the speed of USB8451 is slower than clockrate in labview

[benfg]

I have use a usb8451 spi interface to transfer data from hardware to pc. the clockrate is set as 4Mhz. but when i use "ni845xSpiWriteRead" to transfer data (16bit * 16K ). the total time cost (in labivew) is about 0.78s. instead of 128ms (basd on the clockrate).

is there any speed ratio between clock rate and actual speed? or it is my stupid mistake

[Raj_V]

Hi benfg,

Are you seeing this behavior with 8-bit data instead of 16-bit as well? I came across the FAQ section of the USB 8451 which talks about delays that occur due to hardware limitations. It can be found in the link below?

http://zone.ni.com/devzone/cda/tut/p/id/7448

Is your situation similar to that described?

[benfg]

hi Raj

thanks for your response. actually i am wondering what is the difference between one byte and two bytes.

could i just implement "NI845X read write.vi" in labview with one tyte format to cancel t3?

it looks for my external hardware but i am not sure with labview

[Raj_V]

benfg,

I'm a little confused by your question. In terms of the difference between one and two byte transmissions, the developer zone article here shows how SPI communication works by transmitting 8,16, or 32 bits at a time. I'm not sure what you mean by cancel t3 by using the "NI 845x Read Write.vi". Can you clarify exactly what you are trying to accomplish?