FT232H写命令之间的延迟

Question

I want to use an FT232H IC in SPI mode to drive a display. I have set the clock frequency to 5 MHz. In testing my code, I noticed that even in a tight loop, commands are executed about 120 microseconds apart. In the code below, I issue a command to write 4 bytes. I time the action with a stopwatch in VB and also watch the signals on a scope. The code takes about 200 microseconds to execute once and about 320 microseconds to execute twice in a row, 450 microseconds for 3 times, and so forth. The actual sending of the bytes takes only about 7 microseconds each time. Nothing happens for the rest of the time, ie 120 microseconds appears to be wasted with each transmission. Questions: Is this inactive time just recovery of the routines in the FT232H? Am I missing something here? Is there a better command to use? I want to use SPI to clock data into an ILI9341 display driver chip as fast as possible. I know others have done it. Suggestions would be welcome!

 'Start
    'Data transmit, no receive
    SendBuffer(0) = &H10    'Output on rising clock, no input, MSB first, clock a number of bytes out
    SendBuffer(1) = &H3     'Length L
    SendBuffer(2) = &H0     'Length H
    SendBuffer(3) = &HA
    SendBuffer(4) = &HAA
    SendBuffer(5) = &HA
    SendBuffer(6) = &HAA

    'About 1-3 microseconds to this point

    FT_Status = FT_Write_Bytes(FT_Handle, SendBuffer(0), 7, BytesWritten)   ' Write buffer to the device

    '201 microseconds to this point

    'Data transmit, no receive
    SendBuffer(0) = &H10    'Output on rising clock, no input, MSB first, clock a number of bytes out
    SendBuffer(1) = &H3     'Length L
    SendBuffer(2) = &H0     'Length H
    SendBuffer(3) = &HA
    SendBuffer(4) = &HAA
    SendBuffer(5) = &HA
    SendBuffer(6) = &HAA
    FT_Status = FT_Write_Bytes(FT_Handle, SendBuffer(0), 7, BytesWritten)   ' Write buffer to the device

    '321 microseconds to here

    'Data transmit, no receive
    SendBuffer(0) = &H10    'Output on rising clock, no input, MSB first, clock a number of bytes out
    SendBuffer(1) = &H3     'Length L
    SendBuffer(2) = &H0     'Length H
    SendBuffer(3) = &HA
    SendBuffer(4) = &HAA
    SendBuffer(5) = &HA
    SendBuffer(6) = &HAA
    FT_Status = FT_Write_Bytes(FT_Handle, SendBuffer(0), 7, BytesWritten)   ' Write buffer to the device

    '450 microseconds to here

Answer 1

do not have experience with your chip ( FT232H ) but here some possibilities in general:

1. some ICs have separate clock for internal communication

   if set too low then you are waiting until the command is send to the SPI module and not on the SPI transfer itself.

2. Interrupts timing

   If you are using interrupts then either your ISR is called with delay (also interrupt module has it s own clock sometimes) or you are blocked by other process like timer/counters in background or USB/DMA transfer or configuration or by another ISR .

3. Debug interface

   If you are using debugging interface (like JTAG ) you could be stopped by it. In such case try raw application without such interface and measure by oscilloscope just to rule out this.

4. Power management

   To save power some chips have powered down unused modules and before using they have to be started which takes some time. This can be the case even if you are changing configuration of the module.

5. Bug in the chip

   Well with nowadays forced/rushed new chips to market by management there are higher possibilities to leave bugs in the chips (much higher then in the past). So do not rule this out .. I stumbled on this few times already. Usually try to contact chip producer and or check its datasheet for updates/errata and known bug lists.

Answer 2

I learned from the chip factory that the delay is related to USB bulk transfer considerations. I'll work with the timing by buffering as much data as possible with each transfer.