Page 120 - PDA Robotics Using Your Personal Digital Assistant to Control Your Robot

PDA 05 5/30/03 11:35 AM Page 96
PDA Robotics
When no prescaler is used,
Using Timer0 with an External Clock.
the external clock input is the same as the prescaler output. The syn-
chronization of T0CKI with the internal phase clocks is accomplished
by sampling the prescaler output on the Q2 and Q4 cycles of the inter-
nal phase clocks. Therefore, it is necessary for T0CKI to be high for at
least 2Tosc (and a small RC delay of 20 ns) and low for at least 2Tosc
(and a small RC delay of 20 ns).
Prescaler. There is only one prescaler available, which is mutually
exclusively shared between the Timer0 module and the WDT. A
prescaler assignment for the Timer0 module means that there is no
prescaler for the WDT, and vice versa. This prescaler is not readable or
writable (see Figure 5.39).

The PSA and PS2:PS0 bits (OPTION_REG<3:0>) determine the
prescaler assignment and prescale ratio. When assigned to the Timer0
module, all instructions writing to the TMR0 register (e.g., CLRF 1,
MOVWF 1, BSF 1, etc.) will clear the prescaler. When assigned to
WDT, a CLRWDT instruction will clear the prescaler along with the
WDT. The prescaler is not readable or writable.
Note: Writing to TMR0, when the prescaler is assigned to Timer0,
will clear the prescaler count, but will not change the prescaler
assignment.

The L298 Dual Full-Bridge Driver
(PDA Robot Motor Controller)

• Operating supply voltage up to 46 V
• Total DC current up to 4 A

• Low saturation voltage
• Over temperature protection
• Logical ”0” Input voltage up to 1.5 V (high noise immunity)

Figure 5.41 shows two of the three available packages that the L298
comes in. In this project, we are using the vertical package shown on
the left.

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