SX Micro Controller IO Ports and Port Setup

Data is read or written to the ports by reading or writing the Rx (RA, RB, RC, etc... No "!" in front) registers. Yes, you can use clrb and setb, but don't! At 75Mhz, Read/Modify/Write problems are no small thing. RMW problems get worse at higher speeds and also get worse at lower voltages.

For example mov RA,w will move the data from the working register to the RA port. However, the ports are multipurpose: They can be input ports, output ports, support TTL or CMOS logic levels and many other options exist. These are selected via the port control registers.

The MODE register serves as an index to all port control registers. The control registers for the ports are denoted as !Rx (!RA, !RB, or !RC, like the actual data ports but with a "!" in front) but writing to !RA may set the input/output configuration or the TTL/CMOS, or the pull-up/hi-z configuration, etc... depending on the setting of the MODE register.

In order to write or read any ports control register, the MODE register must contain the appropriate value to point to that port register. The W register is used to write into the MODE register.

The format of the MODE register is shown below. The four least significant bits serve as the pointer to the set of port registers to be accessed. Upon reset, the MODE register is initialized to 0Fh in the SX18 and 28 and to 1Fh in the SX48 and 52. In this default configuration, the program can write the !RA, !RB, and !RC (data direction) registers.

In order for the program to gain access to the other port control registers (to set the pullup resistor, input voltage levels, Schmitt triggers, etc.), the program must write a different value into the MODE register. The mov m,w command will actually exchange the value of the register with w, allowing the program to know the previous state of the port control register. This is how the MODE register controls access to the port registers in the SX18/28:

DIR (aka TRIS) Mode = xF writing to !RA, !RB or !RC will affect the direction of the port pins. A 1 will set that port pin to an input. A 0 will set it to an output

PLP: Mode = xE writing to !RA, !RB or !RC will affect the use of the weak pullup resister on this port. A 1 will disable it and a 0 will enable it.

LVL: Mode = xD writing to !RA, !RB or !RC will affect the input level for each port pin. A 1 will set the level to TTL and a 0 will set it to CMOS.

ST: Mode = xC writing to !RB or !RC will affect the trigger mode of the port pin. A 1 will disable Schmitt-Trigger input and a 0 will enable it.

WKEN: Mode = xB writing to !RB will affect the Wake Up mode for that pin. A 1 will disable multi-input wake up (MIWU). mov m, #$0B

WKED: Mode = xA writing to !RB will affect the Wake Up Edge Selection for that pin. A 1 will select falling edge detection and a 0 will selecte rising edge detection. mov m, #$0A

WKPND: Mode = x9 !RB Multi Input Wake Up Pending Register. mov m, #$09

CMP: Mode = x8 !RB Comparator Enable Register. Bit 7 is the comparator enable bit. A 1 will disable the comparator and a 0 will enable it if port B pins 1 and 2 are configured as inputs. Pin 1 is used as the negative input and pin 2 as the positive input. Bit 0 of the !RB register is the result of the comparison. Bit 6 is the comparator output enable. A 1 will disable output of the comparitor result on port B pin 0 and a 0 will enable it if pin 0 has been configured as an output. Bit 0 of the !RB register is the result of the comparison even if output on pin 0 is not enabled by Bit 6

In the SX48/52 M register indexs the following port control registers.

Note

Since mov w,#$1F mov m,w will resut in MODE being set to 0Fh in the SX 18/28 and 1Fh in the SX 48/52, and mov m,#$1F will assemble as mov m,#$0F on the SXKey52 v 1.07 to 1.09 it should be used in place of  mov m,#$0F in all cases.

Example:

The following example shows common code used to configure the ports:

mov W,#$1F	;don't mov m,#$1F, SXKey52 will assemble mov m,#$0F instead
mov M,W 	;Set up MODE for Data
		;Direction configuration
mov !RA,#$03 	;RA3, RA2 are Outputs,
		;RA1, RA0 are Inputs
mov W,#$1E
mov M,W 	;Set up MODE for Pull-Up
		;configuration
mov !RA,#$01 	;RA3, RA1 Unaffected,
		;Pull-up enabled on RA0
mov W,#$1D
mov M,W 	;Set up MODE for
		;TTL/CMOS configuration
mov !RA,#$02 	;RA3,RA2,RA0 unaffected,
		;RA1 has CMOS voltage level

The following Macro will setup the ports using a simpler command:

in	EQU	$F00
out	EQU	$FFF
pull	EQU	$E00
float	EQU	$EFF
cmos	EQU	$D00
ttl	EQU	$DFF

porthelp MACRO
	ERROR 'USAGE: port r[a,b,c,d,e] [in,out,pull,float,cmos,ttl] bits'
	ENDM

port	MACRO	3
	noexpand
IF \1=RA OR \1=RB OR \1=RC OR \1=RD OR \1=RE
ELSE
	porthelp
ENDIF
IF \2=in OR \2=out OR \2=pull OR \2=float OR \2=cmos OR \2=ttl
ELSE
	porthelp
ENDIF
	expand
	_PortMode = (\2 / $100) | $10
	 mov w, #_PortMode
	 mov m,w
	_Port = \1
	_PortMask = (\2//$100)^\3
	 mov !_Port, #_PortMask

	noexpand
	ENDM
	port ra, out, %10101010

Questions:

• ~NOSPAM~del_wright at hotmail.com asks:

  I am trying to implement a MAC instruction for a digital FIR filter. Although, the SX28AC does not have a MAC instruction, can I still implement code to realize it?

  Yes, in general you can.... but the exact definition of the MAC instruction will be necessary

Unused Pins. +

weak pullup resister +

Syncronous Input Enable

Input / Output Port Expansion

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