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8XC196Kx,8XC196Jx,87C196CA microcontroller family user's manual.1995.pdf
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8XC196Kx, Jx, CA USER’S MANUAL

 

 

XTAL1

 

 

 

 

CLKOUT

 

 

 

 

ALE

 

 

 

 

BUSWIDTH

 

 

 

 

Bus

 

Address Out

 

Address Out

AD15:8

 

 

 

 

 

 

Bus

Address

Low data in

Address

High data in

AD7:0

Out

+1 Out

(Read)

 

 

 

 

RD#

 

 

 

 

INST

 

 

 

 

Bus

Address

 

Address

 

AD7:0

Low data out

High data out

(Write)

Out

+1 Out

 

 

 

 

 

 

WR#

 

 

 

 

 

 

 

 

A3075-01

Figure 15-6. Timings for 8-bit Buses

15.4 WAIT STATES (READY CONTROL)

An external device can use the READY input to request wait states in addition to the wait states that are generated internally by the 87C196CA, 8XC196Jx, Kx device. When an address is placed on the bus for an external bus cycle, the external device can pull the READY signal low to indicate it is not ready. In response, the bus controller inserts wait states to lengthen the bus cycle until the external device raises the READY signal. Each wait state adds one CLKOUT period (i.e., one state time or 2TOSC) to the bus cycle.

15-14

INTERFACING WITH EXTERNAL MEMORY

After reset and until CCB1 is read, the bus controller always inserts three wait states into bus cycles. Then, until P5.6 has been configured to operate as the READY signal, the internal ready control bits (IRC2:0) control the wait states. If IRC2:0 are all set during CCB0 and CCB1 fetch, READY (P5.6) is configured as a special-function input. If port 5 is initialized after reset, you must ensure that P5.6 remains configured as the READY input. If P5.6 is configured as a port pin, the READY input to the device is equal to zero. This will cause an infinite number of wait states to be inserted into bus cycles and the chip to lock up.

After the CCB1 fetch, the internal ready control circuitry allows slow external memory devices to increase the length of the read and write bus cycles. If the external memory device is not ready for access, it pulls the READY signal low and holds it low until it is ready to complete the operation, at which time it releases READY. While READY is low, the bus controller inserts wait states into the bus cycle.

The internal ready control bits (IRC2:0) define the maximum number of wait states that will be inserted. (The IRC2:0 bits are defined in Figures 15-1 and 15-2.) When all three bits are set, the bus controller inserts wait states until the external memory device releases the READY signal. Otherwise, the bus controller inserts wait states until either the external memory device releases the READY signal or the number of wait states equals the number (0, 1, 2, or 3) specified by the CCB bit settings.

When selecting infinite wait states, be sure to add external hardware to count wait states and release READY within a specified period of time. Otherwise, a defective external device could tie up the address/data bus indefinitely.

NOTE

Ready control is valid only for external memory; you cannot add wait states when accessing internal ROM.

Setup and hold timings must be met when using the READY signal to insert wait states into a bus cycle (see Table 15-2 and Figure 15-7). Because a decoded, valid address is used to generate the READY signal, the setup time is specified relative to the address being valid. This specification,

TAVYV, indicates how much time one has to decode the address and assert READY after the address is valid. The READY signal must be held valid until the TCLYX timing specification is met.

Typically, this is a minimum of 0 ns from the time CLKOUT goes low. Do not exceed the maximum TCLYX specification or additional (unwanted) wait states might be added. In all cases, refer to the data sheets for the current specifications for TAVYV and TCLYX.

15-15

8XC196Kx, Jx, CA USER’S MANUAL

.

Table 15-2. READY Signal Timing Definitions

Symbol

Definition

 

 

TCLYX

READY Hold after CLKOUT Low

Minimum hold time is typically 0 ns. If maximum specification is exceeded, additional wait states will occur.

TAVYV

Address Valid to READY Setup

Maximum time the memory system has to assert READY after the device outputs the address to guarantee that at least one wait state will occur.

CLKOUT

 

 

 

ALE

 

TCLYX

 

 

(MIN)

 

 

 

TCLYX

 

 

 

(MAX)

 

READY

 

 

 

 

TAVYV

 

 

RD#

 

 

 

AD15:0

Address Out

 

Data

WR#

 

 

 

AD15:0

Address Out

Data Out

Address

 

 

 

A3076-01

Figure 15-7. READY Timing Diagram

15-16

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