Need to flip order of bits MSB to LSB re: MCP23S17

[Ryan7777]

Hello all!

I am using a couple of MCP23S17's with a PIC18F2320 in order to communicate with a circuit that converts a 24 bit parallel output from the MCP23S17's to a fiber optic signal.

The fiber signal requires timing that the PIC just cannot do, unfortunately, so a bunch of standard logic does the fast stuff, and the PIC handles feeding values in parallel via the port expanders (MCP23S17's)

I'm using some code that I borrowed from here: http://www.picbasic.co.uk/forum/content.php?r=471-MCP23S17-Three-different-approaches-ShiftIn-ShiftOut-MSSP-PBP-MSSP-ASM

The first variation to be exact. I've modified it to work with my circuit, as it includes two MCP23S17's, one of them only using 8 bits and the other using all 16 bits

The MCP23S17 that handles 8 bit's of the full 24 bit packet is easy to deal with because the data is pretty much static and each of the 8 bits is handled as an individual, so no problem there.

The problem is that the other 16 bits need to be handled as a singular WORD for a 16 bit numerical value. It's nice and easy doing math on an integer, etc.

The way the code example works is that it sends everything to the MCP23S17 MSB first. My value has to be transmitted LSB first!

So to the code %1000000000000000 looks like "32768" but if I send that, my circuit thinks it's "1"!!!

So how do I go about reversing the bit order just before I send it on its way to the MCP23S17?

I already have the PCB made, so I can't simply change my bit order by wiring or I would just do that. Trust me, I already have a "blue wire" or two "fixing" a bug or two, one you'll notice with PORTA.4, so I just shared a line with the other chip select, since the MCP's are hardware addressed anyway.

The following is test code, far from complete and fraught with other issues to work through.


' --------------------CHIP FUSES-------------------------------

@ __CONFIG _CONFIG1H, _HS_OSC_1H & _FSCM_OFF_1H & _IESO_OFF_1H
@ __CONFIG _CONFIG2L, _PWRT_OFF_2L & _BOR_OFF_2L
@ __CONFIG _CONFIG2H, _WDT_OFF_2H & _WDTPS_1_2H
@ __CONFIG _CONFIG4L, _STVR_OFF_4L & _LVP_OFF_4L & _DEBUG_OFF_4L
@ __CONFIG _CONFIG3H, _CCP2MX_OFF_3H & _PBAD_DIG_3H & _MCLRE_ON_3H

DEFINE OSC 40
INTCON = 0
ADCON1.0 = 1 ' Turn off analog inputs
ADCON1.1 = 1
ADCON1.2 = 1
ADCON1.3 = 1
ADCON0.0 = 0 ' Turn off analog converter
CMCON.0 = 1
CMCON.1 = 1
CMCON.2 = 1








' --------------------Ports---------------------------------------
'76543210 '76543210
TRISA = %00000000: PORTA = %00000000
TRISB = %00000011: PORTB = %00000000
TRISC = %00000000: PORTC = %00000000

' ALIAS & MODIFIERS
' ================================================== ==============
nCSA VAR PORTA.3
nCSB var PORTA.4 '- BAD - OPEN COLLECTOR! DO NOT USE
nRST VAR PORTA.2
SCLK VAR PORTA.1
SDO VAR PORTA.0
xmit Var PORTA.5
UPbtn VAR PORTB.0
DWNbtn VAR PORTB.1

' VARIABLES & COSTANTS
' ================================================== ==============
MCP23A CON $42 'Register address to MCP23S17 Config
MCP23B CON $40 'Register address to MCP23S17 PortA(1)
IOCON CON $0A 'Register address to MCP23S17 Config
IODIRA CON $00 'Register address to MCP23S17 PortA(1)
IODIRB CON $01 'Register address to MCP23S17 PortB(2)
GPIOA CON $12 'Register address to MCP23S17 GPIOA(1)
GPIOB CON $13 'Register address to MCP23S17 GPIOB(2)
OLATA CON $14 'Register address to MCP23S17 LATA(1)
OLATB CON $15 'Register address to MCP23S17 LATB(2)


DataOut var BYTE
Voltage VAR WORD 'Variable for manipulating Voltage as full 16 bit word
VoltageFst VAR Voltage.BYTE0 'Variable for Voltage Byte 0 to Send to MCP23S17A-A
VoltageSec VAR Voltage.BYTE1 'Variable for Voltage Byte 1 to Send to MCP23S17A-B
Param Var BYTE 'Variable for Parameters Byte to Send to MCP23S17B-A
RD VAR Param.0 'Varible to set read bit on IP Board - 0 = Read
WR VAR Param.1 'Varible to set write bit on IP Board - 1 = Write
O4 VAR Param.2 'Varible unused 4
RY VAR Param.3 'Varible to set Relay bit on IP Board - 1 = Relay Pulled
O3 VAR Param.4 'Varible unused 3
O2 VAR Param.5 'Varible unused 2
CH1 VAR Param.6 'Varible bit 0 of ADC channel select
CH0 VAR Param.7 'Varible bit 1 of ADC channel select
AuxPort Var BYTE 'Variable for Aux Port Byte to Send to the MCP23S17B-B
UPbtnvar Var BYTE 'Variable for UP Button
DWNbtnvar VAR BYTE 'Variable for DOWN Button
MCPReg VAR BYTE 'Variable for the Register to Send to the MCP23S17
MCPWRT var BYTE 'Command o write in MCP23S17 Address $00
MCPRED var BYTE 'Command o read from MCP23S17 Address $00
' INIT
' ================================================== ==============
UPbtnvar = 0
DWNbtnvar = 0

ncsa = 1
'ncsb = 1
nRST = 0 'Reset the port expanders
PAUSE 1
nRST = 1

MCPReg = IOCON 'Configures MCP23S17-A
DataOut = $28
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,MCPReg,DataOut]
nCSA = 1 'Disable MCP23S17-A

MCPReg = IODIRA 'Configures PortA(1) All Output
DataOut = $00
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,MCPReg,DataOut]
nCSA = 1 'Disable MCP23S17-A

MCPReg = IODIRB 'Configures PortB(2) All Output
DataOut = $00
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,MCPReg,DataOut]
nCSA = 1 'Disable MCP23S17-A

MCPReg = IOCON 'Configures the MCP23S17
DataOut = $28
nCSA = 0 'Enable MCP23S17-B
SHIFTOUT SDO, SCLK, 1, [MCP23B,MCPReg,DataOut]
nCSA = 1 'Disable the MCP23S17

MCPReg = IODIRA 'Configures PortA(1) All Output
DataOut = $00
nCSA = 0 'Enable MCP23S17-B
SHIFTOUT SDO, SCLK, 1, [MCP23B,MCPReg,DataOut]
nCSA = 1 'Disable the MCP23S17

MCPReg = IODIRB 'Configures PortB(2) All Output
DataOut = $00
nCSA = 0 'Enable MCP23S17-B
SHIFTOUT SDO, SCLK, 1, [MCP23B,MCPReg,DataOut]
nCSA = 1 'Disable the MCP23S17

AuxPort = %00000000
GOSUB SEND_AuxPort

' _
' |CH|--|RY|-|W|R
Param = %00001010 ' 00 00 1 0 1 0
GOSUB SEND_Param

Voltage = 0
GOSUB SEND_Voltage
' MAIN LOOP
' ================================================== =============

MAIN:

BUTTON DWNbtn,1,255,255,DWNbtnvar,1,BtnDwn

BUTTON UPbtn,1,255,255,UPbtnvar,1,BtnUp

GOSUB Transmit

GOTO MAIN

' SUB - ROTINES
' ================================================== ==============
BtnUp:

IF Voltage >= 65535 THEN
gOTO MAIN
ENDIF
Voltage = (Voltage + 1)
GOSUB SEND_Voltage
'pause 50
GOTO MAIN

BtnDwn:

IF Voltage = 0 THEN
GOTO MAIN
ENDIF
Voltage = (Voltage - 1)
GOSUB SEND_Voltage
'pause 50
GOTO MAIN

Transmit:
xmit = 1
PAUSEUS 1
xmit = 0
pause 50
RETURN

SEND_Voltage:

Voltage = Voltage REV 16

nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA,VoltageFst]
nCSA = 1 'Disable the MCP23S17
pause 1
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATB,VoltageSec]
nCSA = 1 'Disable the MCP23S17
RETURN

SEND_Param:
nCSA = 0 'Enable MCP23S17-B
SHIFTOUT SDO, SCLK, 1, [MCP23B,OLATA,Param]
nCSA = 1 'Disable the MCP23S17
RETURN

SEND_AuxPort:
nCSA = 0 'Enable MCP23S17-B
SHIFTOUT SDO, SCLK, 1, [MCP23B,OLATB,Auxport]
nCSA = 1 'Disable the MCP23S17
RETURN

goto main

END

[Art]

Can’t you reverse the same 16 bits again to look at the correct value?

[Art]

I see.. you don’t know if it’s reversed because you aren’t tracking if it’s been reversed or not.
It would only take one more bit as a flag for if reversed or not, but I would copy to useless variable to send.



useless var word ‘ don’t use for anything other than sending

IF Voltage = 0 THEN
GOTO MAIN
ENDIF
Voltage = (Voltage - 1)

useless = Voltage REV 16
GOSUB SEND_useless

'pause 50
GOTO MAIN

[Ryan7777]

I have an o'scope on the fiber transmitter and say I want the fiber signal to look like this (in ones and zeros, the fiber signal is actually two different widths to represent 1 or 0):

000000000000000101010000 - that is 24 bits, the last eight to the right are OK because they are the 8 from the MCP23S17 that is just static setup data and I can handle those... and they are on the proper side of the string and in the proper order. so no problem with those. BUT....

The first 16 bits are: 0000000000000001 - Decimal "1"

Well, if I use my code to send those 16 bits - the fiber output ends up looking more like this: 100000000000000001010000 - The first 16 bits are opposite of what I need! They equal decimal "32768"

Far from the "1" I was expecting!

It's actually worse than that, because the MCP23S17 is not acting like a 16 bit device, but rather as two 8 bit devices... hence the splitting of "Voltage" into two bytes. But I think I just need to configure the MCP23S17 correctly for 16 bit mode, as soon as I wrap my brain around its datasheet.

But I still need to know how to send just the 16 bit WORD "Voltage" in my code LSB first, without having to do crazy math every time I work with the variable in my MAIN loop

I tried the operator "REV" and it doesn't seem to work as expected.

I think the MCP23S17 must have it's data fed to it MSB first, but my variable has to go LSB first!

I wonder if I could just get away with this? :confused: :


nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA] ' i.e. SHIFTOUT SDO, SCLK, MSB,[MCP23A,OLATA]
SHIFTOUT SDO, SCLK, 0, [VoltageFst] ' i.e. SHIFTOUT SDO, SCLK, LSB,[VoltageFst]
nCSA = 1 'Disable the MCP23S17
pause 1
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATB] ' i.e. SHIFTOUT SDO, SCLK, MSB,[MCP23A,OLATB]
SHIFTOUT SDO, SCLK, 0, [VoltageSec] ' i.e. SHIFTOUT SDO, SCLK, LSB,[VoltageSec]
nCSA = 1 'Disable the MCP23S17
RETURN

[Art]

Using two bytes (PBP alias) I don’t think costs you any time even behind the scenes,
and then just addressing them in the order you want would be the next best thing to swapping the hardware.
I’m not familiar with the device, but if REV didn’t work as you expected, it sound like swapping bytes was the solution?

If that’s wrong, and it’s back to reversing bits, there’s no math to do it,
it’s always whacky procedural code emulating what a Human would do.
You can use the above method first (almost free), and then logical operators,
but it’s essentially the same goal as moving a ball around a table with paper cups.

[Art]

I’ve got to do it this weekend, and don’t get to use REV, so this is what I came up with (untested).
There’s another thread about REV somewhere.. it was decompiled to find out instruction time,
and I don’t know right now If I’m beating it.



sixteenbit var word
revsixteen var word

leasteight var sixteenbit.byte0
mosteight var sixteenbit.byte1
leastrev var revsixteen.byte0
mostrev var revsixteen.byte1

count var byte


reversesixteen:

for count = 0 to 7

leastrev.bit7 = mosteight.bit0
mosteight << 1
leastrev >> 1

mostrev.bit7 = leasteight.bit0
leasteight << 1
mostrev >> 1

next count

return

[Tabsoft]

I tried the operator "REV" and it doesn't seem to work as expected.

The following snippit displays what I would expect.



myWord var word
myTest var word


Low PORTE.2 ' LCD R/W low = write
Pause 10 ' Wait for LCD to startup

main:
lcdout $fe, 1
pause 1000
myWord = $1
myTest = myWord rev 16

lcdout $fe, $80, ihex4 myWord, ":",ihex4 myTest," ","W:T" 'Store reversed value in separate var

myWord = $8000
myTest = myWord rev 16

lcdout $fe, $c0, ihex4 myWord,":",ihex4 myTest," ","W:T" 'Store reversed value in separate var

pause 1000
lcdout $fe, 1

myWord = $1
lcdout $fe, $80, ihex4 myWord, ":"
myWord = myWord rev 16
lcdout $fe, $80+6, ihex4 myWord, " ", "W:W" 'Store reversed value in same var

pause 1000
myWord = $8000
lcdout $fe, $c0, ihex4 myWord, ":"
myWord = myWord rev 16
lcdout $fe, $c0+6, ihex4 myWord, " ", "W:W" 'Store reversed value in same var

pause 1000
goto main


Display
7843
7844

Also, I think you are right in that you should be able to use the " SHIFTOUT SDO, SCLK, 0, [VoltageFst]"

[tumbleweed]

Here's a few tips

First off, don't REV anything. That's only going to confuse the issue.

The MCP23S17 expects the bytes to be sent MSB first, that is the first bit clocked out appears at GPx7 and the last bit appears at GPx0.
That's what you should get if you specify MSBFIRST in the SHIFTOUT instruction

SHIFTOUT SDO, SCLK, 1, [databyte]

If you need to reverse the bits at the output port, then specify LSBFIRST in the SHIFTOUT instruction

SHIFTOUT SDO, SCLK, 0, [databyte]


Now, on to the ordering of bytes. When you have a word declaration like

Voltage VAR WORD
VoltageFst VAR Voltage.BYTE0 'this is the lower byte of the word
VoltageSec VAR Voltage.BYTE1 'this is the upper byte of the word

Voltage = $8001That would give you VoltageFst = $01, and VoltageSec = $80


So, if you used your send routine with those values


nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA] ' i.e. SHIFTOUT SDO, SCLK, MSB,[MCP23A,OLATA]
SHIFTOUT SDO, SCLK, 0, [VoltageFst] ' i.e. SHIFTOUT SDO, SCLK, LSB,[VoltageFst]
nCSA = 1 'Disable the MCP23S17
pause 1
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATB] ' i.e. SHIFTOUT SDO, SCLK, MSB,[MCP23A,OLATB]
SHIFTOUT SDO, SCLK, 0, [VoltageSec] ' i.e. SHIFTOUT SDO, SCLK, LSB,[VoltageSec]
nCSA = 1 'Disable the MCP23S17
RETURN


You end up with:
OLATA7:OLATA0 = $80 ' VoltageFst is $01, but was sent out LSB first so it swapped order
OLATB7:OLATB0 = $01 ' VoltageSec is $80, but was sent out LSB first so it swapped order

If that's not what you need then you'll either have to send VoltageFst to OLATB/VoltageSec to OLATA to swap the bytes,
and/or use MSB in the SHIFTOUT instruction, but that depends on exactly how the output ports are connected to the fiber interface.

In your case I wouldn't bother with trying to using 16-bit mode with the MCP23S17, at least not right now. That'll change all of the
register mapping addresses and likely confuse things even more. 16-bit mode can be faster, but since you're using SHIFTOUT instead
of the hardware MSSP it's already slow.

[Ryan7777]

Awesome stuff guys!

Yes, when I got in and tried the thought that I had of just clocking in the set up register stuff first:


nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA] ' i.e. SHIFTOUT SDO, SCLK, MSB,[MCP23A,OLATA]
SHIFTOUT SDO, SCLK, 0, [VoltageFst] ' i.e. SHIFTOUT SDO, SCLK, LSB,[VoltageFst]
nCSA = 1 'Disable the MCP23S17

It worked just like Tabsoft and Tumbleweed both surmised (or tested!) and yes, the byte order was wrong as well, so I just swapped the two alias bytes:


nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATA]
SHIFTOUT SDO, SCLK, 0, [VoltageSec]
nCSA = 1 'Disable the MCP23S17
pause 1
nCSA = 0 'Enable MCP23S17-A
SHIFTOUT SDO, SCLK, 1, [MCP23A,OLATB]
SHIFTOUT SDO, SCLK, 0, [VoltageFst]
nCSA = 1 'Disable the MCP23S17

And that gave me the full picture!

So yay for my tired little brain at least showing some spark of life there with my hunch! Sometimes it's just good to put stuff up here to get the right thought patterns going.

And thank all of you for helping to fill in the blanks! You guys rock!

Now to get to tackling all the other stuff this thing needs to do.

Like I posted above, the PIC is handling a setup byte and then a 16 bit value that ends up setting a 16 bit DAC at the other end of a fiber optic link. The card my circuit is talking to is an OEM ADC/DAC card that uses two pulse widths to represent a one or zero. The widths are tight and the entire pulse train is pretty narrow, so the PIC couldn't begin to help with that. So its a mess of shift registers and one-shot chips that do the critical timing, the PIC presents the full 24 bits to a string of parallel shift registers via the MCP23S17's and sends an enable pulse to transmit when everything is set.

I'm working on another card that does the RX side by turning the pulse widths back into parallel data, and then I'll read them with a MCP23S17 again. Just 16 bits on the return.

I'll likely end up redoing the board for the send side since it's got some "blue wires" going on, and the fact it requires some trickery in code to map everything just right. But at least I can do some more debugging with it now.

My major mistake was the fact that I breadboarded the digital logic circuitry to figure that all out and never bothered to do the PIC stuff on there as well!!!

I've learned my lesson!

Thanks again guys! You are a wealth of knowledge and way more help than anyone could hope for! :D