buf[i] != PIOS_IBUS_SYNCBYTE) {
continue;
}
// byte 1: type field should match:
if (ibus_dev->buf_pos == 1 && buf[i] != PIOS_IBUS_TYPEBYTE) {
// not the correct type of data, restart from byte 0
ibus_dev->buf_pos = 0;
continue;
}
ibus_dev->rx_buf[ibus_dev->buf_pos++] = buf[i];
if (ibus_dev->buf_pos <= PIOS_IBUS_BUFLEN - 2) {
@ -238,10 +273,12 @@ static void PIOS_IBUS_Supervisor(uint32_t context)
PIOS_Assert(PIOS_IBUS_Validate(ibus_dev));
// clear rx buffer after 4.8 ms without input
if (++ibus_dev->rx_timer > 3) {
PIOS_IBUS_ResetBuffer(ibus_dev);
}
// mark all channels invalid after 51.2 ms without valid data
if (++ibus_dev->failsafe_timer > 32) {
PIOS_IBUS_SetAllChannels(ibus_dev, PIOS_RCVR_TIMEOUT);
}