On 21/04/17 02:17, Rodolfo Giometti wrote: > AFAIK the multidrop mode is supported also by Freescale (IMX6 and maybe > IMX7) even if by using directly 9 bits transmission. The SENDA trick > (that is autogically sending the next first byte as an address while the > following ones as data) is Atmel specific. Atmel solution is really > smart (even if limited to 1 byte addresses long) because it allows > developers to send a message without any delay between the address and > the data bytes. I've seen similar features on the NXP LPC81x series of microcontrollers. http://www.nxp.com/documents/user_manual/UM10601.pdf page 187 onwards covers the USART for these chips. The data register is natively 32-bits wide, so can accommodate the 9th bit easily. On Atmel AVR, I've also seen it, there the 9th bit is held in a separate register from what I recall, don't have a datasheet in front of me to look it up. The 9-bit mode sounded like a good idea, and I thought of how to possibly implement it, but then I had to contend with where would I put the extra bit in an 8-bit byte? Given its use as an address, maybe one option is if the kernel could have a small table of address bytes/masks that it could watch for on receive and the ability to set a "preamble" to prepend to packets. Sort of like subnets in IP; only the mask can be any arrangement of bits. Supposing we had a userspace program that communicated on RS-485 and used two bytes for addressing; one being the destination address and the other being a command. In that way, the userspace program can tell the kernel: - Preamble length: 2 bytes - MSB denotes preamble byte - Ignore all received packets except those starting with 0x0000 (mask 0xff00) and 0x1200 (mask 0xff00) - For all future frames, send preamble 0x12ff IF the kernel driver hears: [1] 0100 0111 [1] 0001 0110 [0] 0011 0000 … it'll ignore the frame until it sees another byte with an initial MSB, because 0x4716 & 0xff00 ≠ 0x1200. If it hears: [1] 0001 0010 [1] 0001 0110 [0] 0011 0000 … we pass the preamble and any subsequent non-preamble bytes to userspace (minus MSBs). Perhaps we can have an IOCTL call or something that can tell the userspace before a read how many bytes are part of the preamble. In the Atmel example above, the driver can use the SENDA feature if the preamble length is 1 byte. Food for thought? Regards, -- Stuart Longland (aka Redhatter, VK4MSL) I haven't lost my mind... ...it's backed up on a tape somewhere. -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
