Virtual com port with JB16

[eckhard]

Hello,

has someone tried to implemant a virtual com ort with a 68hc908jb16 to use it as a rs232 to usb converter ? What has to be done on the windows side to get this working ?

Eckhard

[l456789]

I look at cypress. it have the "Virtual com port" .use low speed usb mcu and HID class. who can move it to hc08?

It's speed:600-56K baud (800 bytes/sec)

http://www.cypress.com/portal/server.pt?space=CommunityPage&control=SetCommunity&CommunityID=209&Pag...

[rocco]

I looked into this for a USB project, but determined that it would not work well.

The reason is that the JB16 does not support full-speed (12 mbps), it only supports low-speed (1.5 mbps). Though this sounds fast enough, the USB spec puts restrictions on low-speed operation:

1) You can only use control messages and interrupt messages. Isochronous messages (what USB-to-Serial adapters usually use) are not available to low-speed devices.

2) A low-speed device cannot issue interrupt messages more often than every 10 millisecond. Also, the payload for those messages cannot be more than eight bytes.

This meant that I could not push more than 800 bytes per second across the USB. That equated to a maximum baud rate of 8 Kbaud, way too slow for my needs.

I ended up using an FDDI chip (a very simple solution) hooked up to a HC08GP32. The FDDI chip is fully supported at the host side, and simply shows up as another COMM port.

[eckhard]

Hello,

maybe thats why i haven´t found anything about that. Hard to believe that the JB16 cannot handle this and the FTDi chip can do it so easy.

Eckhard

[rocco]

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eckhard wrote:

. . . and the FTDi chip can do it so easy.

Eckhard

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Then again, that is all that the FTDI chip does, so it does it well.

Freescale uses FTDI's FT232BM chip on one of their ZigBee boards (DIG528), connected to the MC9S08GT60. They have the schematic on the website (somewhere, Document Number 80000528000_R0203.DSN) if you need a reference design.

Message Edited by rocco on 02-10-200611:30 AM