Using a 74HCT259 8 Bit Addressable Latch with BasicX

Transcription

1 Basic Express Application Note Using a 74HCT259 8 Bit Addressable Latch with BasicX Introduction This application note illustrates how to use a 74HCT259 8-bit addressable latch with a BasicX system. The latch allows you to effectively expand the number of output lines available on a processor. The chip also functions as an I/O buffer. To control the chip, you need 3 address lines, 1 data line and 2 control lines, for a total of 6 output lines from the BasicX system. The latch provides 8 output lines. Hardware interface Figure 1 illustrates the pinouts for the device (Radio Shack part number ): Figure 1 74HCT259 pinout

2 Figure 2 shows you how to connect the chip to a BX-24 system. Other BasicX systems use similar connections, except that pin numbers may be different: Figure 2 -- schematic 2

3 Operation The device has 3 active modes and 1 reset mode: Active Modes Demultiplexer Mode Memory Mode Addressable Latch Mode Reset Mode Reset Mode These 4 modes are explained below: Demultiplexer Mode Latch Enable (LE) -- Low Master Reset (MR) -- Low Memory Mode Action -- The addressed latch follows the data input. All other output lines are low. Latch Enable (LE) -- High Master Reset (MR) -- High Addressable Latch Mode Action -- All outputs isolated from data input, all latches hold last data as defined before LE transition from low to high Latch Enable (LE) -- Low Master Reset (MR) -- High Reset Mode Action -- Addressed latch follows data line, all other latches unaffected. Latch Enable (LE) -- High Master Reset (MR) -- Low Action -- All outputs low 3

4 Software Interface The attached demonstration program LatchDemo exercises all modes except the demultiplexer mode. Low level control. Most of the low level work is done by these 4 procedures: Procedure IsolateOutputs ActivatePin ClearAllPins Action Sets MR high, LE high (memory mode), decouples output lines from data and address lines Sets MR high, LE low (addressable latch mode) Sets LE high, MR low (reset mode), then calls IsolateOutputs to decouple the output lines WriteAddress Writes a 3 bit address to lines A0, A1, A2. High level control. Procedure PutPin259 controls individual output lines. This procedure first calls IsolateOutputs in order to disconnect all outputs from the data and address lines. Then WriteAddress is called, which writes a 3 bit address to the address lines A0, A1 and A2. Next, procedure PutPin is called to write to the data line. ActivatePin is then called, which puts the latch into addressable latch mode and causes the desired output pin to follow the data line. The last step is to call IsolateOutputs, which decouples output lines again. Reading pin states. Although the latch chip is write-only, the software driver includes code that allows you to effectively read the state of each output line. Variable ShadowState maintains an 8 bit copy of the output states. Function GetPin259 reads ShadowState and returns the state of the specified output line. In addition, function GetState259 is similar to GetPin259, except it returns all 8 bits of ShadowState. Program listing. The program LatchDemo consists of the following 2 modules, which are included as separate files along with this application note: LatchDemo.bas Driver74HCT259.bas -- Main program -- API for chip Each file is listed on the following pages. The program is written for a BX-24 system. If you want to port the program to other BasicX systems, most changes should be limited to renumbering I/O pins in module Driver74HCT259. 4

5 Module LatchDemo Option Explicit Public Sub Main() Dim Pin As Byte Initialize259 Debug.Print Debug.Print "Test of 74HCT259 8 bit addressable latch" Debug.Print Do Pulse each pin in sequence. For Pin = 0 To 7 Debug.Print "Pin "; CStr(Pin) Call PutPin259(Pin, True) Delay 0.1 Call PutPin259(Pin, False) Next Debug.Print Delay 0.4 Loop 5

6 Module Driver74HCT259 Option Explicit Driver for 74HCT259 8 bit addressable latch (Radio Shack P/N ). Summary of available modes: Master Latch Reset Enable Action High Low Addressed pin follows data pin, other pins hold their last values High High All pins hold their last values, data and address pins ignored Low Low Addressed pin follows data pin, other pins low Low High All pins low, data and address pins ignored BX-24 I/O pin assignments. These generally need to be changed depending on the BasicX system type. Private Const LatchEnablePin As Byte = 13 Private Const MasterResetPin As Byte = 14 Private Const DataPin As Byte = 15 Private Const Addr0Pin As Byte = 16 Private Const Addr1Pin As Byte = 17 Private Const Addr2Pin As Byte = 18 Private ShadowState As Byte Public Sub Initialize259() Call ClearAllPins Call PutPin(Addr0Pin, bxoutputlow) Call PutPin(Addr1Pin, bxoutputlow) Call PutPin(Addr2Pin, bxoutputlow) Public Sub Terminate259() Initialize259 Public Sub Clear259() Sets all outputs low. Call ClearAllPins 6

7 Public Sub PutPin259( _ ByVal Address As Byte, _ ByVal Value As Boolean) Const MaxAddress As Byte = 7 Safety check. If (Address > MaxAddress) Then Exit Sub Call IsolateOutputs Call WriteAddress(Address) If (Value) Then Call PutPin(DataPin, bxoutputhigh) Call PutBitB(ShadowState, Address, 1) Call PutPin(DataPin, bxoutputlow) Call PutBitB(ShadowState, Address, 0) Call ActivatePin Call IsolateOutputs Public Function GetPin259( _ ByVal Address As Byte) As Boolean Const MaxAddress As Byte = 7 Safety check. If (Address > MaxAddress) Then GetPin259 = False Exit Function GetPin259 = GetBitB(ShadowState, Address) = 1 End Function Public Function GetState259() As Byte GetState259 = ShadowState End Function 7

8 Private Sub WriteAddress( _ ByVal Address As Byte) Const Bit0 As Byte = bx0000_0001 Const Bit1 As Byte = bx0000_0010 Const Bit2 As Byte = bx0000_0100 If ((Address And Bit0) = Bit0) Then Call PutPin(Addr0Pin, bxoutputhigh) Call PutPin(Addr0Pin, bxoutputlow) If ((Address And Bit1) = Bit1) Then Call PutPin(Addr1Pin, bxoutputhigh) Call PutPin(Addr1Pin, bxoutputlow) If ((Address And Bit2) = Bit2) Then Call PutPin(Addr2Pin, bxoutputhigh) Call PutPin(Addr2Pin, bxoutputlow) Private Sub IsolateOutputs() Cause the address pins to be ignored. All output pins hold their last values. Call PutPin(MasterResetPin, bxoutputhigh) Call PutPin(LatchEnablePin, bxoutputhigh) Private Sub ActivatePin() Cause the addressed pin to follow the data pin. All other pins hold their last values. Call PutPin(MasterResetPin, bxoutputhigh) Call PutPin(LatchEnablePin, bxoutputlow) Private Sub ClearAllPins() Call PutPin(LatchEnablePin, bxoutputhigh) Call PutPin(MasterResetPin, bxoutputlow) Call IsolateOutputs ShadowState = 0 8

9 Public Function GetBitB( _ ByVal Operand As Byte, _ ByVal BitNumber As Byte) As Byte Returns the value of the specified bit. This function is similar to GetBit in the system library. Dim Mask As Byte Dim N As Byte Safety check. If (BitNumber > 7) Then GetBitB = 0 Exit Function Mask = bx0000_0001 For N = 1 To BitNumber Right shift mask. Mask = Mask * 2 Next If ((Operand And Mask) = Mask) Then GetBitB = 1 GetBitB = 0 End Function 9

10 Public Sub PutBitB( _ ByRef Operand As Byte, _ ByVal BitNumber As Byte, _ ByVal Value As Byte) Sets the specified bit to 0 or 1 depending on Value. This procedure is similar to PutBit in the system library. Dim Mask As Byte Dim N As Byte Safety check. If (BitNumber > 7) Then Exit Sub Mask = bx0000_0001 For N = 1 To BitNumber Right shift mask. Mask = Mask * 2 Next If (Value = 0) Then Operand = Operand And (Not Mask) Clear bit. Operand = Operand Or Mask Set bit by NetMedia, Inc. All rights reserved. Basic Express, BasicX, BX-01, BX-24 and BX-35 are trademarks of NetMedia, Inc. All other trademarks are the property of their respective owners A 10