Serial port on Pi

I have more than one piece of hardware connected: one is a GPS module which continually spams the Pi’s RXDATA pin with serial data at 9600baud and one is a PaPiRus display which uses the RXDATA and TXDATA pins as GPIO outputs to control the border pixels and to discharge the diode pumps.

[initial condition is serial port receiving data at 9600baud with no problem]

Step 1 is to disable the GPS module and set BCM 14 (TXD) and BCM15 (RXD) to GPIO outputs.

Step 2 is to update the PaPiRus display.

Step 3 resets BCM 14 and 15 to their UART function and reenables the GPS module.

Sometimes though the serial data don’t get received correctly after steps 1 to 3 – I suspect that my step 3 (shown below) is not quite right. Any ideas please?

 6840 REM Need to reenable GPS
 6850 SYS "GPIO_WriteData",22,1:REM Set pin 22 to HIGH
 6860 SYS "GPIO_WriteOE",22,0:REM set pin 22 to output
      REM GPIO 22 is connected to GPS module 'enable'
 6870 REM Need to make sure GPIO 4 is high
 6880 SYS "GPIO_WriteData",4,1:REM Set pin 4 to HIGH
 6890 SYS "GPIO_WriteOE",4,0:REM set pin 4 to output
 6900 REM Need to re-enable serial pins
 6910 SYS "GPIO_WriteMode",14,5:REM UART
 6920 SYS "GPIO_WriteMode",15,5:REM UART
 6930 :
 6940 *fx21,1
 6950 SYS "OS_SerialOp",5,3,0:REM Baud rate to slow
 6960 SYS "OS_SerialOp",1,0:REM Set 8bit, 1 stop bit, no parity
 6970 SYS "OS_SerialOp",0,&3E:REM Set no handshaking
 6980 SYS "OS_Byte",2,2:REM Enable serial input buffer/kbd input
 6990 *FX 21,1
 7000 SYS "OS_SerialOp",5,7,0:REM Baud rate to 9600

Any ideas please?

From someone on the outside looking in, the obvious thing would be to try setting up the serial before gps is sending data. I’m guessing the example way is working for you in your normal gps startup though.

The serial port is all set up and working at start up, the caveat here is that the GPS module starts spamming the RXDATA pin with 9600 baud serial data about 1s after power is applied and it is not until much later (about 11s after power on) that my application checks SerialDeviceSupport is present and does lines 6960 to 7000 as part of its initialisation. Nearly always that works but sometimes it doesn’t (I receive zero and top-bit-set characters over the serial port) and after noticing that I do lines 6950 to 7000 (i.e. change serial port speed down and back again) which flushes the serial port and it then works.

Hence it is not possible to set up the serial port before the gps sends data.

In the case of updating the PaPiRus, once I have finished updating it, I restore the TXDATA and RXDATA pins to UART function. I agree that I should put lines 6850 and 6860 after line 7000, so at least the serial port tinkering is all done before telling the GPS to start again. What I am really asking is would that be sufficient? What is the correct way to resume?

Hence it is not possible to set up the serial port before the gps sends data.

I have a copy of the original Pi direct serial driver by Terry, if you wish to try bypassing the system serial port software to see if there is a bug with the SerialOp method. Being direct, it doesn’t need any support modules.
I dont have your email address.
I copied a line from old Acorn comm software

SYS"Os_SerialOp,"2   :REM otherwise unreliable

Many thanks. That does have an interesting effect.

After the sequence described above (steps 1 to 3) the serial port falls silent, nothing received (despite the GPS module sending data).

Trying my original method to stir it into life is ineffective.

*RMReinit Serial just gives an error ‘serial stream(s) in use’.

Adding SYS “OS_SerialOp”,2,20 at line 7001 causes a complete hang of the machine. ALT-Break does nothing but ALT-Break followed by CTRL-Break says ‘press the return key to stop the program or Press SPACE or click mouse to continue’ but then it is stiffed completely.

Puzzled.

I think I can focus this down a bit. With the PaPiRus display removed but with the GPS module continually spamming the RXDATA pin with 9600 buad the following will duplicate the problem:

Set RXDATA and TXDATA to GPIO outputs.
Restore RXDATA and TXDATA to UART function.
Serial port does not receive data via OS_SerialOp,4.
Change speed and restore 9600 baud.
(still no data)
OS_SerialOp,2,20
machine hangs.

OS_SerialOp,2,20
machine hangs.

Just reading Stronghelp and OS_SerialOP 2 sets the ACIA to transmit a break. (sounds hardware-ish)
It may not be implemented on Pi. I have only used it on Iyonix.
Edit: What is the result of trying to send any character to the GPS, it possibly cant open a write channel, maybe crashing.

Indeed. Why are you sending a break?

What I don’t notice in your list of instructions is the part where you flush the buffers…

Also – the important thing with serial comms is not to make any assumptions about prior state. Flush buffers, set data format, set flow control (probably “none”), etc etc.

Ronald – some GPS devices accept data to configure parameters such as reporting frequency and/or uploading assistance data to help get a quicker fix on location.

the important thing with serial comms is not to make any assumptions about prior state

I have not.

Why are you sending a break?

No idea, but I’ll try any suggestion.

I think *FX 21,1 and a change of serial speed both flush the buffers?

Anyway my work around is not to update the PaPiRus display on start up so the serial port works from start up as before. That is only update it when just about to turn the computer off.