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MSP430F6779: EMDC Commands - 1ms Between Bytes??? Can we go faster?

BenH1
Prodigy 150 points
Part Number: MSP430F6779
My team and I are running into a problem.
EMDC only appears to be able to receive command packets at a speed of 1 byte per 1 ms, even at 400 kBaud rate for the UART.
We need to speed this up quite a bit.
Our application of the MSP430F6779, with EMDC, goes something like this:
- We will have a master-controller PC, which will need to communicate quickly with multiple MSP430F6779, all running EMDC (to perform metering calcs).
- We will be sharing a bus with 40-100 MSP430 EMDC devices at the same time, and we would like to query 1 device at a time in order to get a full read-out (via a device ID in our packets).
Unfortunately the MSP430F6779 has a 1-byte UART buffer.
ByteQueue_push() is called from Comm_uartReceiveHandler() which in turn is called indirectly via the receive interrupt code.
Each byte triggers the Pragma Vector Interrupt Service Routine (ISR) in EMDC, which must be handled completely before it will listen to any other bytes.
This is where things appear to get SLOW.
If we send our commands 1-byte at a time, with 1ms between bytes (to allow EMDC some time to catch-up), it responds OK to commands! Hurray!
Except not so fast...
If I reduce the time between command-bytes, it appears to drop bytes, and no-dice. It then ignores commands.
The less time between bytes, the less reliable it gets.
It frequently will not respond, unless I have at least 1ms between bytes in each command statement.
Now here's the problem... Say we have...
11-byte command packet,
100 devices...
1ms between bytes...
=== 1 second!!!
That simply will not work.
We would only be sending command bytes, with no time to receive data back.
Increasing the speed to 400 kBaud doesn't appear to help the issue.
How can we speed up EMDC's UART routine so it can receive bytes faster?
Is it possible to speed up EMDC so the UART ISR can process a received byte in less than 1ms ?
Are there any interrupts in EMDC's calculations that we can disable temporarily perhaps, to give better UART priority?

What's the best way to speed up the UART, so we can send command bytes faster than 1ms per byte?
  • 0
    Prodigy 150 points

    Hi everyone,

    To do some additional debugging, I've acquired a $12 logic analyzer, which is compatible with Saleae software.
    www.amazon.com/.../

    I've attached my recording at the bottom of this post, which you can open via the Saleae logic software:
    https://ideas.saleae.com/f/changelog/

    The below image shows what we are experiencing. I've taken the main routines (EM_foreground and HMI_statemachine), as well as the interrupts, and added code to status pins up at the beginning of the routines, and down after it has completed the routine. I connected 7 channels to monitor what was going on.

    When I try to reduce my command byte 1ms delay, by a factor of 10 (since I'm only at a whopping 1,000 bytes per second, a 10x improvement shouldn't be too bad), I notice the SD24B_VECTOR appears to frequently take 115us to complete the call, during which time, 2 bytes could be sent and one will be dropped.

    How can we work around this, so we can send command bytes to the device more quickly?

    https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/166/2021_2D00_02_2D00_03-MSP430-Dropped-Data.7z

  • 0 in reply to BenH1
    TI__Intellectual 2585 points

    Hi, 

    From your logic analyzer's graph, the UART will be influnced by SD24 interrupt. 

    Solution 1. Please check the SD24ISR, is there any long term calculation inside? Please optimized the calculation and move the calculation to main loop (Out of the interrupt.) 

    Solution 2. Please enable the nesting interrupt in SD24 ISR. 

    Solution 3: You can try DMA with UART. Enable the DMA trigger from UART. The data can be stored in other address and you can process the received byte later and the data could not be lost. 

    Generally, I think it can be solved in many ways. Please try a best method. 

  • 0 in reply to Hawken Li
    Prodigy 150 points

    Thanks Hawken for the ideas,

    I think Solution 2 probably is the quickest to implement.

    If there is an interrupt inside of the SD24 ISR, will that cause incorrect calculations for the metering routines in EMDC, or will this be okay?

  • 0 in reply to BenH1
    TI__Intellectual 2585 points

    Hi, 

    Depends. 

    Please firstly check the UART interrupt cycle time. If there is no multi nesting interrupt, it would be OK. But if the UART interrupt is too long, it would caused a multi nesting, please try solution 1. 

    You need aware of every interrupt cycle time and decide the 'smart' solution. 

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