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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.

0 0 0 0 M3 M2 M1 M0
bit 7 bit 0

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.

MODE Reg. mov !RA,W mov !RB,W mov !RC,W mov !RD,W mov !RE,W
00h Read T1CPL Read T2CPL
01h Read T1CPH Read T2CPH
02h Read T1R2CML Read T2R2CML
03h Read T1R2CMH Read T2R2CMH
04h Read T1R1CML Read T2R1CML
05h Read T1R1CMH Read T2R1CMH
06h Read T1CNTB Read T2CNTB
07h Read T1CNTA Read T2CNTA
08h Exchange CMP_B
09h Exchange WKPND_B
0Ah Write WKED_B
0Bh Write WKEN_B
0Ch Read ST_B Read ST_C Read ST_D Read ST_E
0Dh Read LVL_A Read LVL_B Read LVL_C Read LVL_D Read LVL_E
0Eh Read PLP_A Read PLP_B Read PLP_C Read PLP_D Read PLP_E
0Fh Read RA Direction Read RB Direction Read RC Direction Read RD Direction Read RE Direction
10h Clear Timer T1 Clear Timer T2
12h Write T1R2CML Write T2R2CML
13h Write T1R2CMH Write T2R2CMH
14h Write T1R1CML Write T2R1CML
15h Write T1R1CMH Write T2R1CMH
16h Write T1CNTB Write T2CNTB
17h Write T1CNTA Write T2CNTA
18h Exchange CMP_B
19h Exchange WKPND_B
1Ah Write WKED_B
1Bh Write WKEN_B
1Ch Write ST_B Write ST_C Write ST_D Write ST_E
1Dh Write LVL_A Write LVL_B Write LVL_C Write LVL_D Write LVL_E
1Eh Write PLP_A Write PLP_B Write PLP_C Write PLP_D Write PLP_E
1Fh Write RA Direction Write RB Direction Write RC Direction Write RD Direction Write RE Direction


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.


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
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'

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

	port ra, out, %10101010


Unused Pins. +

weak pullup resister +

Syncronous Input Enable

Input / Output Port Expansion

file: /Techref/scenix/sxports.htm, 12KB, , updated: 2007/9/20 15:07, local time: 2024/7/18 04:46,

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