Down in the core of the program is a mechanism for setting the Tx command byte (bTxCmd in the Tx data block) and then waiting for an echo. This consists of a subroutine that is called with the required command byte in the X register, and a semaphore that is set only after the SX10509 has echoed the command. The steps for using this are:

1. Load the required command byte into X
2. Call the subroutine
3. Wait for the “all done” semaphore to get set.

The SX10509 will only respond to a different command to the last one processed. For several commands of the same type in a row, the trick is to toggle (invert) bit 7 of the command byte. The subroutine below takes care of that under the hood.

Here’s the first part of the code. This is the subroutine that gets called in step 2 above, with X set to the command byte.

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SCH_SetCommand:
        Push
        Recall          bTxCmd
        Swap
        Store           bTxCmd
        Compare
        GoIfNZ          SCH_SC1         ; g/ new <> old
        NotS            7,bTxCmd        ; old = new ... toggle bit 7
SCH_SC1:
        ClrS            sSCH_GotEcho    ;Clear the Echo semaphore
        LaunchTask      tsk_SCH_AckWait ;Task that watches out for an echo.
        Return

sSCH_GotEcho:            defSEM          ;Signals completion of a SCH/Xwire command

The above code compares the new command with the previous one, which it retrieves from the Tx data block. It then toggles bit 7 if they are the same. Once that is done is clears the “all done” semaphore sSCH_GotEcho, and launches a task tsk_SCH_AckWait that is going to watch for a match between the sent command bTxCmd and the received echo byte bRxCmdEcho.

Here’s the “wait task” tsk_SCH_AckWait:

tsk_SCH_AckWait:
        YieldTask
        Recall          bTxCmd          ;Get back the command just set
        Recall          bRxCmdEcho      ;read the Rx echo location
        Compare
        GoIfNZ          tsk_SCH_AckWait ; g/ different
        SetS            sSCH_GotEcho    ;Signal that the echo is received
        KillTask                        ;Job done!

This task just sits looping until the echo matches what was sent, then sets the semaphore and kills itself.