The chip in question is PIC24FJ256GB210
It is a 100-Pin TQFP form factor
We have an embedded system with two microprocessors.
The two microprocessors use a UART to communicate Which is (according to me) mapped to UART #3 on the PIC24.
I place 4 bytes into UART #3. All goes well. The 5th byte will not go in.
I say FIFO backup.
My local hardware expert says that if I turn off flow control that the bytes will go out no matter what.
Is this true ? I've never heard that one before. I thought it was a hardware signal on the other side; i.e., a read signal has to occur on the other side before the FIFO buffer would allow room on this side.
His definition of "turn off flow control" is for me to not use PPS (Peripheral Pin Select) to map either the RTS (Request to Send) or CTS (Clear To Send) pins to their corresponding physical pin on the board.
I did that. Result: no change; the FIFO buffer still fills up. The "#UTXBF" bit never clears after the fourth byte goes in.
I have the schematic diagram with physical pins numbered and labeled.
I have the source code and MpLab showing the executable down at the register level, right at the assembly language instructions themselves.
I am mapping the pins of UART #3 exactly and identically to the manner that I am mapping UART #2 and UART #1, and both of those other two work perfectly.
While the numbers are different, the instruction sequences are identical. The numbers match the pins.
I am debugging this for the third time, watching each bit in each register and comparing them against the manual to make sure that I have the correct corresponding numbers in the correct bit positions in the correct special function registers.
This is from MpLab's disassembler window where the opcodes show exactly which bits are set and cleared.
206CC1 mov.w #0x6cc,0x0002 Mov #Uart_3_Tx_PPS_Output_Register, W1 ;This is the register we want
21C002 mov.w #0x1c00,0x0004 Mov #Uart_3_Tx_Or_In_Bit_Pattern, W2 ;These are the bits we want on
2C0FF3 mov.w #0xc0ff,0x0006 Mov #Uart_3_Tx_And_Off_Bit_Pattern, W3 ;These are the bits we want off
780211 mov.w [0x0002],0x0008 Mov [W1], W4 ;The existing pattern
618204 and.w 0x0006,0x0008,0x0008 And W3, W4, W4 ;Turn existing bits off
710204 ior.w 0x0004,0x0008,0x0008 Ior W2, W4, W4 ;Turn Desired bits on
780884 mov.w 0x0008,[0x0002] Mov W4, [W1] ;And that's all there is to it
After execution, RPOR6
(which is the Uart_3_Tx_PPS_Output_Register
) contains 0x1C06
This is from the inc file that is used to create the masks and patterns. (I try to avoid hard coding numbers in the source files which have the actual instructions.)
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; ;;
;; ;;
;; Map UART # 3 Tx Pin ;;
;; ;;
;; Docs for this are: Manual DS39975A ;;
;; ;;
;; Find "TABLE 2: COMPLETE PIN FUNCTION DESCRIPTIONS FOR 100-PIN DEVICES" ;;
;; in Manual DS39975A, Page 8, where We find the secret PIC Pin Names for ;;
;; the actual physical pin numbers ;;
;; ;;
;; TABLE 10-4: SELECTABLE OUTPUT SOURCES (MAPS FUNCTION TO OUTPUT) ;;
;; Page 160, We find the output function numbers ;;
;; ;;
;; ;;
;; ;;
;; PIC Associated Output ;;
;; Circuit Physical PIN Control Actual Func. ;;
;; Function Pin NAME Reg Bits Number ;;
;; ------------ ------ ----- ------- ---- ----- ;;
;; ;;
;; UART #3, TX Pin 23 RP13 RPOR6 3F00 28 Output ;;
;; ;;
;; ;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
I combined that knowledge with these notes from the family data sheet to create the constants with meaningful names.
.Equiv Uart_3_Tx_PPS_Output_Register, RPOR6 ;Register 10-35, Page 177
.Equiv Uart_3_Tx_Reg_Control_Bits, 0x3F00 ;Look for "RP13R" in the big include file ;;;DEBUG DEBUG Date: 2013-02-05 Time: 20:47:02
.Equiv Uart_3_Tx_Output_Func_Number, 28 ;From Table 10-4, P. 160
.Equiv Uart_3_Tx_And_Off_Bit_Pattern, ~(Uart_3_Tx_Reg_Control_Bits)
.Equiv Uart_3_Tx_Or_In_Bit_Pattern, ( Uart_3_Tx_Output_Func_Number << RP13R0 )
From the File: "p24FJ256GB210.inc" (no quotes)
;----- RPOR6 Bits -----------------------------------------------------
.equiv RP12R0, 0x0000
.equiv RP12R1, 0x0001
.equiv RP12R2, 0x0002
.equiv RP12R3, 0x0003
.equiv RP12R4, 0x0004
.equiv RP12R5, 0x0005
.equiv RP13R0, 0x0008 ;;; <<<<<----- RP13 is in the right place
.equiv RP13R1, 0x0009
.equiv RP13R2, 0x000A
.equiv RP13R3, 0x000B
.equiv RP13R4, 0x000C
.equiv RP13R5, 0x000D
After all is said and done, with or without RTS
or CTS
enabled, the PIC on the other side of the UART apparently never sees the first byte that I put in on this side.
Does anyone see anything where I put the wrong bit in the wrong place ?
At this moment, I cannot confidently answer yes or no to this question: Is the UART #3 TX function correctly connected to physical Pin 23 on a 100-Pin TQFP configured PIC24FJ256GB210 ?
Thanks a ton if you can identify what's going on here.