MCP2200 Effectively Limited to 240000bps
This is more of an observation than anything since this isn't really impairing the product I am working on. While experimenting with my MCP2200 demo board, I had my USBee ZX on there to get a feel for timing accuracy. I am pleased that the MCP2200 bit timing is pretty accurate with the standard baud settings, up to 921600 (versus the Prolific USB-serial adapter it was talking to...|error| of >2.5% at the same rates).
Anyway, while the bit timing is dead-on, the actual data throughput rate seems to hit an actual limit at 230400bps (or 240kbps more likely). The linked USBee shots are probably faster to get the message from. In short, setting the baud rate higher than this reduces bit time, but the "stop bit" gets longer as the rate goes up, so byte timing remains the same. I assume the disguised PIC18 badged as the MCP2200 is dealing with "something" internally during this pause. All shots are with the same timescale and trigger position, with 10 bytes of value 0 being sent from the MCP2200.
The prolific device does not have this pause at any baud rate. It just pumps out data bytes back-to-back.
I also noted that at these higher baud rates (230400bps & above), the MCP2200 seemed to "lose" data bytes. I made a VB app to test comm in both directions. I set the number of bytes to transmit, sent a (bytes to transmit)-long byte array out one COM port and counted how many came in the other COM port.
- When the MCP2200 transmitted a packet, the Prolific device generally picked them all up at 230400bps. (10/10, 100/100, 500/500). At 921600bps, it would catch ~490/500 bytes.
- With the Prolific device as the source though, the MCP2200 would consistently miss some at 230400bps (7/10, 96/100, 478/500). Above this baud rate, it all went downhill. At 921600bps, the MCP2200 would pick up ~70/500.
Anyway, this seems to be due to a mixture of the Prolific device's out-of-spec timing, and at higher data rates, primarily the big inter-byte dead space of the MCP2200. I just wanted to share my $0.02. Basically, I am very happy with the MCP2200 at 115200bps. It works like a charm, and I haven't had to mess with any Win7 hotfixes or anything. I installed the Microchip drivers, and it worked right away.
post edited by bmwman91 - 2011/02/21 00:06:04